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From Mike Powell@618:250/1 to All on Sun Jul 7 18:26:00 2024
FOUS30 KWBC 072034
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 PM EDT Sun Jul 7 2024
Day 1 - Valid 16Z Sun Jul 07 2024 - 12Z Mon Jul 08 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
COASTAL TEXAS...
16Z Update...
...Coastal Texas into Louisiana...
Beryl still has a landfall forecast prior to 12Z Monday on the
middle TX Coast (please see the NHC website for the latest
information). The most outer band is roughly along the entirety of
the TX Coast here at 16Z (only spotty heavy rain with this though) with
the inner core picked up on regional NEXRAD heading toward the
middle TX Coast. Updated timing from recent HRRRs allows some
updates to the Day 1 coverages. The inner core rainfall should
exceed 7" over much of the middle TX Coast by 12Z, warranting the
Moderate Risk from Rockport to Freeport an points north. However,
there is increased confidence on inflow bands to the right of the
track spreading up as far east as Galveston Bay and up through the
Houston metro starting around 08Z. In coordination with WFO HGX,
the Moderate Risk has been expanded through the immediate Houston
metro and now extends from San Antonio Bay to Galveston Bay.
The Marginal Risk still does not extend down all of Padre Island
with the 12Z CAM consensus for heaviest rainfall to be east from
Oso Bay where the Slight Risk begins. On the east side, far outer
bands will cross southwest LA, warranting maintenance of the
Marginal Risk there as well as the Slight Risk from Galveston Bay
to the Sabine River/TX-LA border.
...Southern Plains to the central High Plains and to the Upper Midwest...
Organized convection/MCS is over central OK and tracking southeast
which is more progressive/farther southeast than most any HREF CAM
guidance had depicted. This should focus the heaviest
redevelopment later today to be farther south over southwest OK
into Northwest TX this afternoon/evening and less so over KS. The
best guidance so far seems to be the 12Z NAMnest if it is
considered 3-4 hours too slow this morning. This served as a guide
to remove the Slight Risk for most of KS and maintain the Slight
Risk for much of OK into Northwest TX where southerly low level
flow looks to maintain 1.75" PW.
Farther north, greater synoptic forcing is present with the upper
trough extending from an upper low over northern MN to the
southwest over the central Dakotas and extending to the central
High Plains. Redevelopment of activity over the central High Plains
this afternoon/evening looks to track south over eastern CO where
the Marginal Risk was expanded. Downstream is a broken swath of
expected heavy rainfall in the moist southwesterly flow ahead of
the trough axis over eastern KS, northern MO, much of IA and into
southwest WI and western IL where the Marginal Risk was expanded a
bit. Even farther north, ahead of the upper low, redevelopment
around western Lake Superior raises an isolated flash flood threat
for the Arrowhead of MN, around through Duluth to northern WI and
far western U.P. of MI where a Marginal Risk was introduced.
...Southeast...
A stalled frontal boundary across the region will be the focus for
convection which will maintain an isolated threat for flash
flooding concerns. PWs will remain above the climatological 90th
percentile, and layered IVT suggest we may see some mid/upper level
moisture connection to Beryl. The intrusion of tropical moisture
may help bolster rainfall efficiency in what will already be a
favorable environment for heavy rainfall rates. A Marginal Risk remains
in effect for a majority of the Southeast region for this period
with a minor expansion over western NC terrain up to the VA border
per recent HRRR runs.
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR MOST OF
EASTERN TEXAS...
21Z Update...
...East Texas through Arkansas...
12Z HREF consensus along with the 18Z HRRR feature feature a
considerable swath of heavy rain ahead and east of the Beryl track
which starts from just inland from the Middle TX coast at 12Z Mon.
An areal average of 3-8" is featured from the upper TX Coast (from
inflow bands right of the track) and in the inner core rainfall
north from Houston-Bryan TX then into southwest AR and far
southeast OK. The Moderate Risk is expanded a bit more northeast
through the rest of TX as well as farther east toward the LA border for
the inflow bands right of the track. At least locally considerable
impacts can be expected. Fortunately this is a fairly progressive
system per NHC forecast forward speed of at least 10kt. Given
increased confidence in the heavy rain swath extending through
western/central AR and far eastern OK warrants expansion of the
Slight Risk up to the MO/AR border. A trend in the track to the
right continues and will need to be monitored in future outlooks,
especially considering how much inflow into the Ozark
terrain/topographical enhancement will occur.
Elsewhere, tropical moisture from Beryl into a cold frontal
boundary draped over the southern Plains should focus scattered
heavy rainfall over much of OK into the TX Panhandle where a
Marginal Risk is maintained/expanded into northwest TX.
...Southeast...
Convection in proximity to the lingering frontal boundary within PW
anomalies of 2 to 3 sigma will persist over the southern
Appalachians, much of GA, southern SC and northeast FL where a
Marginal Risk is maintained. Recent rains have increased soil
saturation levels, so this additional activity allows a threat for
isolated flash flooding.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS THE MIDDLE
MISSISSIPPI VALLEY...
...Mid-South through Midwest...
Beryl is expected to quickly phase with an upper trough as it lifts
northeast from Arkansas Tuesday. A swath of heavy rainfall is
expected to persist through this extratropical transition with the
12Z GFS/ECMWF in good agreement with another shift to the right
with the track which is in line with NHC forecasts. 3-6 inches over
southeast MO, south-central IL to central IN (along with an
additional 1.5" or so over northern AR) warrants a Slight Risk that
includes the St. Louis metro. Increased confidence in central Gulf
coast precip, but limited inflow banding between the coast and the
remnants over the Mid-South warrants trimming the Marginal Risk on
the south side, while the more progressive and right trend in the
track warrants bringing the Marginal Risk into western OH.
...Southwest...
Monsoonal moisture feeds over eastern AZ through the southern
Rockies warranting maintenance of the Marginal Risk for most of
New Mexico, southeast Arizona into southern Colorado with
particularly considerations for burn scars and areas that have
seen ample rainfall recently.
...Southeast...
Maintained the Marginal Risk over southern Georgia through the
northern FL Peninsula given the abundant moisture over the region
and diurnal forcing for locally heavy rainfall, especially for
northeast Florida. General consensus of 12Z guidance continues to
suggest 1 to 2 inches with local maximums of 3 to 4+ inches.
...Northeast...
Some moisture from the Southeast and ahead of Beryl interacts with
a front over the Northeast. Guidance is rather light with activity
mainly in New England, so for now the Marginal Risk was trimmed
back to north of NYC.
Jackson
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)
From Mike Powell@618:250/1 to All on Mon Jul 8 08:29:00 2024
FOUS30 KWBC 080838
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Mon Jul 8 2024
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR MOST OF
EASTERN TEXAS...
The National Hurricane Center anticipates Hurricane Beryl to make
landfall near Matagorda Beach early this morning and continue to
track northward. It is expected to be a fairly compact storm as it
tracks through eastern Texas so the QPF gradient will be somewhat
tight with its 3 to 8 inches swath slightly east of the track with
local maximums possibly approaching 15 inches. The shift in Beryl's
track resulted in an east and northward adjustment to all of the
inherited Excessive Rainfall Outlook areas. The Moderate Risk
spans from Brazosport Area to south of Lake Calcasieu and northward
into southwest Arkansas. The Slight Risk spans from Matagorda Bay
to west of Vermilion Bay and northward into central Arkansas. The
Marginal Risk spans from San Antonio Bay to Atchafalaya Bay and
from the Texas Panhandle to Southern Missouri. Tropical moisture
from Beryl into a cold frontal boundary draped over the Southern
Plains should focus scattered heavy rainfall over much of Oklahoma
into the Texas Panhandle.
...Southeast...
Convection in proximity to the lingering frontal boundary will
persist across the Southeast. PW values on the order of +2 to +3
standard deviations will be pooled over the region and will
bolster rainfall efficiency. Recent rains have increased soil
saturation levels and with additional showers and thunderstorms
expected the threat for isolated flooding concerns will remain
elevated for this part of the country. The latest guidance trended
higher with the thunderstorm potential and QPF amounts across
portions of northeast/central Florida. The Marginal Risk area was
expanded south to account for this trend and new WPC forecast.
...THERE ARE SLIGHT RISKS FOR EXCESSIVE RAINFALL ACROSS THE MIDDLE
MISSISSIPPI VALLEY AND PORTIONS OF THE LOUISIANA COAST...
...Mississippi Valley and Midwest...
Beryl is still expected to quickly phase with an upper trough and
surface front to the north while in transition to being extra
tropical. This should lead to the potential for heavy rain to
blanket part of the central U.S. causing flash flooding concerns
from Arkansas to Indiana during this period. Some of the highest
QPF amounts of 3+ inches are likely to focus over the Ozarks
region and points northeast to the Illinois/Indiana border. Areal
averages are expected to be around 2 to 4 inches with local
maximums upwards of 8 inches. A Slight Risk extends from central
Arkansas to western Indiana. A Marginal Risk stretches from the
central Gulf Coast northward to southern Michigan. Again, an
future adjustments will likely be dependent on the track for
Beryl.
In addition, some of the CAMs hint that some type of southwest to
northeast band could set up along or just inland of south-central
Louisiana coast to produce a few inches of rain over some areas
that could be a little more sensitive to heavy rain impacts. A
Slight Risk area was raised for this stretch of the coastline north
to about I-10.
...Southwest...
Monsoonal moisture feeds over eastern Arizona through the southern
Rockies warranting maintenance of the Marginal Risk for most of
New Mexico, southeast Arizona into southern Colorado with
particularly considerations for burn scars and areas that have
seen ample rainfall recently.
...Southeast...
Convection will persist during this period thanks to the abundant
moisture hanging around and diurnal forcing. Some of the guidance
continues to suggest areas of locally heavy rainfall especially
along the eastern coastline of Florida. General consensus suggests
1 to 2 inches although local maximums of 3 to 4+ inches will
certainly possible. A Marginal Risk remains in effect for portions
of central/northern Florida and southeast Georgia.
...Northeast...
Interaction of moisture ahead of Beryl and a front settling over
the Mid-Atlantic/southern New England may favor heavy rainfall
potential over this region; although guidance is rather light with
activity mainly in New England. Some focus of the convective rain
ahead of the front(s) and sufficient instability for heavy rain
rates in the northern Mid-Atlantic to Northeast favor of a Marginal
Risk.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES REGION AND THE NORTHEAST...
...Midwest to Northeast...
There is still a fair amount of spread in the guidance with respect
to the evolution/track for post-tropical Beryl and exactly how
this upper level energy ejects into the overall Great
Lakes/Midwest mean trough. The flow of tropical moisture should
bring the highest precipitable water anomalies into the Lower
Great Lakes and Northeast during this period. Some PWAT anomalies
within this region will likely exceed the 90th percentile and west-
east oriented fronts north/east of post- tropical Beryl may promote
training. The exact location of the heaviest rainfall will be
dependent on the location of the frontal boundary but there is a
growing signal for a broad area to be impacted by the heavy rains.
A Slight Risk is in effect from northern Indiana to New Hampshire
while a Marginal Risk area spans from Illinois to Maine. As details
resolve and get closer in time, there may be the need to upgrade
the risk level, but at this time there is too much uncertainty.
...Central Rockies and portions of the Southwest...
Continuation of the monsoonal moisture and diurnal heating will
keep convection possible across portions of the Central and
Southern Rockies and southwest New Mexico during this period. The
inherited Marginal Risk was maintained with westward modification for
New Mexico. Rain in proximity to burn scars and other sensitive areas
that have seen ample rainfall recently will have an elevated risk
for flash flooding.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MIDWEST AS WELL AS FOR THE CENTRAL GULF COAST...
...Mississippi Valley and Midwest...
Tropical moisture associated with Beryl will spread a swath of
heavy rain across the Mississippi Valley and into the Midwest as a
cold front advances east through the region. The last few runs of
guidance have trended toward a more narrow axis setting up right
along the front and QPF values increasing further northeast across Illinois/Indiana and southern Michigan. A sizable swath of 3+
inches continues to span from the Missouri Ozarks to northwest
Indiana with local maximums upwards of 8 inches.
A Slight Risk extends from central Arkansas to southern Michigan.
A Marginal Risk stretches from the central Arkansas northward to
southern Michigan and central Ohio.
...Central Gulf Coast and Southeast...
Deep, onshore flow will continue to drive convection to form along
the Gulf Coast, with the potential for heavy down pours. Areal
average rainfall is expected to be in the 1 to 2 inch range however
the CAM guidance suggests very localized maximums of 3 to 5 inches
possible. A Slight Risk is in effect for portions of the Gulf Coast
from eastern Louisiana to the Florida Panhandle.
Daily diurnal storms are likely once again due to collisions of
sea breezes as well as any outliers associated with the stalled
front along the north Gulf Coast.Some of the guidance continues to
suggest areas of locally heavy rainfall especially along the
eastern coastline of Florida. General consensus suggests 1 to 2
inches although local maximums of 3 to 4+ inches will certainly
possible.
...Southwest...
Isolated to widely scattered convection is likely to form along
the mountains once during the afternoon and evening, which could
very well move over flood-sensitive areas. A Marginal Risk remains
in effect for portions of Arizona, New Mexico, southern Colorado
and extreme western Texas.
...Rio Grande of Texas...
Convection that forms along the mountains of Mexico will have
enough of an eastward push that some may cross into the Del Rio and
Eagle Pass areas. Isolated instances of flash flooding may develop,
therefore a Marginal Risk remains in effect.
...Southern Appalachians...
Storms are likely to form in the deep tropical moisture indirectly
associated with Beryl with the mountains acting as the primary
forcing. Numerous flood sensitive areas in western NC may support
flash flooding with any slow moving or stationary storms. A
Marginal Risk area remains in effect.
...Northeast...
Deep tropical moisture pumping north well ahead of Beryl will make
for anomalous PWATs across New England. The Appalachians of this
region may also act as forcing for widely scattered thunderstorms
capable of heavy rainfall. A Marginal Risk area is in effect for
across this region.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE GREAT LAKES REGION INTO THE NORTHEAST AND FOR CENTRAL NEW
MEXICO...
...Northeast...
Training of storms will likely develop along the west-east
orientated front across New York, Vermont and New Hampshire. The
heaviest rains are expected into the Adirondacks primarily...where
the mountains will provide the greatest additional lift being
oriented largely orthogonal to the WSW flow. PWATs will be more
than 3 sigma above the mean, which compared with GFS climatology
would break the daily record. Thus...there will be unusual amounts
of moisture available for Beryl's remnants to convert to rainfall.
A Slight Risk is in effect from northern Indiana to Maine while a
Marginal Risk area spans from Illinois to Maine. Currently, much of
northern New York and Vermont will be a higher end Slight Risk. As
details resolve and get closer in time, there may be the need to
upgrade to a Moderate Risk.
There will be similarly abundant atmospheric moisture all along
the Eastern Seaboard Wednesday and Wednesday night, but the forcing
further south will be much more limited, resulting in less
widespread convection. However, given the amount of moisture in
place...even local forcings should be enough to result in some
storms.
...Central Rockies and portions of the Southwest...
Continuation of the monsoonal moisture and diurnal heating will
keep convection possible across portions of the Central and
Southern Rockies and southwest New Mexico during this period.
Higher QPF is expected to concentrate in the vicinity of the
Sacramento Mountains where there have been a few recent wildfires.
The burn scar complex (Blue-2, South FOrk, Salt and McBride)
is very sensitive to anything more than a gentle, light rain. QPF
is forecast to range from 0.25 to 1 inch across portions of central
New Mexico which elevates the threat for flash flooding and debris
flows. A Slight Risk was raised for this part of the state given
the aforementioned sensitivity.
...Central Gulf Coast...
A stalled frontal boundary along the Gulf Coast may continue to
provide focus for convection capable or producing heavy rainfall. A
Marginal Risk area remains in effect.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NEW ENGLAND...
...East Coast...
The low pressure system and associated tropical moisture will
progress through New England during this period with its trailing
cold front sweeping through the Mid-Atlantic. Lingering showers and thunderstorms will persist over New England while maintaining an
elevated threat for excessive rainfall and flash flooding. A
Slight Risk remains in effect for much of Vermont, New Hampshire
and central Maine. There will be a better concentration of
convection producing higher QPF across southeast Virginia and
eastern North Carolina. Model consensus had areal averages of 1 to
2 inches for this area although local maximums up to 5 inches may
be possible. A Marginal Risk spans from South Carolina northward to
Maine for this period.
...Southwest...
The monsoonal pattern of diurnal convection will persist thus
maintaining an elevated threat for localized flash flooding. A
Marginal Risk remains in effect for portions of eastern Arizona and
much of New Mexico.m
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
UPSTATE NEW YORK THROUGH MUCH OF VERMONT AND NORTHERN NEW
HAMPSHIRE...
...Northeast...
Remnant moisture and mid-level energy from Beryl will translate
northeastward out the Ohio Valley and Great Lakes creating a
heightened threat for widespread heavy rain and flash flooding
across Southeast Michigan through New York and Central/Northern
New England. At the surface, a warm front will bisect much of New
York through Central New England, marked very well by a sharp
theta-E gradient. This will be a focal point for convection prior
to the main circulation moving overhead, as well as the axis of
where the highest PWATs will be confined during the unfolding of
the synoptic scale event. Low- level instability will be highest
within the confines of the warm front and points south with the
best upper level forcing likely along and north of the boundary
creating a dynamic scenario with a zone of highest heavy rain
potential where all three characteristics overlap. The highest
heavy rain prospects lay within the Adirondacks and points east
into north-central Vermont and New Hampshire, including the Green
and White Mountains in the respective states.
12z HREF EAS probabilities were much more aggressive in the signals
for at least 2" and 3" across the aforementioned areas with a
50-80% probability for at least 2" within the Adirondacks to just
south of the Champlain Valley, a strong signal for higher totals
given the necessary overlap of CAMs to exhibit such a larger
probability. Historically, when an EAS signal is above 70%, the
expectation for widespread coverage of that value of rain or more
is very certain and regardless a higher risk consideration if
the areal FFG indices allow. HREF EAS for 3" was not as strong, but
still manages a large coverage of 20-35% with the highest potential
across the Adirondacks and northern Mohawk Valley. This is right
within the inflection of where the warm front is forecast to
reside, creating a zone of higher confidence for heavy rainfall.
The main prospects for flash flooding will occur between 18z
Wednesday until about 06z across New York state until the end of
the period for points further north and east.
Heavy rain will be possible all the way into Maine where the
elevated PWAT anomalies between +2 to +3 standard deviations will
be recognized leading to a higher end SLGT risk residing from
Southeast Michigan all the way into western and central Maine. Totals
of 2-4" are anticipated with local maxima to 6" plausible within
the above areas in the Moderate Risk. 1-3" will be possible as far
south as central and northeastern PA up through the NY Capital
District due to convection developing along and ahead of the
trailing cold front moving through the area tomorrow afternoon and
evening.
Kleebauer
...Central Rockies and portions of the Southwest...
A Slight Risk for excessive rainfall remains in effect across the
higher terrain of central New Mexico. During this period monsoonal
moisture and diurnal heating will maintain convection across
portions of the Central and Southern Rockies and southwest New
Mexico. Some of the highest forecast QPF is expected to focus in
the vicinity of the Sacramento Mountains where there have been a
few recent wildfires. he burn scar complex
(Blue-2, South Fork, Salt and McBride) are highly sensitive to
rainfall and can easily lead to flooding and debris flows,
especially with the 0.25 to 1 inch QPF that is forecast.
...Central Gulf Coast...
Training convection expected to continue along the stalled frontal
boundary. Very high FFGs within the area highlighted by a Marginal
Risk which will limit flash flooding to very isolated and mainly
within larger urban zones like New Orleans and towns based in the
southeastern Parishes. Given the limited threat, should the
convection shift further south offshore there may be the need to
downgrade in future updates.
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHEAST THROUGH THE MID ATLANTIC, AS WELL AS THE SOUTHWEST
U.S...
...East Coast...
Much of the remnant moisture and ascent from Beryl will be out of
the region however additional rainfall of upwards of 1 to 1.5
inches will be possible across New England, mainly during the
morning hours. Further south, the trailing cold front will be the
focus for convective potential with the highest risk of flash
flooding aligning within the Del-Mar-Va Coastal Plain down through
the eastern Carolinas. An upper level disturbance of the Atlantic will
approach the region and enhance upper forcing and moisture as it
moves closer to the Carolina coast. Meanwhile, a area of
convergence in proximity to a slow-moving surface front will help
trigger the development of widespread convection. The deep layer of
moisture over this region will help bolster rainfall and with the
expected training of cells, 2 to 4 inches may spread over the Mid-
Atlantic. Should the guidance persist with this set up and/or
trend up with amounts, there may be the need to upgrade Risk level.
Kleebauer/Campbell
...Southwest...
The persistent monsoonal pattern of diurnal convection will
maintain an elevated threat for localized heavy rainfall and flash
flooding concerns. Local totals of up to 1" signal a lower-end
threat, however it remains within the Marginal Risk threshold.
Campbell
Day 3
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND NORTH CAROLINA...
The training of thunderstorms capable of producing several inches
of rain will continue through this period. Rain from the day 2
period will have helped to raise soil saturation along the Mid-
Atlantic and Carolina region thus lowering FFG. Most of the
guidance is suggesting additional amounts of 1 to 3 inches from
South Carolina to southern New Jersey with a couple showing local
maximums up to 5 inches along this corridor. A Slight Risk was
maintained for this part of the East Coast. A Marginal Risk spans
from South Carolina northward to southern New hampshire.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR SOUTHEAST
VIRGINIA AND EASTERN CAROLINAS...
...Coastal Mid Atlantic...
Stalled frontal boundary will bisect portions of the Eastern
Seaboard with elevated PWATs mirroring the alignment of the front
to points east. Local environment will be primed for convective
potential, but will have some assistance in upper level support as
a meandering wave off the Southeast coast lifts northwest on the
western fringe of the ridge in the Atlantic, acting as a beneficial
forcing mechanism to enhance regional rainfall potential. The best
chance will lie from the Southern Delmarva through the Virginia
Tidewater down into the Eastern Carolinas. Latest HREF
probabilities are fairly aggressive in their signature for local
totals between 2-5" as neighborhood probs for 2-3" totals are
running between 60-90% with an area of 5" probabilities between
50-70% across Eastern North Carolina. This area has been very dry
as of late, and is well reflected within the FFG indices present
for 1/3/6 hour markers. Isolated to scattered urbanized flooding
and/or flash ponding possible for 2-4 inches/hour rates as per the
latest HREF mean hourly QPF output. A Slight Risk was raised for
the North Carolina coast and a very small potion of southeast
Virginia and northeast South Carolina. A Marginal Risk spans from
South Carolina to southern New Jersey.
Kleebauer/Campbell
...Northern Maine...
Lingering convective threat across Northern Maine could induce
some local flash flood concerns, mainly for northern and Downeast
Maine. The latest guidance continues to signal the highest QPF across
Downeast Maine where the highest PWAT anomalies remain leading to
some deterministic output indicating 1-2" of rainfall within the
first 3-6 hours. The Marginal Risk was kept for this issuance, but
may be considered for removal is convection dissipates sooner than
forecast.
...Midwest...
Trailing shortwave trough pivoting southeast around the backside of
the mean trough over the Great Lakes and Ohio Valley will
strengthen as it moves across Iowa into Illinois by tomorrow
afternoon with large scale forcing increasing across the Central
Midwest and adjacent Mid-Mississippi Valley. Convection is expected
to fire up across Illinois, Missouri and Indiana and within these
storms some will be stronger and capable to producing hourly rates
of 1 to 2 inches/hour. Much of the region was soaked by Beryl as is
passed through and soils have not fully recovered. Antecedent
moisture in the top layer of the soils is well-defined with NASA
SPoRT moisture percentiles relaying the 0-10cm layer running
between 75-90% leading to higher runoff capabilities. Given the
sensitive nature of soils and the potential for a few inches, a
Marginal Risk for excessive rainfall was maintained for this
period.
...Southwest...
The persistent monsoonal pattern will continue to support highly
isolated convection with the primary areas of interest being the
burn scars in New Mexico and small urban threat of towns within
complex terrain across Southeast Arizona through much of New
Mexico. A Marginal Risk remains in effect for southeast Arizona and
a majority of New Mexico.
Campbell
Day 2
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC AND THE CAROLINAS...
A second round of slow moving convection is expected near the
stalled frontal boundary. The greater impacts of scattered
instances of flash ponding and flooding be primarily be focused
over urbanized areas and areas that see repeat heavy rains from
the period prior. The ridge will assist the convection spreading
rains further inland. It's possible the northern extent of the
Slight Risk could be scaled back given some small trends in lower
QPF focused north of the Virginia Tidewater. Trends in that stretch
of the coast will be monitored with future updates. The Eastern
Carolinas will remain the primary target for heavier rainfall with
some areas in the two day period potentially receiving over 5
inches from the evolving event. The Slight Risk spans from
northeast South Carolina to southern New Jersey and the Marginal
stretches from South Carolina to southern New Hampshire.
Campbell
Wanted to also make mention of monitoring the progression and
convective evolution within a migrating inverted trough axis
through South Texas into the Rio Grande. Current ensemble forecast
is relatively tame within the realm of QPF, however the
environment is more than favorable for higher convective impacts,
including heavy rain within the Rio Grande and points just inland.
First Guess fields do pinpoint a small MRGL located within the
corridor from Eagle Pass down through Laredo, so perhaps if
guidance ramps up the QPF signal in future updates, there could be
a targeted Marginal Risk area added to the central RGV.
Kleebauer
Day 3
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024
...THERE ARE MARGINAL RISKS IN THE SOUTHWEST AND ACROSS NORTH
DAKOTA AND MINNESOTA...
There will be a resurgence of monsoonal flow back into the
Southwest. Diurnal heating will help fire up convection during the
afternoon and evening hours with the potential for higher rainfall
fall rates. Maintained the Marginal Risk area for much of Arizona,
northwest New Mexico and southwest Colorado for this period.
...North Dakota and Minnesota...
A shortwave trough will be tracking east across Canada and will
send a surface cold front south-southeast through the Canadian
Prairie and into the northern tier U.S.. Showers and thunderstorms
are expected to form along and ahead of this feature which will
predominately focus over North Dakota and northern Minnesota
during this period. This part of the country has been above/well-
above normal on moisture for the past few months and remain
sensitive to additional rainfall. A Marginal Risk is in effect for
eastern North Dakota and northern Minnesota.
From Mike Powell@618:250/1 to All on Fri Jul 12 09:35:00 2024
FOUS30 KWBC 120815
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Fri Jul 12 2024
Day 1
Valid 12Z Fri Jul 12 2024 - 12Z Sat Jul 13 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC...
...Mid-Atlantic...
No major changes were made with this update, but certainly the
active pattern typical of July continues. A strong front featuring
an impressive moisture gradient is set up along the eastern slopes
of the Appalachians this morning. Meanwhile, a shortwave in the
upper levels will lift up the Ohio Valley and into the Northeast
today through tonight. On the warm side of the surface front, PWATs
are near their maximum for this time of year, with amounts above
2.25 inches over southeast Virginia and the Carolinas. Instability
is somewhat subdued over land, but values quickly spike just off
the coast to over 3,000 J/kg, with a strong southeasterly wind
helping advect it northwestward. Rates with the stronger cells this
morning are routinely exceeding 1 inch per hour.
As the band moves northwestward, it will eventually run into the
front near the I-95 corridor, resulting in a slowing and eventual
stalling of the band. With the upper level shortwave shifting
northeastward, expect multiple rounds of storms training
northeastward along the front today into tonight. The Chesapeake
and Delaware Bay breezes may help to focus that convection by
locally adding forcing, with HREF probabilities over the far
northern Delmarva of 30% for 8 inches of rain in 24 hours as well
as a 30% chance of reaching 100 year ARIs in the 40 km neighborhood probabilities. For the ERO, the area remains in a high-end Slight,
but should those 8 inch probabilities realize, expect flash
flooding. The guidance suggests overnight tonight is the best time
for the heaviest rain for northeastern MD, northern DE, far
southeastern PA, and southwest NJ where the highest threat remains.
It's possible a targeted MDT may be needed for this region, as it's
the one part of MD not in a drought as noted in the latest drought
monitor. The DC-Baltimore I-95 corridor's forecast rainfall totals
have also increased with a small northwestward adjustment to the
axis of heaviest rainfall in the forecast, so urban and small
stream concerns have also increased if the band over eastern MD
stalls there later today into tonight. There remains a high
likelihood of a sharp gradient of rainfall totals on the north and
western side of the front due to the rapid dropoff of atmospheric
moisture on the dry side of the front. Thus, much of western MD,
eastern WV, and western VA will see very little if any rainfall
from this event.
For the Carolinas, many areas can expect 2 separate rounds of rain
as the front pushes west today, then retreats back to the east
overnight. This should overall reduce the flooding threat, though
some areas along the Pamlico and Albemarle Sounds have seen some
heavy rain in recent days which may locally increase the flash
flooding threat.
...Southern Missouri...
A Marginal Risk area was introduced with this update for continued
convection through the morning associated with the current
convection over southeastern Kansas. The storms are very slow-
moving, but some of the guidance does speed it up through the
morning with increasing westerly flow. Much of southeastern
Missouri was recently hard hit with Beryl's remnants, and therefore
have lower FFGs and therefore somewhat more favorable antecedent
conditions.
...Southeast Texas...
Also due to the remnants of Beryl, the potential for typical daily
afternoon convection over this hard hit area may cause isolated
flash flooding in the possible event any stronger cells remain
nearly stationary. A small Marginal Risk was introduced for this
reason.
Wegman
Day 2
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR SOUTHERN NEW
ENGLAND...
...Mid-Atlantic Coast into New England...
In coordination with BOX/Taunton, MA and OKX/Upton, NY forecast
offices, a Slight Risk area was introduced with this update. A
strong front draped across New England will be invigorated by the
passage of a shortwave trough in the upper levels late Friday night
through Saturday morning. While the shortwave will eventually help
the front to slide eastward, this will be after it adds lift to the
ongoing convection from NYC north and east. Light and variable
Corfidi Vectors will become SW or parallel to the front at 10-15
kt. This will support training convection across southern New
England. NASA Sport soil moisture imagery shows much of southern
New England is about average for soil moisture conditions. However,
abnormal moisture with PWATs over 2 inches are expected Saturday
morning. With expected instability also being advected northward
with the front, expect locally intense rainfall rates with the
stronger cells.
Lighter and less organized convection is likely to continue
Saturday morning along much of the Mid-Atlantic coastline as the
front that brought the heavier rain into the I-95 corridor on
Friday pushes east. The Marginal risk was extended to include all
of the Mid-Atlantic coast assuming that rainfall from Friday will
lower FFGs.
...Marginal Risks...
Upper Midwest: The inherited Marginal Risk was shifted east a fair
bit based on the latest guidance. A cold front crossing over
Minnesota, northern Wisconsin, and the western U.P. should lead to
mostly progressive storms across this region, but given favorable
antecedent conditions from recent heavy rainfall and the potential
for training cells on the southern end of any segments that
develop, the Marginal risk still looks good with no plans for any
upgrades.
...Southeast Texas...
Continued soil sensitivity in this region post-Beryl and another
round of afternoon convection may once again cause isolated flash
flooding. A Marginal Risk was introduced with this update but is a
lower-end Marginal.
...Arizona/New Mexico...
Increasing moisture associated with the monsoon should result in
some increase in convective coverage Saturday afternoon.
Nonetheless much of the activity should be confined to northwestern
Arizona and around the Sangre de Cristo Mountains of north-central
New Mexico, where burn scars may increase the potential for flash
flooding.
Wegman
Day 3
Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
GREAT LAKES AS WELL AS FOR ARIZONA AND PORTIONS OF ADJACENT UTAH,
NEW MEXICO, AND COLORADO...
...Great Lakes...
A series of weak shortwave troughs will move across the Great Lakes
and southwesterly flow of Gulf moisture advects into the region.
Expect widely scattered showers to impact the area, resulting in
isolated flash flooding.
...Four Corners...
Increasing moisture with the monsoon will advect northward into the
Mogollon Rim of Arizona. Meanwhile storms that develop over the
mountains of Mexico may advect northwestward across the border near
Nogales, where there's good agreement on over an inch of new
rainfall. While coordination with the associated offices resulted
in agreement to remain a Marginal, it's likely a Slight Risk will
be needed for portions of Arizona with future updates for Sunday
afternoon.
From Mike Powell@618:250/1 to All on Sat Jul 13 10:05:00 2024
FOUS30 KWBC 131126
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
726 AM EDT Sat Jul 13 2024
Day 1
Valid 12Z Sat Jul 13 2024 - 12Z Sun Jul 14 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EASTERN SEABOARD, UPPER MIDWEST, PINEY WOODS, AND FOUR CORNERS REGIONS...
...Missouri... Early morning convection has been more expansive
than earlier guidance, and will likely continue for the first
several hours of the D1 period. See MPD#614 for more information.
...Eastern Seaboard...
In coordination with PHI/Mt. Holly, NJ forecast office, the
inherited Slight was downgraded to a Marginal with this forecast
update. A slow-moving front and disturbance located along the east
coast will gradually move north this morning. Associated showers
and storms will continue today, but the guidance has come down
somewhat on the amounts as the primary area of rain shifts
northeast more quickly this afternoon. There should be widely
scattered instances where a few storms may produce an inch or 2 of
rainfall through this morning, but given both antecedent dry
conditions and general disorganization of the rainfall, expect only
isolated instances of flash flooding today. The disturbance will
drift further east away from the coast by tonight, largely ending
the flash flooding threat.
...Upper Midwest...
Showers and storms may train/backbuild from Minnesota south and
east into the Chicagoland region today as southwesterly flow of
moisture runs into storms tracking southeastward today. There is
considerable disagreement as to where these storms will track and
how much training will occur, with a broad ensemble consensus
suggesting 2 separate lines of fairly fast moving storms moving
across the region today. Given the urbanization on the western
shore of Lake Michigan and some southward trending...expanded the
Marginal to include the Chicagoland region.
...Piney Woods of Southeast Texas...
Widely scattered but slow moving storms may cause isolated flash
flooding in portions of southeast Texas again later today due in
part to some sensitivity of the soils post-Beryl.
...Four Corners...
Monsoonal moisture may cause slow moving storms to form along the
terrain which may cause isolated flash flooding from the Mogollon
Rim over to the Sangre de Cristos of central NM. There has been
little change in the overall coverage and intensity of the storms
in the guidance in the Marginal Risk area, but a notable decrease
further south into southern Arizona, so the Marginal Risk was
trimmed a bit.
Wegman
Day 2
Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE NORTHERN PLAINS AND THE GREAT LAKES AS WELL AS FOR PORTIONS OF
THE SOUTHWEST...
...Northern Plains and Great Lakes...
On Sunday the axis of rainfall across the region will shift a bit
towards the east across the Great Lakes on the periphery of the
building ridge across the middle of the country. Organization of
the storms remains poor in the guidance, so any areas where a
little training may result in higher rainfall totals remains
obscured. Thus, while a small area of Slight risk impacts may be
embedded in the Marginal risk region, uncertainty is too great to
separate out the area of that risk at this time. Further, since
there's good agreement that there will be more rain in portions of
the area (Wisconsin) today, should higher rainfall totals occur
then those areas may be able to be highlighted with a Slight with
future updates.
...Southwest...
Atmospheric moisture will increase across the region as compared to
today by Sunday. This will result in both an increase in coverage
and intensity of the resultant storms across this area today. That
said, the trends in the guidance have been flat. There's somewhat
higher certainty that storms will concentrate along the Mogollon
Rim of Arizona, so that region is in a higher-end Marginal with
potential for an upgrade to a Slight with future updates. Meanwhile
the signals for heavy rain have decreased in far southern Arizona.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER GREAT LAKES AND FOUR CORNERS REGION...
...Upper Great Lakes...
"Ridge running" storms will continue across the upper Great Lakes
again on Monday. The storms should be fast-moving which will
greatly limit the flash flooding potential, but antecedent
conditions after potentially multiple afternoons of storms for some
areas may support a future Slight risk for this area. Nonetheless
there remains too much uncertainty for such an upgrade at this
time.
...Four Corners...
Monsoonal flow may shift a little bit east away from northwestern
Arizona and southern Utah and focus closer to the AZ/NM border
while extending north across much of central Colorado. Storms that
form will once again likely be tied to the terrain, which with burn
scars may result in locally significant flash flooding if the
storms are strong enough and remain stationary, but with still
minimal organization there is not enough certainty to upgrade to a
Slight at this time, but the potential is there for future updates.
From Mike Powell@618:250/1 to All on Sun Jul 14 08:03:00 2024
FOUS30 KWBC 140900
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
500 AM EDT Sun Jul 14 2024
Day 1
Valid 12Z Sun Jul 14 2024 - 12Z Mon Jul 15 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN ILLINOIS...
...Northern Plains and Midwest...
In coordination with LOT/Chicago, IL forecast office, a Slight Risk
upgrade was included with this update for portions of northern
Illinois and far northwestern Indiana.
A series of shortwave disturbances will track roughly west to east
across the northern Plains and Midwest today through tonight. The
latest wave has sparked several MCS's over the Upper Midwest this
morning. The one currently moving across MI/IN is expected to
continue weakening as it moves southeast...however there is a
second one tracking southeast down the Mississippi River between
MN and WI. This one following close behind the first will raise
additional flooding concerns across northern IL and Chicagoland
this morning before moving into MI/IN and dying out later this
morning. These MCSs have a history of producing locally significant
flash flooding, particularly in those areas where cell mergers and
the back edge of the line of storms get hung up, resulting in a
prolonged period of heavy rainfall. A few spots saw 3 inch per hour
rainfall rates. On the large scale, expect these MCSs to continue
moving quickly...but small portions of the storms are likely to get
hung up and cause locally significant flash flooding.
CAMs guidance has been very poor with the handling of these storms,
almost across the board too light and small with coverage. Thus,
there is lower than normal confidence on how the storms likely to
develop once again tonight will behave. Given the new flooding
sensitivities across much of northern Illinois, even comparatively
disorganized and fast moving storms redeveloping tonight over these
same areas will have a high enough potential to cause flash
flooding as to justify a Slight risk upgrade. It should be noted
that HREF guidance is much lower on the potential for FFG
exceedance in this area...though it's all but certain to be using
pre-storms FFGs. Thus, it is assumed the FFGs now across northern
Illinois are much lower and will therefore be much more easily
exceeded with additional shower and thunderstorm activity expected
tonight.
...Southwest...
An influx of drier air at the lower levels around the 4 Corners
region prompted a reduction in the Marginal Risk area for today.
Guidance suggests the greatest moisture has pushed south and west
and will be focused into Arizona for today. Otherwise, expect
typical monsoonal coverage of thunderstorms this afternoon and
evening. There will be widely scattered thunderstorms over much of
the Intermountain West, but the prevalence of low level dry air
should keep most of the precipitation from those storms as virga
except into Arizona where the lower level moisture will remain. Of
course, particularly persistent storms especially those tied to
terrain may produce enough rainfall for long enough to overcome the
dry air, but those are expected to be isolated incidences.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN ILLINOIS INTO SOUTHWESTERN MICHIGAN...
...Midwest...
On Monday evening, a surge of moisture associated with the typical strengthening of the low-level jet (LLJ) will run into a cold front
supported by a strong and expansive area of Canadian high pressure
moving south across Saskatchewan and Manitoba behind a strong upper
level low over western Ontario. The clash of abnormally moist and
warm air pushing northeastward on the LLJ from the central Plains
into the Great Lakes and much cooler and drier air associated with
the Canadian high is likely to occur from the Mississippi River
east across northern Illinois and into southwestern Michigan. This
area has been hard hit with flooding rains overnight tonight, and
while there will be a good 36 hours between these events (barring
any rain tonight), saturated soils across northern Illinois will
certainly support the development of additional flash flooding
across this region. As always with stalled out front, the exact
placement of the front will determine who sees the most rain and
subsequent flash flooding. However, both saturated soils towards
Rockford and urbanization in and around Chicago are lowering FFGs,
such that the potential for multiple inches of rain in just a
couple hours could result in locally significant flash flooding.
With portions of northern Illinois having been hard hit this
morning, expecting some rain again tonight, and then following that
up with potential for multiple separate rounds of training
thunderstorms Monday night, that area has been highlighted for a
higher-end Slight, which depending on the behavior of the storms
tonight may potentially need to be upgraded further.
As the night wears on Monday night, the LLJ is expected to follow
its typical nighttime evolution turning more westerly with time.
This would align the resultant storms parallel to the front which
should move them more quickly, and with less resistance may allow
the front to continue to sag southward, but the potential for
multiple rounds of organized training thunderstorms will persist,
gradually moving south with time.
...Marginal Risks...
South-central Colorado: The guidance is marginally more optimistic
for heavy rain producing thunderstorms to develop across the
mountains of southern Colorado, particularly the San Juans on
Monday afternoon as compared with Sunday. A bit more westerly flow
in the upper levels may allow the storms to have a bit more upslope
support on the west facing sides of the mountains, which are the
areas most likely to see flash-flooding producing thunderstorms
Monday afternoon. That said, the chances are still on the lower-end
Marginal side, and could be downgraded should drying trends
continue. For now, have divided up the inherited Marginal to only
highlight the areas the guidance has better agreement for heavy
rains.
Southern Arizona: Pacific moisture streaming northward from the
Gulf of California will shift a bit to the east on Monday as
compared with Sunday. This should allow somewhat more organized
convection to develop off the mountains of Mexico...impacting far
southeastern Arizona the most Monday afternoon and evening. The
southerly flow of moisture will support storms drifting north out
of Mexico into the southern Arizona desert. Further north, upslope
lifting along the Mogollon Rim will create a secondary maximum of
rainfall across central Arizona, though flash flooding will be
isolated.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST...
...Mid Mississippi Valley to the Northeast...
Ongoing convection over the northern Plains is forecast to
weaken as it drops southeast across the upper into the mid
Mississippi Valley later this morning. However, while differing on
the details, most of the hi-res guidance shows redevelopment
during the afternoon as mid-level energy sliding southeast into the
region begins to interact with deepening moisture (PWs ~2.25
inches) and increasing instability supported by significant
southwesterly low-level inflow. Confidence in the details is
limited by the notable spread in the guidance in how this
convection will evolve. However, despite differences in timing,
most hi-res guidance members show a period of training convection
along a slow-moving boundary, producing locally heavy amounts. A
Slight Risk was maintained from eastern Iowa to northern Indiana
and southwest Michigan. This reflects the area where the 00Z HREF
continues to show higher neighborhood probabilities for
accumulations of 2 inches or more. Based on the trends of the
deterministic runs, believe the greater threat for heavy
accumulations and flash flooding concerns will center in the
western portion of the Slight Risk area, in the vicinity of
southeastern Iowa into central Illinois, where the models show the
stronger signal for training storms.
Further east, the shortwave associated with ongoing convection
moving through the southern Great Lakes is forecast to track
northeast, through the eastern Great Lakes into the St Lawrence
Valley. Storms, redeveloping and intensifying with the return
daytime heating, should be overall fast-moving. However, brief
heavy downpours may pose isolated runoff concerns, especially
across urbanized areas.
...Southwest...
Monsoon moisture will fuel another day of afternoon showers and
thunderstorms, with isolated flash flooding remaining a concern
from southeastern Arizona and southwestern New Mexico into the
Mogollon Rim. Models have been underplaying the convection
development further to the west into southern California. With
little change in pattern and environment expected, a Marginal Risk
was also added across the Peninsular into the southern Transverse
ranges.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MID MISSISSIPPI AND OHIO VALLEYS...
...Central Plains to the Northeast...
As a broad upper trough begins to amplify over the Great Lakes, the
associated surface boundary and accompanying moisture pool will
become a focus for showers and storms and the potential for heavy
rainfall amounts. Southwesterly inflow into the front will support
PWs climbing upwards of 2 inches from the mid Mississippi Valley
eastward into the Ohio Valley. This is also where the guidance
shows the best mid-to-upper level forcing, raising the potential
for west-to-east training storms and heavy accumulations. The
general model consensus shows the greater threat has shifted a
little further south and east, but is overall close to the previous
axis. Therefore, a Slight Risk was maintained from central
Missouri eastward to far western Pennsylvania. Some of the
deterministic guidance do show amounts that would raise Moderate
Risk concerns. However, given the model spread, opted not to
include any upgrades at this point.
While the model consensus indicates the greater threat for
widespread moderate to heavy amounts is further east, there is a
notable signal for locally heavy amounts developing over the
central High Plains, especially over eastern Colorado and
western Kansas. Mid-level energy spilling off the top of the ridge
and interacting with upslope flow and moisture pooling along the
western edge of the surface front will likely support storms
developing along the high terrain before moving eastward. While
differing in the details, most models show at least locally heavy
rainfall amounts, raising at least isolated flash flooding
concerns across the region.
...Southwest...
Greater coverage of isolated to scattered diurnal convection is
expected as the upper high currently in place weakens. This will
allow more showers and storms to develop further to the north,
extending the threat for isolated flash flooding across a greater
portion of northern Arizona and New Mexico.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
COLORADO AND NEW MEXICO...
...Southwest to the central and southern Rockies...
A Slight Risk was introduced from south-central Colorado to central
New Mexico, where moist upslope flow will support increasing
coverage of showers and storms and locally heavy amounts. This will
raise the potential for flash flooding, especially across
vulnerable areas. Guidance indicates a notable increase in low
level moisture transport, raising PW anomalies to 1.5 standard
deviations above normal across central New Mexico. This moisture
along with weak flow aloft and daytime heating, is expected to
support slow-moving storms capable of producing heavy rainfall
rates. Vulnerable areas, including burn scars, urbanized areas, and
areas of complex terrain will be most susceptible to flash
flooding.
Elsewhere, the potential for diurnal convection producing at least
isolated concerns for heavy rainfall and flash flooding will
continue to expand across the Southwest.
...Southern Plains to the Northeast...
Widespread coverage of moderate to locally heavy amounts,
extending from Oklahoma and northern Texas all the way to the Mid
Atlantic and southern New England, can be expected as a cold front
continues to drop slowly south through the central and eastern
U.S. this period. Model spread contributed to the decision to
maintain a broad Marginal Risk across this region for now. However,
there are some model signals that an upgrade to a Slight Risk may
be required across some areas at some point. This includes areas
from eastern Oklahoma through northern Arkansas, where some of the deterministic models show heavy amounts developing as mid-level
energy moving through the base of the trough interacts with a
deeper moisture pool (PWs ~2 inches) along the front. Other areas
include portions of the Ohio Valley and the Mid Atlantic, where
there are signals for at least locally heavy amounts as well.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR ACROSS
PORTIONS OF NORTHERN ILLINOIS, INCLUDING THE CHICAGO METRO...
...Midwest into the Ohio Valley...
...0100Z Update...
Few changes were made to the previous (special) ERO update, based
on the latest observational trends along with recent HREF and HRRR
guidance. MCS across IA-WI-IL continues to grow upscale, with
cloud tops getting up to around -75C per the latest longwave IR
loops. Plenty of instability in place out in front of the MCS...to
the tune of 5000 J/Kg ML CAPEs downshear across northern to east-
central IL and central IN. Given the current low-mid level shear
profiles, expect the QLCS to remain progressive for a while,
however the rapid refresh guidance (RAP and HRRR) continue to show
increasing low level inflow with time overnight, while aligning
nearly parallel to the mean 850-300 mb flow. This should allow for
more upwind propagation by the time the activity reaches central IL
and northeast MO. As a result, we did expand the Slight and
Marginal Risk areas slightly southward across these areas, though
the Moderate Risk area continues as was, due in large part because
of recent heavy rainfall (antecedent saturated soils) while the
northern portion of the QLCS is poised to cross the area this
evening.
Hurley
...Southwest...
Monsoon moisture will fuel another day of afternoon showers and
thunderstorms, with isolated flash flooding remaining a concern
from southeastern Arizona and southwestern New Mexico into the
Mogollon Rim. Models have been underplaying the convection
development further to the west into southern California.
Considering a somewhat decent signal with the SBCAPE fields off the
recent CAMs and PWAT anomalies within the interior of SoCal and
Southern NV, have expanded the MRGL risk and connected the
separated risk areas to become a more uniform MRGL over the region.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF KANSAS
THROUGH PORTIONS OF THE MID MISSISSIPPI AND OHIO VALLEYS...
...20z Update Summary...
The previous SLGT risk was expanded west to include the rest of the
southern half of Missouri all the back into the High Plains of
Western Kansas. A high-end SLGT is forecast over the Ohio Valley
with the primary area of focus located from Southeast MO across
Southern IN/IL/OH and adjacent Northern Kentucky. MRGL risks over
the Southwest and Northeast U.S were adjusted slightly based on the
latest QPF and instability trends, along with some local
collaboration with impacted WFOs.
Kleebauer
...Central Plains to the Northeast...
As a broad upper trough begins to amplify over the Great Lakes, the
associated surface boundary and accompanying moisture pool will
become a focus for showers and storms and the potential for heavy
rainfall amounts. Southwesterly inflow into the front will support
PWs climbing upwards of 2 inches from the mid Mississippi Valley
eastward into the Ohio Valley. This is also where the guidance
shows the best mid-to-upper level forcing, raising the potential
for west-to-east training storms and heavy accumulations.
Recent HREF neighborhood probabilities are officially in range of
the full threat and are depicting some elevated signals for heavy
rain totals positioned across much of the Ohio Valley within the
corridor extending from Southern IL/IN all the way into Southern OH
where the >3" probability is solidly within 40-70% across the
entire aforementioned area of focus. The >5" probability was also
on the higher side considering the setup with 25-40% probs nestled
across Southern IL/IN into Northeast KY. This is likely due to the
increased LLJ interaction with the front as it bisects the area and
runs parallel to the boundary. Mean wind between 925-700mb is
aligned out of the west-southwest with small mid-level
perturbations ejecting eastward across the area. The combination of
all these variables will create an environment conducive for
thunderstorm development and training regime that would exacerbate
the flood threat in any given area along and near the front. This
area was solidly in the SLGT risk prior, but the area was extended
a bit further south to account for a typical northern bias and
likely southern surges of the front due to outflow generation from
the convection nearby.
Across the Central Plains, the front will progress south into KS by
tomorrow afternoon while a shortwave out of the Rockies will dive
southeastward around the eastern flank of a ridge west of the
Continental Divide. The interaction between the front and the
shortwave trough will allow for convective initiation across the
Southeast CO Front Range over into Southwestern KS where the best
instability and deep layered moisture will be co-located. Guidance
has ramped up the coverage and expected intensity of the
thunderstorm development in-of the above area with correlating
higher probabilities in both the 3" (40-70%) and 5" (25-35%)
neighborhood probs with rainfall rates projected between 2-3"/hr in
the strongest cells. There was enough of a signal and agreement
within the deterministic and ensemble QPF distribution to upgrade
that area with a SLGT and link with the threat further east.
Pereira/Kleebauer
...Southwest...
Greater coverage of isolated to scattered diurnal convection is
expected as the upper high currently in place weakens. This will
allow more showers and storms to develop further to the north,
extending the threat for isolated flash flooding across a greater
portion of northern Arizona and New Mexico. Isolated flash flood
threat will be plausible within the complex terrain of any of the
highlighted regions within the risk, as well as targeted areas of
focus in the copious amounts of burn scars present within northern
and central NM and AZ.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL HIGH PLAINS TO THE OHIO VALLEY...
...1145Z Update...
The Marginal Risk has been expanded northward to account for
ongoing thunderstorms over northeast Kansas and northern Missouri.
WPC recently posted MPD #0628 with a breakdown of the mesoscale
forcings that are driving the ongoing rounds of convection there.
Please refer to MPD #0628 for additional details on the Excessive
Rainfall threat in northeast Kansas and northern Missouri.
Mullinax
---Previous Discussion---
...Mid-Mississippi and Ohio Valleys...
Strengthening southwesterly flow ahead a shortwave moving across
Kansas will support deepening moisture ahead of a cold front
sliding southeast into the mid Mississippi Valley this morning and
the lower Ohio Valley later today. Persistent inflow into this
slow-moving boundary along with favorable forcing aloft is
expected to support periods of training storms, with areas of heavy
rainfall likely, especially from central and southern Missouri
eastward into the lower Ohio Valley. Excessive rainfall is
especially a concern from central and southern Missouri through
southern Illinois to the western Indiana and Kentucky border, where
the 00Z HREF is showing high neighborhood probabilities for
accumulations of 3 inches or more through the period.
...Central Plains...
Moist upslope flow behind a cold front settling south through the
central Plains will support storms developing over the high terrain
before these storms spread east into the Plains during the
evening. Merging storms along with increasing organization ahead of
a shortwave digging southeast through the Plains is expected to
support locally heavy amounts from southeastern Colorado through
southwestern Kansas and northwestern Oklahoma, where the HREF is
also showing some notable probabilities for 3 inches or more.
...Upper Ohio Valley to the Northeast...
Ample moisture ahead of an amplifying upper trough over the Great
Lakes and its associated cold front will fuel additional storms
later today. Overall, storms that develop are expected to be
progressive in nature. However, intense rainfall rates may produce
isolated runoff concerns.
...Southwest...
An upper high sitting over the Four Corners will allow moisture to
funnel across the region, with an expansion of convective activity
expected today, bringing the risk for isolated heavy amounts and
flash flooding to a greater portion of Arizona and New Mexico.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
COLORADO AND NEW MEXICO, AND THE ARKANSAS AND RED RIVER BASINS TO MID-SOUTH...
....Southwest to the Central to southern Rockies and High
Plains...
As the previously noted front continues to slide to the
south through the Plains, increasing moisture afforded by low-
level easterly flow will support an increasing threat for locally
heavy rainfall and flash flooding concerns. Ample moisture along
with increasing lift will support storms capable of producing heavy
rainfall rates, that will develop and drop south across the
region. The greater threat continues to center from the Colorado
Sangre de Cristos into the northern and central New Mexico ranges,
where models show some of the better low level inflow and greater
PW anomalies.
Elsewhere, a more typical monsoon pattern will continue, with
diurnal convection generating isolated heavy rainfall and flash
flooding concerns across much of the Southwest.
...Southern Plains to the Northeast...
An upper trough will continue to amplify over the Great Lakes,
pushing a cold front that will extend from the Northeast to the
southern Plains this period further south and east. While locally
heavy rainfall and an isolated threat for heavy rainfall cannot be
ruled out for any area along the front, deeper moisture and
stronger forcing are expected to elevate the threat for some areas.
This includes portions of the Red River and Arkansas basins into
the Mid-South. Deepening moisture and increasing ascent ahead of a
shortwave approaching from the northeast is expected to support
organized heavy rainfall across the region. With plenty of typical
difference in the details, several of the overnight deterministic
models indicate locally heavy amounts of 2-3 inches within the
Slight Risk area.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
SOUTHEASTERN VIRGINIA AND EASTERN NORTH CAROLINA AND THE SOUTHERN
ROCKIES...
...Southern Mid-Atlantic and the Southeast to the lower Mississippi
Valley...
The previously noted cold front will continue to settle further
south and east, bringing heavy rain and the threat for flash
flooding into portions of the southern Mid Atlantic, the Southeast,
and the lower Mississippi Valley. A Slight Risk was maintained
across portions of southeastern Virginia and eastern North
Carolina, where PWs are forecast to increase to ~2.25 inches as the
front settles and begins to stall across the region. This moisture
along with some mid-to-upper level support is expected to fuel
heavy amounts across the region.
Elsewhere, models suggest less organized storms with more isolated
threats for flash flooding further west along the front into the
lower Mississippi Valley.
...Southwest to the central and southern Rockies...
With no significant change to overall pattern expected and plenty
of moisture remaining in place, maintained much of the same outlook
areas from Day 2 into Day 3, including a Slight Risk centered over north-central New Mexico extending into far south-central Colorado.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL HIGH PLAINS TO THE OHIO VALLEY...
...01Z Update...
Was able to trim the Slight Risk area out of much of the Ohio
Valley as convection in that part of the country weakened with the
loss of daytime heating. The Slight risk area remained in place
from parts of Arkansas and southern Missouri westward to the
Central High Plains as additional convection rolls off the higher
terrain and propagates along and immediately north of a quasi-
stationary boundary...with increased chances of heavy rainfall as
storms encounter a strengthening low level jet. Still lingering
convection in northern New England and the Southwest US that should
gradually weaken (in both areas) later this evening.
Bann
...16Z Update...
Adjustments this forecast cycle were to expand the Slight Risk area
over the Central Plains farther south to include more of the
northern TX Panhandle, northern OK, and northern AR. Latest
guidance has shown the nearby stationary front inching a little
more south compared to overnight guidance, which would allow for
segments of storms tonight being farther south than initially
progged over northern OK and even as far west as the TX Panhandle.
Closer to northern AR, strong surface based heating that further
destabilizes the atmosphere ahead of the front may lead to strong
storms by itself given the 89F convective temp via the 12Z LZK
sounding. Then there is the concern for outflows from thunderstorm
activity over northern OK that may approach and act as another
trigger tonight over the Ozarks.
In the Northeast, the soils are more primed in parts of NY's North
Country and around the Finger Lakes following yesterday's stormy
afternoon. Another MCV approaching from the west may lead to a near
carbon-copy setup for today, but storm motions should remain
progressive enough to limit the areal extent of flash flooding to
be more localized. One wildcard to factor in would be areas where
more extensive tree damage occurred that could be more prone to
flash flooding. The 12Z HREF also showed moderate-to-high
probabilities (50-70%) for 3-hr QPF surpassing 3-hr FFGs just east
of Lake Ontario and along the Tug Hill. Still, the progressive
nature of the storms kept any Slight Risk from being issued for the
time being. One adjustment was to expand the Marginal Risk across
most of northern New England as the soils (particularly in northern
ME) are a little more saturated (NASA SPoRT-LIS sows >80% soil
moisture percentiles). Given their added sensitivities and PWATs
that are topping 1.5" (>90th climatological percentile according ot
ECMWF SATs), the Marginal Risk extension was decided upon this
forecast cycle.
Mullinax
---Previous Discussion---
...Mid-Mississippi and Ohio Valleys...
Strengthening southwesterly flow ahead a shortwave moving across
Kansas will support deepening moisture ahead of a cold front
sliding southeast into the mid Mississippi Valley this morning and
the lower Ohio Valley later today. Persistent inflow into this
slow-moving boundary along with favorable forcing aloft is
expected to support periods of training storms, with areas of heavy
rainfall likely, especially from central and southern Missouri
eastward into the lower Ohio Valley. Excessive rainfall is
especially a concern from central and southern Missouri through
southern Illinois to the western Indiana and Kentucky border, where
the 00Z HREF is showing high neighborhood probabilities for
accumulations of 3 inches or more through the period.
...Central Plains...
Moist upslope flow behind a cold front settling south through the
central Plains will support storms developing over the high terrain
before these storms spread east into the Plains during the
evening. Merging storms along with increasing organization ahead of
a shortwave digging southeast through the Plains is expected to
support locally heavy amounts from southeastern Colorado through
southwestern Kansas and northwestern Oklahoma, where the HREF is
also showing some notable probabilities for 3 inches or more.
...Upper Ohio Valley to the Northeast...
Ample moisture ahead of an amplifying upper trough over the Great
Lakes and its associated cold front will fuel additional storms
later today. Overall, storms that develop are expected to be
progressive in nature. However, intense rainfall rates may produce
isolated runoff concerns.
...Southwest...
An upper high sitting over the Four Corners will allow moisture to
funnel across the region, with an expansion of convective activity
expected today, bringing the risk for isolated heavy amounts and
flash flooding to a greater portion of Arizona and New Mexico.
...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS
COLORADO AND NEW MEXICO, FROM THE ARKANSAS AND RED RIVER BASINS TO
MID-SOUTH, AND FOR THE NORTHERN MID-ATLANTIC I-95 CORRIDOR...
...2030Z Update...
...Northern Mid-Atlantic I-95 Corridor...
As daytime heating unfolds, the atmosphere will rapidly destabilize
as high temperatures rise into the mid 90s and dew points average
around 70F. Latest CAMs guidance shows thunderstorms firing
initially along a pre-frontal trough Wednesday afternoon with
potential redevelopment Wednesday evening as the cold front
traverses the region. PWs are expected to range between 2.0-2.2",
which is between the 90-97.5th climatological percentile for the
time of year. Soundings in the region also show no shortage of
MUCAPE (at least 2,000 J/kg) and hodographs that are not only
supportive of organized clusters of storms, but potentially capable
of containing organized mesocyclones (which are more efficient
rainfall producers). The storms will have a rather progressive
motion and soils can most definitely use the rain. However, the
potential for >2"/hr rainfall rates along a highly urbanized
corridor that contains no shortage of hydrophobic surfaces is a
recipe for potential flash flooding. The potential for additional
storms in the evening in wake of the initial pre-frontal storms may
also make some soils more sensitive for the final round of storms
along the approaching cold front. Given these reasons, and in
collaboration with LWX and PHI, a Slight Risk was introduced for
Wednesday.
Otherwise, did adjust the Slight Risk in the Lower Mississippi
Valley and ArkLaTex a little farther south given the slightly
farther south progression with the cold front. The rationale behind
the threat area remains unchanged. Made just minor adjustments to
the Marginal and Slight in the Southwest to account for recent QPF
changes and 12Z HREF guidance.
Mullinax
---Previous Discussion---
....Southwest to the Central to southern Rockies and High
Plains...
As the previously noted front continues to slide to the
south through the Plains, increasing moisture afforded by low-
level easterly flow will support an increasing threat for locally
heavy rainfall and flash flooding concerns. Ample moisture along
with increasing lift will support storms capable of producing heavy
rainfall rates, that will develop and drop south across the
region. The greater threat continues to center from the Colorado
Sangre de Cristos into the northern and central New Mexico ranges,
where models show some of the better low level inflow and greater
PW anomalies.
Elsewhere, a more typical monsoon pattern will continue, with
diurnal convection generating isolated heavy rainfall and flash
flooding concerns across much of the Southwest.
...Southern Plains to the Northeast...
An upper trough will continue to amplify over the Great Lakes,
pushing a cold front that will extend from the Northeast to the
southern Plains this period further south and east. While locally
heavy rainfall and an isolated threat for heavy rainfall cannot be
ruled out for any area along the front, deeper moisture and
stronger forcing are expected to elevate the threat for some areas.
This includes portions of the Red River and Arkansas basins into
the Mid-South. Deepening moisture and increasing ascent ahead of a
shortwave approaching from the northeast is expected to support
organized heavy rainfall across the region. With plenty of typical
difference in the details, several of the overnight deterministic
models indicate locally heavy amounts of 2-3 inches within the
Slight Risk area.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, THE EASTERN MID-ATLANTIC REGION, & THE
SABINE RIVER VALLEY...
...Southern Mid-Atlantic & South...
A cold front will advance southeastward into the Mid-Atlantic
region today while the western portion slows across the southern
Plains. Along and ahead of this boundary will remain a focus for
deep moisture (PWATs > 2 inches in some places of the Mid-
Atlantic, Southeast and Gulf Coast states. The greatest threat for
heavy rainfall appears to be with and ahead of the front along
pre-frontal convergence axes from southeastern Virginia into
eastern North Carolina. In addition to the anomalous moisture and
low level convergent axes, upper levels will be favorable within
the right entrance region of a jet positioned east of an upper
trough axis advancing eastward through the Great Lakes, with an
area of upper level diffluence forecast over the eastern Mid-
Atlantic region due to the presence of a ridge off of the Southeast
coast. For the 24 hour period ending 12Z Friday, 00Z HREF guidance
supports EAS probabilities for 3 inches of 30 percent between
Albemarle and Pamlico Sounds along with standard probabilities for
8 inches of 30-40 percent in the same location. While this is
typically a less flash-flood prone region, these probability values
are not super common and could be highlighting the potential for
locally significant rainfall totals somewhere from southeastern
Virginia into eastern North Carolina.
...Eastern Texas into Western Louisiana...
A second and relatively small Slight Risk was maintained for
locations near the Arklatex and southward along the Sabine River to
account for locally greater potential for heavy rain. Upper level
diffluence will be maximized here and greater coverage of
thunderstorms are during the afternoon/overnight near/south of an
850 mb low located near the Arklatex. Individual members of the 00Z
HREF and RRFS indicate localized QPF maxima of 4 to 6+ inches.
...Southwest to the Southern Rockies...
Another day of monsoonal convection is expected for the southern
Rockies into the Southwest as the center of the mid-level ridge
shifts slightly west from the Four Corners region through Friday
morning. The overall pattern, available moisture and instability
are expected to remain largely unchanged from Wednesday, resulting
in similar threats for heavy rainfall and flash flooding across
the region. Rainfall accumulations of 1-2 inches in less than an
hour and localized maxima of 2-3 inches are expected with a focus
across the Rockies where low level moisture flux will be maximized.
Otto/Roth
Day 2
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES AND EASTERN CAROLINAS...
...Carolinas to the Lower Mississippi Valley...
A front will linger across the region, focusing deep moisture and
another day of unsettled weather. The greatest threat for
organized heavy rainfall is expected to focus along the eastern
extent of the boundary ahead of a mid-level shortwave forecast to
track northeastward from Mississippi/Alabama. After settling a bit
further south across the eastern Carolinas, guidance shows a wave
developing along the front bringing deeper moisture back to the
north. Some portions of eastern North Carolina could see back-to-back
days of heavy rain, raising flash flooding concerns across the
region, though it appears the main axis of rainfall will be
generally south for Friday to Saturday compared to the Thursday to
Friday period.
...Southwest to the Southern Rockies...
No significant changes in moisture are expected for Friday into
Saturday compared to the previous day. The mid-level ridge will
begin to show better movement toward the Great Basin but diurnal
convection is expected to again raise at least isolated flash
flooding concerns for many of the same areas as the Thursday to
Friday period. Therefore, the Slight and Marginal Risks remain
similar to the previous day with potential for rainfall
accumulations of 1-2 inches in less than an hour and localized
maxima of 2-3 inches.
...Eastern Nebraska and Adjacent Locations...
A progressive front is expected to bring precipitable water values
over 1.5 inches across eastern Nebraska Friday afternoon/evening
with 850 mb winds of 20-30 kt ahead of a developing low. Sufficient instability, shear and moisture will be in place within the region
of maximized low level moisture transport to support areas of
organized convection. Given the above, hourly rain totals to 2"
with local amounts to 4" are possible. As the front is progressive,
any flash flooding should be limited in scope and a Marginal Risk
was maintained from continuity to cover the possibility.
Otto/Roth
Day 3
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS THE
SOUTHEAST AND FROM THE CENTRAL/SOUTHERN PLAINS INTO THE
CENTRAL/SOUTHERN ROCKIES AND SOUTHWEST...
...Mid-Atlantic to Central Gulf Coast...
A slow moving front will continue to extend from the Mid-Atlantic
coast into portions of the Tennessee Valley with anomalous moisture
(1 to 2 standardized anomalies of precipitable water) in place in
the vicinity of the front to the Gulf and Southeast coasts.
Slightly stronger southwesterly low level winds are expected across
the Carolinas which could support areas of training and/or slow
movement of thunderstorm clusters containing localized rainfall
rates of 2-3 in/hr. Southwesterly flow will be in place to the east
of a longwave trough axis centered near the Mississippi River and
upper level jet support will likely focus convective activity over
southern Virginia into North Carolina ahead of a shortwave to move
across the Mid- Atlantic states during the day on Saturday. A
Marginal Risk remains in place where the highest probabilities for
2+ inches of rain exist in the latest ensemble guidance. There
wasn't enough agreement in the latest deterministic guidance for
anything higher than a Marginal Risk at this time, but upgrades may
be warranted with future forecast cycles.
...Central Plains to Texas Panhandle...
While some minor differences exist in the models, there is broad
agreement that the central to southern Plains will be situated in
northwest flow aloft with a possible closed mid-level center in the
vicinity of Iowa. A weak frontal boundary may exist across northern
Texas but the focus for potentially excessive rainfall will exist
ahead of an 850 mb low/trough extending from near the Mid-Missouri
River Valley into the Texas Panhandle with weakly anomalous
moisture from northwestern Texas into the Missouri River Valley.
Current thinking is for some 2-4 inch totals, which may fall over a
short period of time supporting localized areas of flash flooding.
...Central/Southern Rockies into the Desert Southwest...
Mid-level ridging is expected to migrate westward into the Great
Basin from Saturday to Sunday morning. The translation of the ridge
and possible shortwave trough advancing southeast across the
southern Plains will increase the easterly component of the low to
mid-level flow across New Mexico into eastern Arizona. Moisture is
forecast to be seasonable to above average, with the best
potential for above average moisture setting up from West Texas
into southern New Mexico and southern Arizona. In terms of upper
level forcing and mean steering flow however, nothing stands out as
anything greater than a Marginal Risk of flash flooding at this
time with typical monsoonal convection.
Otto
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)
From Mike Powell@618:250/1 to All on Fri Jul 19 08:44:00 2024
FOUS30 KWBC 190828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Jul 19 2024
Day 1
Valid 12Z Fri Jul 19 2024 - 12Z Sat Jul 20 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES AND EASTERN CAROLINAS...
...Carolinas...
The cold front from the past few days will become quasi-stationary
across the Carolinas with a strong low-level convergence signature
focused from Northeast NC down through Central SC in the Piedmont.
Areal PWATs are sufficient for heavy rainfall potential with
anomalies nestled between 1-1.5 deviations above normal with values
between 1.8-2.2" focused over the above area. Current mean QPF is
right around 1-1.5" with some smaller maxima located across
multiple portions of the Carolinas which is reflective of the
overall convective environment. The primary signature of note for
the potential is heavily based in the latest probability fields
with EAS signatures between 25-40% for at least 1" present through
much of the Eastern Carolinas with the max confined to the eastern
extent of the Piedmont down to around Columbia, an urban area more
prone for flash flooding historically. Neighborhood probabilities
for at least 3" show multiple areas between 50-70% with another max
located within the escarpment of SC/NC/TN where models are
depicting some terrain focused convection later this afternoon.
There are some minor signals for at least 5" totals within the
confines of the Eastern Carolinas as well signaling a general
maximum of up to 6" in any cells. Area hodographs indicate a weak
steering pattern which would lead to either slow moving, or even
stationary storms that could enhance the local threat further as
the storm motions would only be accelerated through outflow
propagation. The SLGT risk from the previous forecast was
maintained with some expansion over the northern and southern
fringes to match the recent trends within the probability fields.
...Southern Rockies...
General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel as noted within
the convective QPF footprint. The area will see another round of
diurnally driven convection with more mid-level perturbations
rounding the eastern flank of the ridge, moving overhead during the
peak of diurnal heating. This will create an enhancement within the
large scale ascent pattern along with the already favorable
instability field with the theta-E ridge bisecting much of Central
and Northern NM. Local totals of 1-2" with a few spots seeing
higher amounts are forecast across the terrain down into the
northern extent of the Bootheel near Gila National Forest.
Probabilities for at least 1" are relatively higher when gauging
the neighborhood probability output with some scattered signals for
at least 2" in portions of Central NM. The combination of multi-day
impact within complex terrain and remnant burn scars exacerbating
the flash flood concerns was plenty of reason to extend the SLGT
risk maintain general continuity.
Kleebauer
Day 2
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES THROUGH MUCH OF NEW MEXICO...
...Southern Rockies...
A pattern of persistence within the mid and upper-levels will lead
to an ongoing threat of convective development in-of the Southern
Rockies and adjacent terrain of NM due to primed environmental
destabilization coinciding with a progression of mid-level
vortices streaming down the eastern flank of the ridge across the
Western U.S. Theta-E ridge is forecast to shift focus across NM
with the tongue of elevated instability generally confined to the
terrain extending from the Bootheel up through the Sangre de
Cristos. Scattered to numerous thunderstorms will be capable of
locally heavy rainfall that will interact with complex terrain and
the burn scars littered across the Southern Rockies down to the
Sacramento Mountains. Areal QPF average between 0.75-1.5" is
forecast with a few locations potentially seeing upwards of 2"
which would easily cause issues no matter where they occur given
the lower FFGs situated across the state. Considering the ongoing
impacts from prior periods and the convective coverage anticipated,
maintained the previous SLGT risk with some broadening of the risk
area to reflect the concerns even outside the terrain focused
areas.
...Southeast to Southern Mid Atlantic...
Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.
Kleebauer
Day 3
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES THROUGH MUCH OF NEW MEXICO...
...Southern Rockies...
Mid-level ridge axis will begin sliding eastward with building
heights centered over the Lower Colorado River Basin into the
southern portion of the Great Basin. A multitude of mid-level
vortices will continue to plunge down the eastern flank of the
ridge and interact with ample surface destabilization located
within the Southern Rockies. With steep low to mid-level lapse
rates and prominent moisture anomalies running close to 1-2
deviations above normal,AND SOUTHWEST convective threats will
induce periods of moderate to heavy rainfall within the general
confines of the Southern Rockies with emphasis on the Sangre de
Cristos and the Northern fringe of the Sacramentos. There will also
be an extension of convective concerns down closer to the NM
Bootheel within the Gila National Forest just to the north of
Silver City. Areal QPF within the means are consistent with
0.75-1.25" with some bias corrected maxes exceeding 2" in spots,
mainly over Northern NM. With the successive period of heavy
rainfall potential within the terrain and remnant burn scars, the
threat remains well within the SLGT risk range inherited from prior
forecast. Thus, outside some adjustment within the fringes to
reflect QPF trends, the SLGT risk was majority continuity.
...Mid-Atlantic to Central Gulf Coast...
A mid-level trough will begin to carve out across the Central
Plains with a lingering stationary front still bisecting much of
the Southeast into the Southern Mid Atlantic. Broad extent of
thunderstorm chances will remain stagnant in terms of placement and
potential impact with greater effects generally relegated to focal
points near higher surface convergence patterns and deep layer
moisture field present in any given area. Scattered to numerous
thunderstorms are forecast across TX to the Central Gulf Coast
during the afternoon on Sunday with locally heavy rainfall
plausible in any cells that develop. General 1-2" are possible in
any convective signal, however the area over the Central Gulf has
the best chance for heavier returns due to a deeper moisture field
located along the Gulf. These areas do have a higher FFG however,
so the threat is on the lower end of MRGL during the period, along
with the area across TX. Continued threat of heavy rainfall is
forecast across the Southern Mid Atlantic where multiple shortwaves
will enter the region and enhance regional ascent within the
convergence corridor near the stationary front. The main area of
impact is subject to some variability at range due to the forecast
frontal placement being a major part of where training convection
could develop. The MRGL risk across the area was broad but worthy
of the current location given the setup. An upgrade is possible
pending the convective evolution over the previous period.
From Mike Powell@618:250/1 to All on Sat Jul 20 10:00:00 2024
FOUS30 KWBC 200818
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
418 AM EDT Sat Jul 20 2024
Day 1
Valid 12Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST, SOUTHERN ROCKIES, AND SOUTHEASTERN VIRGINIA...
...Southeast Virginia...
A well-defined shortwave analyzed across the AL/GA line will advect
northeast through the early portion of the period, eventually
closing off at 85H as it progresses into Southern VA by the early
afternoon period. At the surface, a quasi-stationary front will
lift northward towards the vicinity of the Hampton Roads corridor
with a surface reflection developing over the NC/VA border and
riding along the confines of the boundary. Increasingly backed flow
within the boundary layer will promote the advection of deeper low
to mid-level moisture that will align with the frontal boundary as
the surface wave approaches with generally more favorable support
within the 850-700mb height field. The combination of the above
variables will allow for a stronger low-level convergence zone
aligned within close proximity to the wavy frontal positioning
transposed over the region. The latest HREF and correlating CAMs
output have all increased the signal within the QPF footprint with
the ensemble means generally depicting an axis of heavier precip
from I-85 to the east towards the I-64 corridor and adjacent
coastal areas. QPF maxima in exceedance of 5" is common within the deterministic output along with the HREF blended mean signifying a
stripe of 2-3+" within the confines of Williamsburg down close to
Norfolk. HREF neighborhood probabilities were also fairly stout
when it comes to the upper quartile of the threat as the >3" probs
sit at a widespread 40-70% over the entire stretch of Southeast VA
from I-85 to I-64 with a bullseye of 80% located along I-64. The
5" probability field is less robust out towards the I-85 area up
to Richmond, but there's still a significant corridor of 30-60%
probabilities situated across that I-64 area down close to Hampton
Roads.
The signals were pronounced enough to warrant a targeted upgrade
across the aforementioned area with the alignment of the risk
comprising of the higher forecast probabilities for at least 3"
with the best threat likely over the I-64 area southeast of
Richmond. The general proxy of the front and the aided surface wave
bisecting the area will likely target an area under or just to the
north of the general circulation. The track of any 850mb low
development will only add to the prospects of increasing low-level
convergence signatures that would spawn an axis of very heavy
rainfall (>2"/hr) within the northern periphery of that circulation
as well. A SLGT risk was added to account for the evolving setup.
...Southwest and Southern Rockies...
General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel over into
Southeast AZ as noted within the convective QPF footprint. The
area will see another round of diurnally driven convection with
more mid-level perturbations rounding the eastern flank of the
ridge, moving overhead during the peak of diurnal heating. This
will create an enhancement within the large scale ascent pattern
along with the already favorable instability field with the theta-E
ridge bisecting much of Central and Northern NM. Local totals of
1-2" with some locally enhanced amounts are forecast across the
terrain down into the northern extent of the Bootheel near Gila
National Forest. HREF neighborhood probabilities for at least 1"
are very high with an areal coverage exceeding 70% that encompasses
much of north-central NM, the Sacramento Mountains, and the
Huachucas in Southeastern AZ. The 2" probabilities are also higher
than previous days with an expanse of 15-30% outputs with a few
bullseyes over 30% within the aforementioned zones above. The
combination of multi- day impact within complex terrain and remnant
burn scars exacerbating the flash flood concerns was plenty of
reason to extend the SLGT risk and maintain general continuity.
Kleebauer
Day 2
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND SOUTHWEST...
...Southwest and Southern Rockies...
A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The ensemble mean QPF was
not as robust compared even with the addition of the elevated
deterministic signals leading to less confidence of higher impacts
that would necessitate an expansion of a higher risk across parts
of Central AZ. The previous update does have the eastern most
confines of the Mogollon Rim within the inherited SLGT risk which
does jive with the latest NBM probability fields signaling the best
threat further east away from the ridge center. QPF mean around
0.75-1.25+" was the target precip depiction for where to outline
the SLGT risk which aligns with the lower FFG exceedance markers
across the region. The highest probabilities for those totals
within the NBM are mainly in NM with only some of AZ seeing those
types of better outputs. This solidified the SLGT risk from
previous forecast with some minor modifications based on the QPF
footprint. The primary areas of interest within the current SLGT
will reside within the complex terrain and areas surrounding the
remnant burn scars located within the Sacramentos and Sangre de
Cristos. SBCAPE between 800-1200 J/kg is forecast within the
ensemble means with some max of 1500+ J/kg showing up within the
terrain of NM and AZ. Expect a general continuity for the next
succession of updates with perhaps that westward expansion of the
SLGT into the Mogollon Rim if the short range guidance trends
upward with the heavy rain signal.
...Southeast to Southern Mid Atlantic...
Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.
...Southern Plains...
The eastern extent of the broad ridge axis across the Western U.S
will help send down multiple mid-level perturbations that will
allow for convective development upstream over the Eastern NM High
Plains, eventually propagating south and east along expected
outflow generation and mean steering flow out of the north-
northwest. There's a growing signal for organized multi-cell
clusters to move off the Caprock and enter portions of West TX and
the nearby Upper Trans Pecos. Deterministic output is subject to
variability in the location and magnitude of any convective cluster
that materialized and enters the area. The signal for potential is
present and was enough to expand the western MRGL risk area further
southeast to account for the threat of more organized heavy
rainfall. This is within the lower end of the risk threshold for
now, but will be subject to expansion or removal based on later
trends within numerical output, especially in the hi-res window.
Further to the southeast across Central and Eastern TX, an
approaching front will become a focal point for diurnally driven
convective development in-of the boundary itself as the environment
it moves into becomes increasingly favorable for cell generation.
The convergence signal is not nearly as impressive as areas further
to the east, but the ensemble mean QPF footprint dictates a
smattering of smaller QPF maxima that would indicate some locally
enhanced rainfall potential. Mean QPF is only around 0.75-1.25" in
spatial coverage, but some deterministic output is much more robust
with 2-4+" bullseyes in portions of Hill Country, Concho Valley,
out into the I-35 corridor. This is a lower end MRGL risk
opportunity within the region, but the upper quartile of outcomes
could spell for some local flash flood concerns if everything
breaks right (or wrong depending on perspective).
From Mike Powell@618:250/1 to All on Sat Jul 20 12:08:00 2024
FOUS30 KWBC 201551
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1151 AM EDT Sat Jul 20 2024
Day 1
Valid 16Z Sat Jul 20 2024 - 12Z Sun Jul 21 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST, SOUTHERN ROCKIES, AND SOUTHEASTERN VIRGINIA...
16Z Update...
Overall...changes to the on-going Excessive Rainfall Outlook were
pretty minor. Broad area of deep moisture and resulting instability
remained in place across the Southeast US with weak and difficult-
to-time shortwaves embedded within the mean flow. Flow looks to
remain confluent...which will help support cell development and
fuel the potential for isolated to widely scattered downpours
throughout the day and evening. There were some signals from the
12Z HREF probabilities for 1 to 2 inch per hour rates over portions
of northern Iowa into far southern Minnesota. The previously issued
Marginal did not reach quite that far north...so extended the area
a bit to cover the potential.
Over the western US...models still tended to support the previously
issued Marginal risk area as well as the embedded Slight Risk area
extending from southeast Arizona across much of New Mexico into
southern Colorado. Except for a few nudges...the area remained
unchanged from the initial issuance.
Bann
...Southeast Virginia...
A well-defined shortwave analyzed across the AL/GA line will advect
northeast through the early portion of the period, eventually
closing off at 85H as it progresses into Southern VA by the early
afternoon period. At the surface, a quasi-stationary front will
lift northward towards the vicinity of the Hampton Roads corridor
with a surface reflection developing over the NC/VA border and
riding along the confines of the boundary. Increasingly backed flow
within the boundary layer will promote the advection of deeper low
to mid-level moisture that will align with the frontal boundary as
the surface wave approaches with generally more favorable support
within the 850-700mb height field. The combination of the above
variables will allow for a stronger low-level convergence zone
aligned within close proximity to the wavy frontal positioning
transposed over the region. The latest HREF and correlating CAMs
output have all increased the signal within the QPF footprint with
the ensemble means generally depicting an axis of heavier precip
from I-85 to the east towards the I-64 corridor and adjacent
coastal areas. QPF maxima in exceedance of 5" is common within the deterministic output along with the HREF blended mean signifying a
stripe of 2-3+" within the confines of Williamsburg down close to
Norfolk. HREF neighborhood probabilities were also fairly stout
when it comes to the upper quartile of the threat as the >3" probs
sit at a widespread 40-70% over the entire stretch of Southeast VA
from I-85 to I-64 with a bullseye of 80% located along I-64. The
5" probability field is less robust out towards the I-85 area up
to Richmond, but there's still a significant corridor of 30-60%
probabilities situated across that I-64 area down close to Hampton
Roads.
The signals were pronounced enough to warrant a targeted upgrade
across the aforementioned area with the alignment of the risk
comprising of the higher forecast probabilities for at least 3"
with the best threat likely over the I-64 area southeast of
Richmond. The general proxy of the front and the aided surface wave
bisecting the area will likely target an area under or just to the
north of the general circulation. The track of any 850mb low
development will only add to the prospects of increasing low-level
convergence signatures that would spawn an axis of very heavy
rainfall (>2"/hr) within the northern periphery of that circulation
as well. A SLGT risk was added to account for the evolving setup.
...Southwest and Southern Rockies...
General persistence will create another day of locally heavy
rainfall threats located within the Southern Rockies with a
southern expansion down closer to the NM Bootheel over into
Southeast AZ as noted within the convective QPF footprint. The
area will see another round of diurnally driven convection with
more mid-level perturbations rounding the eastern flank of the
ridge, moving overhead during the peak of diurnal heating. This
will create an enhancement within the large scale ascent pattern
along with the already favorable instability field with the theta-E
ridge bisecting much of Central and Northern NM. Local totals of
1-2" with some locally enhanced amounts are forecast across the
terrain down into the northern extent of the Bootheel near Gila
National Forest. HREF neighborhood probabilities for at least 1"
are very high with an areal coverage exceeding 70% that encompasses
much of north-central NM, the Sacramento Mountains, and the
Huachucas in Southeastern AZ. The 2" probabilities are also higher
than previous days with an expanse of 15-30% outputs with a few
bullseyes over 30% within the aforementioned zones above. The
combination of multi- day impact within complex terrain and remnant
burn scars exacerbating the flash flood concerns was plenty of
reason to extend the SLGT risk and maintain general continuity.
Kleebauer
Day 2
Valid 12Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES AND SOUTHWEST...
...Southwest and Southern Rockies...
A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The ensemble mean QPF was
not as robust compared even with the addition of the elevated
deterministic signals leading to less confidence of higher impacts
that would necessitate an expansion of a higher risk across parts
of Central AZ. The previous update does have the eastern most
confines of the Mogollon Rim within the inherited SLGT risk which
does jive with the latest NBM probability fields signaling the best
threat further east away from the ridge center. QPF mean around
0.75-1.25+" was the target precip depiction for where to outline
the SLGT risk which aligns with the lower FFG exceedance markers
across the region. The highest probabilities for those totals
within the NBM are mainly in NM with only some of AZ seeing those
types of better outputs. This solidified the SLGT risk from
previous forecast with some minor modifications based on the QPF
footprint. The primary areas of interest within the current SLGT
will reside within the complex terrain and areas surrounding the
remnant burn scars located within the Sacramentos and Sangre de
Cristos. SBCAPE between 800-1200 J/kg is forecast within the
ensemble means with some max of 1500+ J/kg showing up within the
terrain of NM and AZ. Expect a general continuity for the next
succession of updates with perhaps that westward expansion of the
SLGT into the Mogollon Rim if the short range guidance trends
upward with the heavy rain signal.
...Southeast to Southern Mid Atlantic...
Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. Unlike the previous period, a lack of stronger
mid-level vorticity will negate the higher end potential as the
ascent pattern is mainly limited to boundary layer destabilization
and low-level convergence within the front. The previous MRGL risk
was maintained with some minor adjustments.
...Southern Plains...
The eastern extent of the broad ridge axis across the Western U.S
will help send down multiple mid-level perturbations that will
allow for convective development upstream over the Eastern NM High
Plains, eventually propagating south and east along expected
outflow generation and mean steering flow out of the north-
northwest. There's a growing signal for organized multi-cell
clusters to move off the Caprock and enter portions of West TX and
the nearby Upper Trans Pecos. Deterministic output is subject to
variability in the location and magnitude of any convective cluster
that materialized and enters the area. The signal for potential is
present and was enough to expand the western MRGL risk area further
southeast to account for the threat of more organized heavy
rainfall. This is within the lower end of the risk threshold for
now, but will be subject to expansion or removal based on later
trends within numerical output, especially in the hi-res window.
Further to the southeast across Central and Eastern TX, an
approaching front will become a focal point for diurnally driven
convective development in-of the boundary itself as the environment
it moves into becomes increasingly favorable for cell generation.
The convergence signal is not nearly as impressive as areas further
to the east, but the ensemble mean QPF footprint dictates a
smattering of smaller QPF maxima that would indicate some locally
enhanced rainfall potential. Mean QPF is only around 0.75-1.25" in
spatial coverage, but some deterministic output is much more robust
with 2-4+" bullseyes in portions of Hill Country, Concho Valley,
out into the I-35 corridor. This is a lower end MRGL risk
opportunity within the region, but the upper quartile of outcomes
could spell for some local flash flood concerns if everything
breaks right (or wrong depending on perspective).
From Mike Powell@618:250/1 to All on Sun Jul 21 12:15:00 2024
FOUS30 KWBC 211555
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1155 AM EDT Sun Jul 21 2024
Day 1 Valid 16Z Sun Jul 21 2024 - 12Z Mon Jul 22 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHWEST AND THE WESTERN CAROLINAS...
...16Z Excessive Rainfall Discussion...
Few changes needed. There was a southeastward extension of the
Slight Risk a bit farther into Texas than before given a subtle
increase in QPF shown by the NBM and reflected by the HREF
probabilities. Otherwise...the 21/12Z suite of guidance largely
supported the on- going ERO/ERD.
Bann
...Western Carolina's...
General pattern persistence across the Southern Mid Atlantic will
yield another round of scattered to widespread convection in-of
the Carolinas with some locations setup to see a greater emphasis
for heavy rainfall. A few weak mid-level perturbations will advect
northeast within the mean flow, encroaching on the Piedmont of the
Carolina's by later this morning. The coupling of increased upper
support with a strong diurnal destabilization pattern will promote
the threat of stronger cell cores with heavy rain potential due to
anomalous moisture lingering along and south of the quasi-
stationary front to the north. Recent trends within the HREF and
associated CAMs have been for an increase in heavy rainfall for
points along and west of I-85 where the mid-level ascent will be
maximized along with the favored instability. Convection will
likely fire along the terrain of the escarpment up through the
Appalachians of NC before drifting to the east and northeast away
from the terrain. Probabilities for locally enhanced rainfall
exceeding 2" is very high (>70%) for much of Western NC down into
the far Upstate portion SC to the north of GSP. There's some hints
of upwards of 5" within the probability fields and individual CAMs
members along the escarpment up through areas like Boone points
east towards Greensboro. This area has seen its fair share of heavy
rain in the past 24-48 hrs at times, so the latest FFGs are lower
than climo. In coordination with the surrounding WFOs that bridge
coverage in the Western Carolina's, have introduced a SLGT risk
across the area encompassing much of the area west of the I-85
corridor.
...Southwest and Southern Rockies...
A pattern of general persistence within the mid and upper-levels
will lead to an ongoing threat of convective development in-of the
Southern Rockies and adjacent terrain of NM due to primed
environmental destabilization coinciding with a progression of mid-
level vortices streaming down the eastern flank of the ridge
across the Western U.S. A slight weakening of the ridge will allow
for an expansion of the convective threat to migrate westward into
the Mogollon Rim with some deterministic output signaling some
formidable amounts within the terrain. The primary areas of
interest within the current SLGT will reside within the complex
terrain and areas surrounding the remnant burn scars located within
the Sacramento's and Sangre de Cristos. Secondary areas of focus
include the NM Bootheel, Southeast AZ terrain around the Huachucas,
as well as the Eastern NM High Plains into the Northwest Permian
Basin of Texas.
The latter of the aforementioned areas is a newer development
within the latest ensemble means with a focus along a remnant
outflow that bisect the Caprock down into the Permian Basin,
outlined by a marginal theta-E gradient in place from Clines
Corner, NM down close to I-20 around the Midland/Odessa corridor. A
shortwave currently analyzed over CO will continue to make headway
to the south around the eastern flank of the ridge eventually
aiding in convective initiation across east-central NM by the
afternoon. Multi-cell cluster of thunderstorms will congeal by
prevailing cold pools and migrate to the southeast, riding right
along the theta-E gradient that will maintain presence through the
afternoon. There's a growing consensus that storms will be able to
hold together and impact a large area encompassing the Caprock of
Eastern NM down through the Northwest Permian Basin before
potentially collapsing in the evening. There is a chance this holds
together to the I-20 corridor and provides some heavier rain
within the confines of the Midland/Odessa area, but the probability
is lower compared to the Northwestern areas up across Southeastern
NM up through the Caprock along the TX/NM state lines. HREF
probabilities are highest for at least 2" within the confines
above, including some >70% output being displayed within the
Northwest Permian up through Lea and Roosevelt Counties in NM.
Despite a very dry signal within the soil moisture availability
over the region, rainfall rates in excess of 1.5"/hr will be
plausible given the elevated moisture presence as noted within the
latest NAEFS PWAT anomalies approaching 1-1.5 deviations above
normal across much of Southeast NM extending southeastward into the quasi-stationary front aligned near I-10.
The previous SLGT risk was maintained, but did allow for an eastern
extension to account for the increasing organized convective threat
aimed for portions of Southeastern NM down into West Texas.
...Oklahoma...
A compact but well-defined surface wave will linger within the
base of the mean trough carved out across the Mid-Mississippi
Valley back down into the northern fringes of the Southern Plains
in OK. Current IR satellite indicates a blossoming of convection
over Northeastern OK this evening that will lead to some locally
heavy rainfall close to Tulsa and surrounding locales. Hi-res
deterministic is consistent on the surface reflection sticking
around through the first half of the period with a short term
degradation of the convective field after 12z, but comes back in
earnest due to the addition of the diurnal destabilization along a
surface trough extending near and south of the surface low. Signals
for increasing low-level convergence within the confines of low
are present within several CAMs outputs later this morning and
afternoon leading to a narrow corridor of heavy rain that could see
totals breach 4" within a short period of time. The main threat is
confined within the small circulation with the northern fringes of
the low being the prime focus for where modest training could
occur. There's a small footprint within the ensemble means of 2+"
just south of the Tulsa metro with accompanying neighborhood
probabilities of >5" up between 25-40% in the same area. Whether
that's the exact location or not, the areal extent of flash flood
concerns is small due to the compact nature of the setup. A MRGL
risk was maintained from previous forecast, but want to make
mention the threat could trend towards more locally significant
impacts where the training convective pattern establishes itself.
Look for future MPD's on the threat as we move towards the late
morning and early afternoon hours.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN ROCKIES, TEXAS THROUGH THE ARKLATEX...
...Southern Plains...
A quasi-stationary front will bisect much of the state of Texas
with a westward extension into the Southwest TX terrain back
through the Big Bend, Edwards Plateau, Hill Country, all the way
out to the ArklaTex. Mid-level energy from the northwest will
navigate southeast towards the higher terrain south of I-20,
migrating slowly eastward within the confines of the front. The
added upper forcing in conjunction with the increasing low-level
convergence along the stationary front will induce a swath of
stronger thunderstorms capable of significant rainfall within any
cell core. Latest NAEFS PWAT anomalies signals a solid +2
deviations from climo for the moisture field present across much of
the central portions of TX back into the Stockton Plateau. Cell
initiation across the higher terrain out west will lead to
convective clustering with outflow generation stemming from areal
thunderstorm coverage, eventually propagating to the east into the
I-35 corridor in Central TX by later Monday afternoon and evening.
Precip totals within the initial cell development are generally
between 1-2", but some higher totals exceeding 4" will be possible
across the Lower Trans Pecos through the Southern Edwards Plateau.
The heaviest core of precip will lie within the eastern extent of
the Edwards Plateau across into Hill Country until it reaches the
I-35 corridor near Austin/San Antonio. This is where ensemble mean
QPF has been the most consistent for totals exceeding 2" with
indications of up to 3" in areal average QPF within Hill Country
and individual deterministic output exceeding 6" in places hit with
repeated cells. Mean storm motions within the confines of the
boundary are weak meaning slow moving convective clusters with
rainfall rates between 2-3"/hr are possible along that frontal
boundary. The SLGT risk from previous forecast was maintained but
expanded to the west to include more of the Stockton Plateau given
the latest trends within the mean QPF and favorable environment
over the area leading into the evolving event.
Further to the northeast through Northeast TX into the ArklaTex,
energy from the southwest will eventually advect northeastward with
convective generation during the afternoon and evening hours
becoming more organized with the additional upper support. Some
cell clusters will be capable of locally heavy rainfall extending
along the stationary front with some convective training plausible
due to similar conditions from upstream. Totals are not as prolific
within the means across the above region, but some totals of 2-4"
are not out of the question, so felt there was no reason to make
significant adjustments to the previously inherited SLGT risk.
...Southwest and Southern Rockies...
Scattered thunderstorms will develop once again within the confines
of the Sangre de Cristos with highest impacts situated over the
complex terrain and any burn scars within the mountain chain.
Totals are not expected to be as prolific compared to recent days
with less of a favorable mid-level pattern and lower SBCAPE
forecast. Regardless, considering the expected convective
development and the very sensitive nature of the flash flooding
potential within the burn scars, there was enough merit to continue
the focused SLGT risk across the Mountains with an extension down
into the Sacramento's due to the ongoing issues caused by the burn
scarring near Ruidoso. This is a lower end SLGT risk threshold with
impact based reasoning for the risk continuity.
Across the rest of the Southwestern U.S, the upper pattern will
shift to have less ridge potency leading to an expansion of the
convective risks a bit further west to include the Lower Colorado
River valley between CA/AZ/NV. The best threat will still be across
the Mogollon Rim where scattered thunderstorm coverage will likely
spawn some totals exceeding 1" within a short period of time along
the terrain of central AZ. The coverage and moisture anomalies were
not primed enough to warrant an upgrade at this time, but the
threat is still a mid to higher end MRGL risk, on the cusp of a
potential upgrade if the setup becomes more pronounced within the
means.
...Southeast to Southern Mid Atlantic...
Persistent surface front bisecting much of the Southeastern U.S up
through the Southern Mid Atlantic will continue the threat of
scattered thunderstorms capable of locally heavy rainfall with some
training prospects within the confines of the quasi-stationary
boundary. The QPF footprint is a shotgun of small QPF maxima
reflective of the potential with less of any organized threat and
more of a widespread convective pattern that favors some locally
greater impacts, but remaining on the low to middle grounds of the
MRGL threshold. The one area of note for a potentially more
impactful setup is across the Central Mid Atlantic where a stronger
shortwave will advect northeast out of the Carolina's with
increased upper forcing traversing the DC/Baltimore metro area
during the middle of peak diurnal instability. Recent deterministic
output is a bit more robust compared to recent forecasts and is in
agreement with the ML output based within the GFS Graphcast and
ECMWF AIFS. It will be interesting to see the trends as we move
into the CAMs window because there could be a targeted SLGT risk
within the urban corridor if the signal holds. A MRGL risk is in
effect, but will be monitoring closely.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN ROCKIES, SOUTHERN PLAINS, MID ATLANTIC, AND THE CAROLINAS...
...Southern Plains...
A quasi-stationary front will bisect much of TX during the period
with a succession of mid-level disturbances entering the region
beginning later this morning, carrying through the rest of the
period as they pivot south-southeast around the western flank of a
mean trough to the north. A deep moisture presence will be situated
across Southwest TX through points east with a stronger low-level
convergence pattern and favorable anomalies tied to the lingering
front. Scattered convection will form across the Davis Mountains
and Stockton Plateau in the early afternoon time frame with cells
across the Edwards Plateau initiating not long after. Considering
the moist anomalies and relatively slow storm motions off the
terrain, some cells will be able to produce locally enhanced
rainfall with rates between 1-2"/hr on average with some of the
stronger cores capable of breaching 2"/hr anywhere across the
aforementioned area(s). Numerous multi-cell clusters will develop
by mid-afternoon with outflow generation likely considering the
higher DCAPE environment forecast across the western half of TX.
Outflow propagation will make progress to the east with more cell
initiation forming in the favorable environment away from the cold
pools.
Storms should develop initially over Hill Country to the
I-35 corridor, but more organized convective clusters will enter
the picture by early evening leading to more widespread heavy rain
chances and flash flood concerns as rates can sufficiently hit
2-3"/hr as reflected in the HREF hourly rate probability fields.
There's a very high spatial coverage of higher probabilities for at
least 1" of rainfall in the HREF EAS fields (30-50%) with the
neighborhood probabilities for at least 3" and 5" depicting a
formidable areal extent of 60-80% and 25-40% respectively for each
threshold. There's even some low-end >8" probabilities reflected in
the HREF output as well, indicative of a locally significant impact
potential across portions of Western and Central TX. The best
probabilities lie within the Edwards Plateau through Hill Country
with a small bullseye showing up in the means to the I-35 corridor.
This is well within the bounds of a SLGT risk with higher end SLGT
risk wording necessary for the current forecast. There is a non-
zero chance for an upgrade somewhere across the above region, but
there is less of a true, organized heavy rain risk to pinpoint, but
the area that will be impacted will see those locally significant
flash flood concerns arise.
...Mid Atlantic and Carolinas...
Persistent troughing to the northwest across the Great Lakes will
edge eastward with southwesterly flow and increasing diffluence for
much of the Mid Atlantic and Carolinas, moving into the Northeast
U.S by the back end of the period. At the surface, a lingering
quasi-stationary front will settle over the VA Tidewater down
through the Carolina's and points west with a stronger low-level
convergence footprint tied to the boundary. PWAT anomalies of 1-2
deviations above normal will spread north as we see a bit more
backing of the flow within the Mid Atlantic with the strongest ties
across the Southern Mid Atlantic thanks to a surface wave riding up
the front once again, reaching the VA/NC coastal areas by the late
afternoon hours.
The area(s) of highest interest are located across the VA Tidewater
and adjacent Northeast NC, the eastern side of the Carolina
Piedmont, and up into the Susquehanna Valley of PA where the HREF
neighborhood probability fields are all lit up for the potential
of seeing >3" in spots with even a strong >5" signature located
along the I-64 corridor in Southeast VA (50-60%). The increased
upper forcing due to better aligned mid-level diffluence and some right-entrance region of a developing upper jet streak just off to
the north will allow for a blossoming of scattered convection
across portions of the Mid Atlantic through Central PA. The primary
initiation points will likely be tied to the terrain at first, but
as cells propagate off the terrain, they will experience some
modestly favorable shear that would help sustain updrafts that
could lead to stronger cell cores and locally heavy rainfall. PWAT
anomalies are most favorable for heavy rain along and east of the
I-99 and US-15 corridors leading to a lot of CAMs members
ratchetingup the potential with isolated pockets of 2-4+" totals
within the deterministic output, and reflected in the HREF blended
mean data set showing the expanse of higher totals and increasing
flash flood concerns. The threat will also reside within the urban
corridor from DC to Philadelphia, but there is some disagreement
within the CAMs on the potential due to a small cull in the precip
field with initiation away from the major metro areas. The UFVS ML
First Guess Field is still insistent on a widespread SLGT risk
through the urban areas extending all the way up to the PA/NY
border. The QPF footprint within ML output is also insistent on
potential maxima within the metro areas, so the best course of
action was have that area within the new SLGT risk proposal with
areas to the north into PA outlined as well given the better
signals and agreement in the First Guess Fields.
Further south into Southeast VA and the Carolinas, the threat is
more defined thanks to the presence of the stationary front leading
to ample low-level convergence potential coinciding with fairly
stout theta-E indices located along and south of I-64. The highest
threat will reside over the Hampton Roads area up to Williamsburg
where several days of heavy rainfall have degraded the FFG indices
considerably leading into today. This signal of degraded indices is
documented down through the NC Piedmont all the way into Columbia,
SC where back-to-back days of significant convective impact have
led to flash flooding reports all across southern and central SC.
The threat for today extends into those areas again thanks to weak
mid-level perturbations ejecting northeast within the mean flow
correlating with the stationary front to provide another focused
area of heavy thunderstorms from basically the GA/SC border and
points northeast. As a result, the SLGT risk addition was also
extended to include part of the Delmarva down through Southeast VA
into portions of the Central and Eastern Carolinas.
...Southwest and Southern Rockies...
Continued Monsoonal convective pattern will impact the Desert
Southwest this period with focus along the terrain in AZ and NM,
including the hotter spots of the Mogollon Rim over into the Sangre
de Cristos and the Sacramento Mountain chains. Probabilities
continue to be modest for 1-2" maxima with some higher end
potential in stronger cores that linger within the terrain. The
continued mid and upper ridging in place has led to slower storm
motions with some training concerns under the weak mean steering
flow. Considering the onslaught of scattered to widespread
convection for the past several periods leading to many Flash Flood
Warning issuances by the WFOs across the Southwest, another SLGT
risk was maintained within the Southern Rockies where the impact
potential is highest thanks to remnant burn scars and persistent
flooding over the past week. A MRGL extended to much of the
Southwestern U.S with the Mogollon Rim as the secondary candidate
for higher totals and impacts for localized flash flooding
concerns. The threat probabilistically is still on the middle to
high end of MRGL for the aforementioned area, but a short term
upgrade is plausible if coverage ends up more sufficient than
currently forecast.
...Upper Midwest...
Shortwave trough across Manitoba will pivot southeast into the
Arrowhead of MN with increasing large scale ascent and accompanying
positive vorticity advection (PVA) within the confines of the area.
Relatively steep mid-level lapse rates within a corridor of modest
instability will result in scattered thunderstorm initiation once
the disturbance crosses the border into the U.S. Progressive storm
motions will limit the threat of flash flooding to more isolated
signals, however some heavier cores with rates between 1-2"/hr will
be plausible given some of the low-end probabilities within the
latest HREF output. Totals are generally within the 0.5-1" in areal
coverage, but the matched mean does indicate a few cells capable of
dropping 2-3" near the northern shores of Lake Superior, including
near DLH where urban flooding is more probable. A MRGL risk was
introduced in coordination with the local Duluth WFO for low-end
flash flooding concerns, but enough to warrant the targeted risk.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
CENTRAL AND SOUTHEAST TEXAS...
...Texas...
Quasi-stationary front will push north and eventually wash out
across the Southern Plains as surface ridging across the Gulf
begins flexing more northwest allowing for deeper moisture presence
to advect into Southeast TX. Scattered convection will develop from
the Lower Trans Pecos through all of Central TX by the afternoon
due to diurnal destabilization with lingering outflow boundaries
from previous periods convection becoming a potential focal point
for heavy rainfall and training over much of the area. Areal QPF
averages are between 0.5-1" across West-Central and Central TX with
some of the upper quartile outcomes closer to 3-4" within different deterministic forecasts. This seems to be towards the upper
threshold of the potential across the region, but after what is
expected today, there's expected to be a lingering flash flood
threat after much of the area becoming primed from previous
rainfall. This was sufficient for a maintenance of the previous
SLGT risk.
Further to the southeast, the additional surge of low-level
moisture out of the Gulf will be accompanied by a weak mid-level
disturbance that will advect north out of the Bay of Campeche,
already causing some convective flare ups down that way when
assessing the latest IR satellite. Ensemble means are becoming more
bullish on the threat of heavier rainfall tied to the Middle and
Upper TX coast with some deterministic outputs pushing 3-5" between
Corpus to the far Upper TX coast, including coastal Houston.
The threat is gaining favorable within the ensemble probability
fields as well with the NBM now depicting a 25-40% chance of >2"
within the zone encompassing Matagorda up towards Port Arthur.
Considering this is working off the mean QPF of the blend, that is
impressive at 2-day leads. The extension of the SLGT was made to
encompass that area of the coast from CRP up to the Southwestern
corner of LA.
...Southwest and Southern Rockies...
Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.
...Midwest...
Shortwave trough over the Northern Great Lakes will dip further
southeast with trailing mid-level vorticity pivoting around the
base of the trough situated over Ontario. Scattered cells in the
more will move southeast, but lose fervor as they enter into MI. A
secondary pulse of convection is expected later Tuesday with a more
organized area of thunderstorms expected to form upstream near Lake
Superior, dropping southeast through WI and the western portions of
the UP. The storms will remain on the progressive side, but the
threat of rates up to 2"/hr could cause some isolated flash flood
concerns in more urbanized zones over Central and Eastern WI. This
includes places like Green Bay, Northern Milwaukee, Oshkosh, and
Sheboygan. The previous MRGL risk was maintained with some minor
adjustments on the western flank of the risk area to account for
QPF shifts in the ensemble means.
...Southeast through the Northeast U.S...
Scattered thunderstorms will continue across the Southeast all the
way up into the Northeastern U.S thanks to persistent synoptic
scale forcing and a relatively modest thermodynamic environment in
place. Smaller mid-level perturbations will be entrenched in the
mean flow and will help trigger some smaller, organized cell
clusters capable of isolated flash flooding basically extending
from the Deep South into the Mid Atlantic. Any cell generation
in the deep, moist environment will be capable of flash flooding
concerns with the highest threat likely within the Carolina's due
to the antecedent wet conditions in place over much of the eastern
2/3's of the region. A broad MRGL risk is in place extending from
the Southeast through the portions of the Northeastern U.S given
the threat.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN PLAINS, LOWER MISSISSIPPI VALLEY, AND THE SOUTHERN MID ATLANTIC...
...Southern Plains and Lower Mississippi Valley...
Two distinct areas of interest for the D1 period with different
synoptic and mesoscale evolutions to impact each area. The first
area will be across Central TX within the Edwards Plateau, Hill
Country, to the I-35 corridor. A persistent shortwave embedded
within the base of the trough located over the Southern Plains will
enhance regional upper forcing through the first half of the
forecast period, interacting with a weak quasi-stationary boundary
bisecting the aforementioned area. As high pressure noses in from
the north, a tightening theta-E gradient will transpire within
the confines of the shortwave impulse. Enough favorable large scale
forcing within a weakly capped environment will allow for the
initiation of a line of convection located within the proxy of the
front. Models are in agreement on the development and overall
impacts expected within the line of convection promoting locally
heavy rainfall within areas that were impacted over the previous 24
hrs. Area FFGs are much lower within the Edwards Plateau out
through Hill Country after yesterdays storms leading to a lower
threshold for impacts from flash flooding as the top layer of soil
moisture now sits between 60-80% according to the latest NASA SPoRT
viewer. Neighborhood probabilities for at least 3" are up around
50-80% within a large portion of the above area, extending
eastward to the I-35 corridor from Killeen down through New
Braunfels. >5" probabilities are also present, but more scattered
in the presentation with percentages closer to 15-25% leading to a
somewhat capped upper potential. This was sufficient for the
current SLGT risk continuity with minimal change from the previous
forecast.
Further southeast into the Texas coastal plain and the adjacent
Lower Mississippi Valley, the pattern will yield a much different
impact scenario thanks to the evolving upper levels leading to a
persistent Gulf moisture advection regime within a developing
coastal trough. Small impulses will round the western periphery of
a mid-level ridge extending through the Gulf, making their presence
known as they enter the proximity of the middle and upper TX
coasts. Bands of heavier rain with tropical origin in the moisture
field will promote efficient rain makers as they work their way
onshore. Just away from the coast, adequate surface buoyancy with
deep moisture presence will allow for the generation of more
thunderstorms just away from the coast leading to a larger QPF
footprint between the two convective initiation standpoints. As of
now, the prospects for flash flooding are highest along the Upper
TX coast between Houston to the southwest corner of LA as all
deterministic, ML, and associated ensemble means have overlapped
with the best focus of heavy rainfall around that corridor in
question. Areal average of 2-4" will be forecast within that span
of the coastal plain with QPF maximum upwards of 7-8" possible if
one particular area sees extended training. Probability fields
indicating at least 5" are running between 25-40% across an area
between Matagorda through Port Arthur right along the coast. The
probabilities are a bit less for the >5" totals away from the
immediate coast, but still generally between 10-20% for places like
Houston and Beaumont. This is bordering on a higher risk category,
but the protrusion of heaviest rainfall being mainly at the coast
leaves this on the fence, but well within the upper bound of the
SLGT risk. Will be monitoring the progression of the setup closely
as a targeted MDT is possible, especially if conditions allow for
the population centers within Houston/Galveston up towards Port
Arthur and Beaumont.
Some of the moisture entrainment within the northwest flank of the
ridge in the Gulf will be pulled inland to the northeast over LA
and MS leading to an expected band of heavy thunderstorms capable
of rates between 2-3"/hr at peak and totals pushing 2-5" locally
extending from Lake Charles up through Jackson, MS. The prospects
for localized flooding within urban corridors has grown from recent
updates allowing for an extension of the SLGT risk over TX to be
pulled more northeast to account for the threat.
...Southern Mid Atlantic...
Another day of scattered to widespread convective impact
anticipated across the Southern Mid Atlantic with the heavy rain
threat continuing within Central and Northeast NC up through the VA
Tidewater. Heavy rain is likely within any cell development thanks
to a persistent +2 deviation PWAT anomaly situated south of the quasi-stationary front bisecting the Central Mid Atlantic. Multiple
smaller impulses will advect from the southwest with the mean flow
aloft along with entering into the RER of an upper jet streak
forming to the north. This will correlate to cell initiation
within more favorable upper dynamics and modestly buoyant
environment in place over the Southeast VA through the Carolinas
creating another threat of heavy rainfall and repeated impact of
cells along the corridor from the SC Piedmont and points northeast.
HREF EAS probabilities between 25-40% for at least 1" covers a
large expanse of the Southern Mid Atlantic, a traditional symbol
for agreement within the CAMs on a widespread convective heavy
rain threat and within the threshold for a SLGT risk upgrade. Based
on the setup, repeated threat continuing of heavy rain and flash
flood concerns, and the probability fields insinuating more 3+"
totals locally in Southern VA and through NC, a SLGT risk was
issued over the aforementioned area(s).
...Southwest and Southern Rockies...
Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of the southern half of NV leading to
more coverage of the MRGL risk across those areas. QPF means are
generally light, but some of the higher end outputs within the
deterministic suite are upwards of 1-2", especially across the
Mogollon Rim. This is a classic Monsoonal setup with the favored
terrain being the primary focus for the period. The MRGL risk
inherited was maintained with full continuity as ensemble mean QPF
did not change much in terms of magnitude and precip placement from
previous forecast.
...Midwest...
Shortwave trough across the Great Lakes will continue to push
southeastward with modest upper forcing under the trough axis.
Modest instability will linger across the Midwest heading into
today with some trailing vorticity maxima entering Central Midwest
by late morning and afternoon. Convection will develop upstream
over the Upper Midwest and pivot south into Central and Southern
WI, IA, Northern IL, and eventually spread eastward into parts of
Michigan. Isolated heavy rain signals within the CAMs backed by
some modest probabilities for 1-2" and locally up to 3" promote a
general continuation of the MRGL risk in place. Only some minor
adjustments were made to the previous forecast given the relatively
stable ensemble QPF footprint.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS, SOUTHWEST LOUISIANA, AS WELL AS THE SOUTHEASTERN U.S...
...Texas and Louisiana...
A continuation of the pattern from the previous period will lead to
more shortwave impulses exiting out of the western Gulf as they
navigate northward between a broad upper trough to the northwest
and the persistent ridge off the southeast in the Gulf and Western
Caribbean. A coastal trough will strengthen at the surface right
along the mid and upper TX coast leading to a more defined axis of
surface convergence in-of the coastal plain. Moisture anomalies
will be poking closer to +3 deviations with area PWATs generally
running between 2.2-2.5 along the TX coast all the way into
Southwest LA. There is still some modest uncertainty as to where
the best axis of heavy rainfall will preside for the D2 period, but
the consensus is slowly merging towards the Upper TX coast with a
grazing of the Mid TX coast for the heaviest rain potential. Recent
ensemble QPF has an axis of 2-4" located right along the immediate
coast beginning near Matagorda, expanding to the northeast to as
far north as Lake Charles. Ensemble probabilities within both the
GEFS and ECENS are targeting the Middle TX coastal areas to the
southwest of Houston with the best probabilities for upwards of 2"
while the NBM has a split maxima of one overlapping the area the
GEFS and ECENS are targeting along with one to the north over Port
Arthur into Lake Charles. The split comes for a discrepancy in the
handling of a more robust shortwave that ejects to the north later
in the period. Due to that discrepancy, a potential for an upgrade
was passed on to take more time for guidance to come into agreement
on where the focal point for the highest totals will occur. There
is enough merit given the environment that if any area is favored
for at least 4" within the mean, and probabilities favor the threat
of over 5-6+", there could be a targeted MDT risk in future
updates. For now, a broad higher end SLGT has been maintained with
emphasis on the Middle and Upper TX coast, including near Houston
proper.
...Southeast...
A stronger mid-level impulse will eject northeast out of LA into
through the Deep South within a corridor of moderate instability
and deep moist access. The favorable upper forcing and accompanying thermodynamic presence has allowed for guidance to generate a
widespread area of higher QPF along the confines of the impulses
path. This is consistent within the ensemble bias corrected QPF
footprint with 1.5-2.5" totals in the ensemble positioned from
east-central AL through Central GA/SC into extreme southern NC.
Considering the prospects for 2-3+"/hr rates within the convective
zone Wednesday afternoon, a SLGT risk addition was warranted to
cover for the potential.
...Southwest and Southern Rockies...
Scattered convection will occur once again across much of the
Southwestern U.S with isolated threats of flash flooding within the
flashy complex terrain, remnant burn scars, and slot canyons
located within the Great Basin. The ridge axis will shift a bit
further to the west aligning more of the convective potential
through eastern CA and much of NV leading to more coverage of the
MRGL risk across those areas. QPF means are generally light, but
some of the higher end outputs within the deterministic suite are
upwards of 1-2", especially across the Mogollon Rim. This is a
classic Monsoonal setup with the favored terrain being the primary
focus for the period. The MRGL risk inherited was maintained with
full continuity as ensemble mean QPF did not change much in terms
of magnitude and precip placement from previous forecast.
...Great Lakes and Ohio Valley...
Surface low across MI will push northeast into Canada with a
trailing cold front progressing east and southeast over the course
of Wednesday. Isolated to scattered thunderstorms will develop
along and ahead of the cold front with some modest cell cores
plausible given the increased surface and upper forcing pattern.
QPF footprint is overall on the weaker side compared to what you
would want to see regarding more considerable flash flood threats,
but the environment is capable for some totals of 2-3" in a very
short time which if it falls within any of the urban corridors
around the Lakes could spell some isolated flash flood concerns.
The threat overall is low-end within the MRGL threshold, but enough
to warrant a continuation from previous forecast.
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ACROSS THE UPPER
TEXAS COAST AND SOUTHWEST LOUISIANA...
...Texas and Louisiana Coast...
After coordination with the local Lake Charles and Houston WFO's, a
targeted Moderate Risk was issued across the Upper Texas coast into Southwestern LA, including Galveston Island. At the surface, a
coastal trough is positioned just off the Upper TX coastal plain
with a deep moist axis aimed orthogonal to the coast between
Galveston up through the Lower Sabine Valley. The entire area
within the coastal plain is positioned within a very anomalous axis
of elevated moisture with PWAT deviations running between +3-4 from
CRP up through all of Southeastern TX into LA. 00z soundings out of
KLCH depict a very tropical airmass with a deep moisture presence
through the column, basically running from the surface to the
tropopause. Warm cloud layer depth is running around 15.5k feet, a
classic signature of heavy rain potential with a greater propensity
to exude very efficient rainfall rates from warm rain processes.
Even within any deep layer convection, this would spell trouble for
local rates breaching 2"/hr with the sampled environment generally
capable for upwards of 4"/hr within the stronger cell cores. The
latest HREF hourly rate probabilities are indicative of just that
with a corridor of elevated probability signatures between 25-40%
for rates exceeding 3" within the confines of the coast stretching
from Galveston up into places like Port Arthur/Beaumont as the
frictional convergence regime begins towards dawn this morning and
maintains prominence through the early afternoon before
dissipating. The probability fields for total rainfall were the
biggest signal for the risk upgrades with the HREF EAS outputs
considerably bullish within the 2" (45-60%) and 3" (15-30%)
markers. Considering the conservative nature of the EAS due to the
process which it's calculated, this is a significant output that is
typically reserved for those higher end potential events.
Assuming the factors involved and the presence of deep, tropical
moisture advection off the Western Gulf, there was enough of a
signature for a MDT risk upgrade in the location with the highest
probabilities and mean QPF footprint indicating a solid 3-6" with
locally as high as 9" possible, indicated with the LPMM HREF.
...Southeast to Southern Mid Atlantic...
The same quasi-stationary front that has been stuck in the Southern
Mid Atlantic down through the Southeastern U.S will continue to be
a boon for the the region as another round of heavy thunderstorms
will develop within an axis of elevated theta-E's located along the
boundary from LA to Southeastern VA. Several small impulses also
within the stagnant upper pattern will aid in the necessary upper
forcing to promote small cell clusters of storms capable of a
larger areal extent of heavy rainfall that would promote flash
flood concerns within more urbanized corridors across the South,
along with any slower moving convection that gets anchored to
lingering cold pools that are running rampant from the previous
days of convection. A deeper moisture flux will be entering into
the Deep South with an embedded stronger mid-level vorticity maxima
pivoting northeastward after it exits the Gulf. This will spur a
more organized convective cluster downstream over places like
MS/AL/GA. This is causing a well defined bullseye of higher precip
within that corridor and is represented within the probability
fields with the >3" neighborhood output hovering between 40-70%
extending from Southern MS across into the SC Low Country. The
previous SLGT risk was expanded further west to account for the
convective signatures all the way into LA.
...Northeast...
A tongue of elevated theta-E's will be advected north on the lee
side of the upper trough pivoting through the Great Lakes. To the
southwest, a blossoming jet streak will develop over the Southern
Ohio Valley and begin nosing into the interior Northeast by later
this afternoon. At the surface, a cold front will be sweeping
eastward out of Ontario through Western NY and PA leading to
increasing low-level convergence within the axis of greater
instability. All this to say that scattered thunderstorms with
rainfall rates between 1-2"/hr will be plausible across the
interior with the highest chance for flash flood concerns within
Central and Upstate NY where the greatest merger of upper forcing
and repeated convective impacts will occur later this afternoon and
evening before the front kicks the threat downstream. HREF
neighborhood probabilities are generally high for at least 2"
across the Adirondacks back towards the eastern Finger Lakes. The
probabilities for at least 3" are still fairly modest within the
neighborhood output with a bullseye of 40-50% located over the
Adirondacks. The signals is scattered for the highest totals
within the deterministic outputs, so the setup is right on the
higher end of the MRGL and on the cusp of a small SLGT risk area.
For now, maintained continuity but will note the threat of a
targeted upgrade if the trends continue upwards in the next update.
...Midwest and Ohio Valley...
Surface low located over the Great Lakes will begin to occlude and
track northeast through Ontario with a trailing cold front swinging
through the Midwest and Ohio Valley later this morning and beyond.
A general consensus within the CAMs for a round of convection to
develop in-of IA and IL later this morning, spreading southeast
along the confines of the front. Flow will be modestly convergent
along the leading edge of the boundary with some favorable large
scale ascent caused by the northern half of the area sitting within
the left exit region (LER) of a blossoming jet max situated over
KY. Observing the latest forecast soundings off some of the CAMs
indicate a signature for locally heavy rainfall with some potential
training as the flow becomes a more parallel to the front as we
move into the evening hrs. The threat will not persistent however,
as the front will be on the move through the entirety of the
forecast. This has limited the overall potential of the setup to
mainly 1-2" maxes with perhaps as high as 3" as per the latest
HREF neighborhood probabilities of >3" settling between 15-25%
across portions of Central IL with lighter 5-10% markers further
east in IN/OH. This was enough for a continuation of the inherited
MRGL risk with only some minor adjustments necessary to reflect the
latest QPF and probability fields.
...Southwest...
Scattered Monsoonal convection will continue for another period
across the Southwestern U.S with the primary heavy rain footprint
aligned within the Mogollon Rim up through the Great Basin. Areal
SBCAPE values will range from 500-1500 J/kg through a large extent
of the west with PWAT anomalies generally +1 deviations across AZ
and NM with +2 and +3 deviations aligned from Northern CA across
into NV. Highest moisture anomalies will advect eastward through
the period as a shortwave trough across the PAC NW begins to
flatten the northern extent of the ridge sending the more prominent
moisture anomalies into Northern NV, Southern ID, and Western UT.
Current QPF signature within guidance is relatively scattered when
it comes to the higher values, but some 1-2+" totals are likely
within the current setup with the best chance focused across the
Mogollon Rim and the terrain west of I-19 in Southern AZ based on
the latest ensemble QPF output and probability fields. The MRGL
risk from previous forecast was maintained with only a few small
modifications to reflect the instability gradient and forecast QPF
footprint.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA, AS WELL AS PORTIONS OF THE
SOUTHEAST AND SOUTHERN MID ATLANTIC...
...Texas and Louisiana...
A continuation of the pattern from the previous period will lead to
more shortwave impulses exiting out of the western Gulf as they
navigate northward between a broad upper trough to the northwest
and the persistent ridge off the southeast in the Gulf and Western
Caribbean. Unlike today, the pattern has less definition to the
heavy rain threat with a more inland protrusion of the heavy rain
footprint when assessing the deterministic globals and ensemble
based QPF. There will likely be an area between the Middle TX coast
up through the Sabine River Valley that will experience some
significant rainfall potential so long as the upper pattern remains
favorable based in the model output. Verbatim, the threat is likely
to impact areas a bit further down the TX coast with the area from
Matagorda up through Houston the primary target initially with the
QPF maximum generally aimed within that corridor on most
deterministic and ML output. Current forecast for 1-3" areal
average is a good base to start prior to having some benefit from
the future CAMs that will be able to exhibit better defined
convective characteristics given the favorable mesoscale cores and
resolution. Expectation is for another round of locally heavy rain
with potentially some significant flash flood concerns within the
hardest hit zones as local maximum could climb upwards of 6" if the
pattern holds. The PWAT anomalies between +2-3 deviations will
still linger across all of Southeast TX through much of LA,
continuing the prominent warm rain processes that will allow for
efficient rainfall accumulations. The previous SLGT risk was
maintained with only some minor adjustments to the norther fringes
to align with the trends of a further inland protrusion of the
heavier precip signals. The risk is a higher end SLGT within the
coastal plain just north of CRP up into Southwestern LA.
...Southeast to Southern Mid Atlantic...
A cold front will migrate to the southeast pressing through the
southern half of the Mid Atlantic while the at the upper levels a
130kt jet streak will be passing over New England leading to better
large scale forcing within the RER of the jet across the
aforementioned area. Ample pre-frontal instability within anomalous
deep layer moisture will also contribute to the favorable
environment for heavy rain potential, especially as the front
approaches and enhances low-level convergence within the confines
of the boundary. After destabilization will aid in priming the
environment prior to the fronts approach with the initial area of
interest lying over Southeastern VA, eventually shifting focus into
the Carolina's as the front progresses south. This is the area of
greatest significance given the best surface to upper level forcing
presence working in tandem to create a period of widespread heavy
rainfall. Ensemble QPF between 2-3" is common within the confines
of the front from the Hampton Roads area down through Eastern and
Central NC/SC with much of the rain occurring within a short time
frame as the front approaches. This is a signature for heavy hourly
rates that will enhance the flood risk over the region. For that,
the SLGT risk was maintained from the previous forecast.
Across the Southeast, deep moisture presence with several mid-level perturbations entering the area through the period will assist in
maintaining a scattered thunderstorm signature with heavy rain
threats continuing thanks to the persistent elevated moisture
anomalies and ample instability. The threat is not as pronounced as
what will occur further to the north, but the setup is sufficient
for any cell producing flash flooding, especially over GA/SC where
the best instability is located.
...Southwest and Southern Rockies...
Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. The area
of interest remains across the Mogollon Rim down into Central AZ
where Locally 1-2" with upwards of 3" is possible currently in
that zone due to a stronger axis of instability under which could
end of being near a 600dm ridge on Thursday afternoon. Across the
north, a more prominent moisture advection pattern will continue
through the Interior Mountain West as a shortwave trough over the
PAC NW continues to squash the northern extent of the ridge and
funnel the moisture further into the interior after it began
navigating out of the CA/NV. More widespread convective coverage is
forecast across UT/Southern ID/Western WY as a result leading to
an expansion of flash flood concerns within those areas. The MRGL
risk was generally maintained from previous forecast but did
expand the risk slightly on the northern periphery to account for
the moisture anomalies and accompanying forecast instability across
those northern zones.
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE MIDDLE
AND UPPER TEXAS COAST THROUGH SOUTHWEST LOUISIANA...
...Texas and Louisiana...
Current upper air and surface analysis across South TX indicates
the development of a weak surface reflection with a closed 85H
(850mb) low present as of the latest observations. This will play
a significant role in the upstream convective potential through the
period, especially along the immediate TX coast from CRP up
through the Southwest coast of LA. There's a near unanimous
consensus on the latest 00z CAMs of a targeted area of significant
rainfall within the region spanning from Matagorda Bay up to just
south of Galveston during the first half of the forecast period.
850mb low across South TX will slowly migrate to the northeast
with a focused LLJ on the eastern flank of the circulation. The
enhanced jet and increasing low level convergence under the
surface and 850mb low(s) will generate a skinny axis of heavy
rainfall with embedded convection given marginally sufficient
MUCAPE located near the coast. 00z sounding out of KCRP was
indicative of deep tropical moisture entrenched within the column
as PWATs nestled just under 2.4", a solid 3 deviations above normal
and within the 99th percentile climatologically, just falling
short of the daily maximum of 2.42". Regardless of a record or not,
the environment is indicative of a moist, tropical airmass that
will exude efficient warm rain processes through any convective
development and the evolving pattern this evening through the
morning is one that will very likely promote intense rainfall
locally within the confines of the surface and closed 850mb
reflections. This setup will slowly migrate to the northeast during
the morning, but the setup for later this afternoon and the
following evening still has the "jury out" on exactly how the 850mb
low behaves.
As of now, the mean QPF output via the HREF and associated
ensembles are depicting a narrow axis of significant rainfall
totals extending from just northeast of Port Aransas up through
Galveston Bay and eventually hitting the Upper Texas coast near the
Sabine River, the same place that was just impacted recently from
significant rainfall yesterday. The probability fields for >3" of
rain are pronounced within that corridor, but the upper quartile
outcomes and associated neighborhood probabilities for at least 5"
(50-80%) and 8" (20-40%) are much higher further south along the
Middle TX coast with the bullseye situated around Port Lavaca up
through Matagorda with the former being the focal point for the
heaviest rainfall as of the latest model output. Further away from
the coast remains a question on the inland extent of the heavy rain
as the QPF from many of the CAMs and even some global deterministic
are all shying away from a larger protrusion inland with the focus
still remaining primarily at the coast. So much of this is
contingent on the behavior of the 850mb low as a longer
maintenance of the low across the coast would likely enhance a
secondary push of convection inland by nightfall thanks to a
redevelopment of the LLJ aimed orthogonal to the coast after 00z
this evening. Models that do have this outcome are more bullish in
a larger impact footprint to the north as the low drifts that
direction and allows for a greater inland focus to arise as we step
into the back end of the forecast period. These solutions also
give credence to higher potential impacts for greater Houston, so
that will be something to monitor in time where there could be
adjustments to the current MDT risk forecast.
For now, the main threat is tied closer to the coastal areas with
the previous MDT risk generally maintained by expanded to the south
to the northeast of CRP, mainly from Matagorda Bay on northeast.
The northeastern extent of the MDT was trimmed back across LA with
guidance becoming less enthusiastic with the push of deeper
moisture inland near Lake Charles and points north and maintaining
a more confined coastal heavy rain threat, similar to where the
impacts were felt this past forecast period.
...Southeast into the Southern Mid Atlantic...
An intensifying upper jet across the Ohio Valley will extend into
the Northeastern U.S with a strengthening core upwards of 130kts
reflecting some fairly significant large scale ascent within the
right entrance region (RER) of the jet. At the surface, a cold
front will translate east and southeastward with progression
through the Mid Atlantic and the Southern Ohio Valley through the
period leading to some modest surface convergence along the front
with scattered to widespread convection out ahead of the boundary
thanks to the present large scale ascent from the aforementioned
jet. Sufficient low-level buoyancy located across Southern VA
through the Carolinas into the Deep South will promote an
opportunity of heavy thunderstorms with the highest probability
located over the Carolinas into Eastern Ga due to the presence of a quasi-stationary front still bisecting the area. This will become
a focal point for low-level convergence and will provide an anchor
for storms to develop and slowly drift within the confines of the
boundary. Rainfall rates of 1-2"/hr will be most common in any
storm development, but a max of 3"/hr is still plausible given the
low probabilistic output of 10-20% over a large portion of the
Carolinas into far Eastern GA for those 3"/hr potentials. Mean QPF
output via the HREF was solidly between 1-2" spatially with the
individual CAMs output signaling some much higher local totals that
could exceed 4" in some cases, a much greater threat for flash
flooding potential.
Considering the incessant nature of the pattern the past week with
copious heavy rainfall episodes contributing to flash flooding
across the Carolinas, a SLGT risk from previous forecast was
maintained with a small shift on the western extent of the risk
area where the probabilities dropped off significantly for the
threat of flooding rainfall, carrying the signal closer to the
SC/GA line than previous forecasts.
...West...
Monsoonal convection will encompass much of the Southwest U.S with
primary coverage in the Great Basin as the mid-level ridge pattern
shifts focus to the west of the Four Corners. Modest moisture
anomalies and relatively formidable instability across much of the
region will allow for scattered thunderstorms with isolated heavy
rain cores that could spell issues if they fall along complex
terrain, burn scars, urban footprints, and slot canyons. More
widespread convective coverage is forecast across UT/Southern
ID/Western WY as a result leading to an expansion of flash flood
concerns within those areas. Neighborhood probabilities for the
higher convective totals >1" are still most prominent across AZ
with the focus areas being over the Mogollon Rim down through the
Southern terrain near the border where the probs are running
between 60-80% for the threat of at least 1". Secondary maxima are
popping up across the Inner Mountains west within Utah and Western
Colorado, but the threat is a little more subdued due to the lower
instability outputs that favor further south under the 500mb ridge.
Still, the moisture anomalies are forecasted to be best to the
north, so there could be some sneaky prospects for locally higher
impacts from convection, especially within the terrain and
adjacent valleys. The MRGL risk was generally maintained from
previous forecast but did expand the risk slightly on the northern
periphery to account for the moisture anomalies and accompanying
forecast instability across those northern zones.
Kleebauer
Day 2 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE CAROLINAS...
...Carolinas...
The cold front from the previous period will finally make progress
off the eastern seaboard across the northern half of NC while
making slow progress to the south through the rest of NC/SC region.
Scattered thunderstorms within the persistent deep moist and
unstable environment will allow for a period of heavy rain
potential, especially along and ahead of the cold front as it
migrates southward. The heaviest rain will align over Eastern NC
into the South Carolina Low Country, areas where FFGs are a bit
higher and potentially more difficult to flood. Previous rainfall
has lowered the FFG indices, especially as you work from Columbia,
SC to points northeast which plays a role in the current SLGT risk
maintenance. Ensemble QPF output is currently between 1-2" across
Southeast NC, including the Wilmington area extending down through
the Low Country and coastal sections of SC. Individual
deterministic output is as expected with some local maxima around
4" within any particular global member leading to ensemble bias
corrected means closer to 2-2.25" over the hardest hit areas. Did
trim back on the northern extent of the previous SLGT with
maintenance further south and southeast across central and eastern
SC up through Southeast NC where the front will slow in forward
progression and become a focal point for Friday afternoon
convection. Best threat for flash flooding will likely occur over
more urbanized zones due to the sandier soils present in the
confines of the current risk area which lead to the continuity,
and not expecting any upgrades in the current setup.
...Southwest through the Inner Mountain West...
Scattered thunderstorms within a favorable moist axis will linger
through the Southwestern U.S with the northern half of the
convective threat shifting further inland into the Interior before
hitting the proverbial wall at the Central Rockies. Any storm
within the period could exude heavy rainfall with rates generally
topping at 1"/hr, an intensity that could easily cause issues
within the complex terrain out west. The increased moisture
advection back over NM will also open the door for more convective
threats within the remnant burn scars in place over the Sangre de
Cristos, so the MRGL risk was promptly positioned to cover for the
threat. General rainfall maxima is expected to be between 1-2" but
a small chance for locally higher remains, especially from any
cells that get anchored to the terrain. The previous MRGL risk was
kept to general continuity, but did pull the MRGL risk a little
further west into NV due to some linger convective potential from a
trailing shortwave migrating through the region during peak diurnal instability, aligned with a further west QPF footprint as of the
latest NBM and ensemble bias corrected outputs.
...East Texas through the Lower Mississippi Valley...
Remnant mid-level energy will linger across Eastern TX into the
Lower Mississippi Valley within a persistent anomalous moisture
presence situated over the region. Sufficient buoyancy and
attendant large scale forcing will likely generate periods of
scattered convection capable of producing locally heavy rainfall
over the aforementioned areas. There are some indications that a
much more defined 500mb low will develop over the Southern Plains
which would increase the general forcing pattern allowing for a
more robust convective initiation over the eastern flank of the
mid-level circulation. This was noted via some of the 00z global
deterministic with the ECMWF being the most aggressive with the
solution. There are some hints of the threat becoming more
pronounced across East TX into parts of LA/AR given the recent GFS
Graphcast and ECMWF AIFS ML output, so a targeted upgrade may occur
over the above areas if the consensus grows and remains stable
in the upcoming model iterations. For now, a MRGL was maintained
from previous forecast.
From Mike Powell@618:250/1 to All on Fri Jul 26 09:20:00 2024
FOUS30 KWBC 260828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Fri Jul 26 2024
Day 1 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
SOUTHEAST TEXAS AND ACROSS PORTIONS OF THE SOUTHEAST US COAST...
...Portions of Southeast Texas...
Radar was showing increasing coverage of showers in a region of
confluent flown off the Gulf of Mexico in the early morning
hours...and the expectation is that convection will be persisting
beyond 12Z. Given some overlap with a region of lower Flash Flood
Guidance values and potential for 1 to 2 inch per hour rates
redeveloping later in the day as shown by the HREF 40 km
neighborhood probability guidance, maintained a Slight Risk from
coastal Texas northeastward along the axis of highest precipitable water/instability. There is some upper support in the form of a
trof axis between 200 mb and 300 mb with a 50 kt speed max rotating
around the east side of the trof axis that looks help draw the
moisture towards the Southern Mississippi valley late in the
period,
...Portions of the Southeast US Coast...
Surface low pressure will linger along the Carolina coastline for
much of the period with an associated frontal boundary helping
provide some focus for convection as mid-level height falls
approach from the north and west. Much of the operational guidance
showed the heaviest rainfall along or immediately off the Carolina coastline...although the NAM maintained some threat of 2 to 5 inch
amounts falling inland. Between that and the fact that the 26/00Z
HREF showed 40 to 60 pct neighborhood probabilities of 1- and
3-hour QPF exceeding flash flood guidance along the immediate
coastline at time of maximum daytime heating...maintained the
Slight risk but continued to shrink the area somewhat from the placement
of the inherited Slight Risk area.
Surrounding the two Slight Risk areas was an expansive Marginals
Risk area. Models depict abundant moisture and instability in this
area but weak forcing mechanism and meager steering flow. Thus
cells that form will be slow moving and efficient rainfall
producers.
...Southwest...
Enough moisture and instability will still be in place over
portions of the Southwest and the Intermountain basin to support
the development of convection in the afternoon and which persists
into the evening...although the best focus will have shifted
eastward since Thursday. There is still concern that any storm
which forms will have the potential for 1+ inch per hour rates and
isolated storm total amounts in the 1 to 2 inch range...enough to
result in flooding and run off problems with the greatest risk
being over recent burn scars and in normally dry washes. Some minor
adjustments were made to the previous MRGL risk area but overall
there was a fair degree of continuity.
Bann
Day 2 Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A LARGE
PORTION OF THE SOUTHERN/SOUTHEASTERN US AND INTERMOUNTAIN REGION...
...Southern and Southeastern United States...
The 26/00Z suite of numerical guidance showed deeper moisture
getting drawn north and eastward from the Texas coast towards the
western part of the Tennessee Valley given persistent south to
southeasterly flow on the east side of an upper level
trough/closed low. With mid-level height rising from the western
Great Lakes to eastern Tennessee Valley, opted tom limit the
eastern extent of the Marginal. Elsewhere...enough instability and
sufficiently deep moisture will be in place for almost any storms
that develop to produce isolated flash flooding due to slow moving
downpours.
...Southwest United States...
Confidence remains below average in the potential and placement of
flash flooding across the West...with the areal coverage shrunk yet
again from the previous issuance. Given the terrain and lingering
deep moisture in much of the West, it's going to be really tough
to completely eliminate the flash flooding potential.
Bann
Day 3 Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PARTS OF THE SOUTHERN/WESTERN UNITED STATES AND UPPER MIDWEST...
...Gulf coast to parts of the Tennessee Valley...
A closed mid- and upper-level low that started to take shape late
Saturday night/early Sunday morning should begin to lift northward
during the day on Sunday and then become an open wave again by
Sunday evening. Lowering mid-level heights should help increase the
coverage of convection within an an atmosphere sufficiently moist
to support isolated downpours in a corridor from the middle Gulf
coast to the Tennessee valley during the afternoon and evening,
...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.
...Upper Midwest...
Scattered convection should develop along a cold front extending
into the northern tier of the US from a system in Canada. With
increasing precipitable water and instability values increasing as
a result of southerly flow ahead of the front...isolated downpours
could result in isolated flooding or run off problems in regions of
poor drainage.
From Mike Powell@618:250/1 to All on Sat Jul 27 09:23:00 2024
FOUS30 KWBC 270828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Sat Jul 27 2024
Day 1
Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TEXAS GULF COAST...
...Texas Coast to Southeast US...
Maintained the Slight Risk area with onshore southerly flow of
deep tropical moisture off the Gulf set to bring another day of
scattered convection capable of producing heavy rain over an area
where flash flood guidance has been lowered by several days of
moderate to heavy rainfall...with the expectation that the
convective organization should be less today compared with previous
days.
Elsewhere, only modest adjustments were made to the expansive
Marginal Risks surrounding the Slight Risk area and eastward to the mid-Mississippi Valley given persistent south to southeasterly
flow on the east side of an upper level trough/closed low. The
trough/upper low could lead to enough upper support to focus and
sustain storms that produce localized downpours...but even
convective initiation along local outflow boundaries will be
forming in an environment supportive of downpours from the
Mississippi Valley into parts of the Southeast U.S...so isolated
instances of multi- inch rainfall totals are quite possible.
...Southwest United States...
Signals from the 27/00Z suite of numerical guidance continues to
show decreasing threat of excessive rainfall as mid level flow
increases across the Intermountain region. The exception remains
over parts of Arizona and New Mexico where enough
moisture/instability linger in a region with meager southwesterly
steering flow...so will be maintain at Marginal risk area.
...Upper Midwest...
Maintained the Marginal Risk area mainly over northern Minnesota as
given the presence of increasing southwesterly low level flow
feeding an airmass with precipitable water values locally in
excess of 1.75 inches ahead of a cold front making its way across
the area. Given spread in forward speed of the front...expanded the
Marginal Risk area but largely maintained continuity in terms of
the area covered.
Bann
Day 2
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST...
...Northern Minnesota...
Rain associated with a cold front moving across the northern tier
of the country should be ongoing across northern Minnesota. While
there is likely to diminish in intensity for a period early in the period...there should be a renewed risk of excessive rainfall later
in the day and evening. With precipitable water values increasing
to 1.75 inches or greater continuing to be fed into the region by
south to southwesterly 850 mb flow with divergence aloft being
provided by the right entrance region of an upper level
jet...the 27/00Z HREF neighborhood probability values of 10 to 15
percent for 2+ inch amounts in an hour seem pretty reasonable.
Assuming that the GFS was too fast with the forward speed of the
front during the period...he risk of training cells and repeat
convection are additional concerns for excessive rainfall in
addition to the intense rainfall rates.
...Tennessee Valley and Surrounding Areas...
A stationary front turned warm front will track northeastward up
the Tennessee Valley through late Sunday night/early Sunday morning.
Increasing atmospheric moisture associated with the advance of an
airmass originating off the Gulf will allow for potential training
and backbuilding of strong storms capable of increasingly heavy
rainfall. The surface warm front will likely slow as it approaches
the southern Appalachians...supporting slower moving storms that
with interactions with the terrain will be capable of flash
flooding. With very efficient warm rain processes at
work...multiple inch per hour rainfall rates . Urban areas are at
a higher risk for localized flash flooding with any repeating
storms.
...Southwest US...
The risk of excessive rainfall continues to get less conducive with
time and gets shunted eastward as mid-level westerly flow
strengthens over much of the West outside of Arizona and New
Mexico. Maintained a Marginal Risk here given the lingering
moisture.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY FOCUSED OVER
THE TENNESSEE VALLEY...
...Southeast US...
Focus for excessive rainfall shifts eastward and is mainly focused
in the central and eastern portion of the Tennessee Valley as mid
level height falls approach from the west and precipitable water
values at or above 1.75 inches will already be in place. While the
height falls are associated with a trough that was beginning to
fill...the diffluent flow located above an instability axis should
be enough to support downpours from slow moving storms in the
afternoon and evening. Elsewhere in the Southeast US...the forcing
remains weak and nearly impossible to time with any accuracy at
this range but localized downpours from any convection that can
develop and was covered by an expansive Marginal Risk area.
...Upper Midwest to far Western Great Lakes...
The same cold front helping to focus heavy to potentially excessive
rainfall on Sunday/Sunday night will continue to make its way
eastward on Monday. The pattern becomes somewhat less favorable for
heavy to excessive rainfall but the potential for some isolated
enhanced amounts linger...especially from far northern Minnesota
into the western Great Lakes where a Marginal Risk was placed.
...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert)...meaning
a slight westward expansion in the Marginal risk area over
portions of Arizona and New Mexico compared with the previous
couple of days.
From Mike Powell@618:250/1 to All on Sun Jul 28 08:29:00 2024
FOUS30 KWBC 280827
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Sun Jul 28 2024
Day 1
Valid 12Z Sun Jul 28 2024 - 12Z Mon Jul 29 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND THE OHIO/TENNESSEE VALLEYS...
...Northern Plains...
Heavy to potentially excessive rainfall is possible as a mid- and
upper system ejects northeastward from the Mid-Mississippi Valley
on Sunday into the western Great Lakes by early Monday morning. The
inherited Slight Risk was once again expanded towards the south and
east as moisture streaming northwards from the Mid-Mississippi
system interacts with the strengthening of mid-level deformation.
With model guidance showing 1 to 3 inches of rainfall...much of
which could be delivered in an hour or two...there is the potential
for flash flooding.
...Central Ohio/Tennessee Valley...
Mid-level height falls approaching from the west should help focus
and support showers and thunderstorms capable of producing some
downpours from the afternoon into the evening...mainly across the
western portion of the Ohio and Tennessee Valley. Northerly flow to
the east and ridging to the east should slow the eastward progress
of storms...at least initially. Overall...the GFS looks to be the
most progressive while the ECMWF was on the slowest side of the guidance...leaving the NAM as a compromise in terms of position and
its 1 to 2 inch amount as being reasonable given the amount of the
instability and moisture.
...Southwest...
The Marginal Risk in this area was extended east to once again
include the Sacramento Mountains of south-central New Mexico, given
the sensitivities of the soils in this area. Expect otherwise
typical afternoon convection in the monsoonal moisture to develop
once again, with several very sensitive areas into southern
Arizona and New Mexico.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE EASTERN OHIO/TENNESSEE VALLEY TO SOUTHERN APPALACHIANS...
The Slight Risk across this area was shifted eastward...again...largely
due to guidance shifts in the axis of heaviest deterministic rainfall,
which now appears to be focused along much of the central and
southern Appalachians, but extending back across much middle and
upper Ohio Valley. Given areas where flash flood guidance has
lowered values around 1.5 inches per hour or 1.9 inches per 3 hours
due to due to recent rainfall...the risk of excessive rainfall
persists given the amount of moisture still in place.
Upper Midwest...
Rainfall should be diminishing in areal coverage and intensity as
deeper moisture and the better dynamics get shunted eastward.
Maintained a Marginal risk area here given some on-going rainfall
early that could overlap with flash flood guidance lowered by rains
on Sunday.
...Southwest US...
Steering flow in the mid and upper levels begin to weaken as
heights begin to rise while precipitable water values linger at or
above an inch (even 1.25 inches in places over the desert). This
warrants keeping the previously issued Marginal risk along/near the
Arizona/New Mexico border.
...There is a Slight Risk of Excessive Rainfall Lingering Over
Portions of the eastern Ohio Valley to the Appalachians...
...Eastern US...
With lingering potential for rainfall over a region of lowered
Flash Flood Guidance in the eastern Ohio/Tennessee Valleys and
adjacent areas of the Appalachians...opted to maintain a Slight
Risk area in a similar position to that of the Slight Risk on
Monday. Model QPF is actually pretty modest...but the environment
remains capable of producing downpours on scales that are difficult
for models to handle this far out.
Surrounding the Slight Risk...kept a Marginal risk area in place
for areas to the north where the ECMWF and GFS show energy in the
northern stream making its way eastward across the eastern Great
Lakes. The model spread on timing this energy is large enough to
warrant a Marginal although an upgrade is certainly in the realm
of possibilities if the large scale pattern supports more
organization over a broader area...especially upstream.
...Southwest...
Steering flow in the mid and upper levels and persistent monsoonal
moisture will still be in place along the Arizona/New Mexico border
that begins to ease northward towards southern Colorado during the
period.
....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY, UPPER TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...
West northwesterly mid to upper level flow will persist during the
upcoming day 1 period from the Northern Plains, east southeastward
through much of the Mississippi Valley, Ohio Valley, Southern to
Central Appalachians, Mid-Atlantic and Southeast. Embedded
shortwaves in this flow will support another day of active
convection across much of these areas where PW values will be
mostly above seasonal averages. A slight risk area was drawn where
the latest HREF neighborhood probabilities were the highest for 1,
2 and 3"+ totals and overlapped the lower FFG values and observed
precip axis from recent rains. This results in a slight risk from
southern Illinois, east-southeast through the Ohio Valley, Upper
Tennessee Valley into the Southern Appalachians. Changes from the
previous issuance were to elongate the previous slight risk farther
westward through the Lower Ohio Valley and southward into the
Southern Appalachians where heavy rains fell over the past 24
hours. The eastern portion of the slight risk over west central NC
was trimmed westward given relatively high FFG values and no precip
overlap potential from past 24 hours and the upcoming day 1
period.
...Florida...
Concentrated the marginal risk area across the southern portion of
the peninsula where model consensus is for heavy rainfall in the
high PW axis along and south of the stationary frontal
boundary/trof currently analyzed off of the FL east coast. HREF
neighborhood probabilities are high for 1, 2 and 3"+ amounts across
the southern half of the Peninsula, supporting isolated runoff
issues, especially in the more urbanized regions.
...Southeast Arizona into southwest New Mexico...
No significant changes made to the marginal risk area across
southeast Arizona into southwest New Mexico. Scattered convection
likely again associated with shortwave energy rotating northward on
the western side of the Southern Plains upper ridge and across
portions of the Southwest. There is the usual low confidence in qpf
details, but potential for isolated heavy totals and localized
runoff issues. The marginal risk area fits well with the axis of
the 1"+ HREF neighborhood probabilities.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN MID-ATLANTIC INTO THE CENTRAL APPALACHIANS...
There is not expected to be any significant changes to the overall
large scale flow across the northern tier of the country from the
Upper Mississippi Valley, east-southeastward across the Great
Lakes, Ohio Valley, Central to Southern Appalachians into much of
the east coast. West northwesterly mid to upper level flow will
again persist across these regions with additional embedded
shortwaves once again supporting potential for large regions of
convective activity. The lead area of height fall expected to
support potentially organized convection across portions of the
Central Appalachians into the northern Mid-Atlantic in an axis of
PW values 1.5-2.5 standard deviations above the mean. The previous
slight risk was expanded northeastward into the northern Mid-
Atlantic to cover this heavy rain potential. The southern portion
of the previous slight risk area was extended farther south into
eastern TN/far western NC to cover the area of qpf overlap day 1
and day 2.
...Ohio Valley into the Upper Mississippi Valley...
To the west and northwest of the slight risk area, there is a lot
of model qpf spread through the Ohio Valley into the Upper
Mississippi Valley in the region where additional mid to upper
level vort will be tracking east southeastward in a continued axis
of above average PW values. To cover the model qpf spread, the
previous marginal risk area was expanded significantly to the
northwest into the Upper Mississippi Valley. At the moment, not a
lot of confidence in any one heavy model qpf, so the risk level was
kept at marginal across these areas.
...Southwest Florida...
The previous marginal risk area was decreased in size and
concentrated over Southwest Florida. This corresponds to where the
HREF neighborhood probabilities for the 12 hour period from 12Z Tue
to 00Z Wed are highest for 2 and 3"+ amounts. Isolated runoff
issues possible, especially over urbanized regions.
...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.
Oravec
Day 3
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST...
The progressive northern stream flow will continue into day 3 with
the next strong area of height falls expected to push eastward
across the Northern Plains and into the Upper Mississippi Valley.
There is potential for additional organized convection/heavy
rainfall to push eastward with these height falls across eastern
portions of the Northern Plains and into the Upper Mississippi
Valley. There are some typical latitude differences with the max
qpf axis. The day 3 slight risk area was drawn to favor the more
southern solutions of the EC, NAM and CMC. The previous slight risk
area over the Upper Mississippi Valley was suppressed more to the
south and southwest to better match the more southern solutions
with the next round of potentially organized convection.
...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 2, will progress across eastern
NY State into New England day 3. There is a typical amount of qpf
spread for the day 3 period, but general consensus for potential of
moderate to locally heavy rainfall amounts in a region of much
above average PW values ahead of these height falls. A broad
marginal risk area was maintained from the previous issuance to
capture the heavy rainfall spread in the models across this area
day 3.
...Southwest...
Similar to the days 1 and 2 period, the mid to upper level ridge
will remain centered over the Southern Plains. Scattered convection
again likely over portions of the Southwest as shortwave energy
again rotates northward on the western side of this ridge and into
the Southwest. Localized heavy rains and isolated runoff issues
again possible for southeast Arizona into southwest to central New
Mexico.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE MID MISSISSIPPI VALLEY, LOWER OHIO VALLEY, UPPER TENNESSEE
VALLEY INTO THE SOUTHERN APPALACHIANS...
...Mid Mississippi Valley, Lower Ohio Valley, Upper Tennessee
Valley and Southern Appalachians...
Another day of active convection likely in the broad west
northwesterly mid to upper level flow from the Mid to Upper
Mississippi Valley, into the Ohio Valley, Tennessee Valley and
Central to Southern Appalachians. Shortwaves in this flow will be
pushing through an area of above average PW values, 1 to 1.5+
standard deviations above the mean. While there is a lot of model
qpf detail spread, there is consensus for a northwest to southeast
oriented axis of potentially heavy rains across these areas. The
two previous slight risk areas were consolidated into a more
elongated slight risk area that corresponds well with the latest
HREF mean and the higher HREF neighborhood probabilities for 1 and
2"+ amounts and the axis of HREF EAS probabilities for 1"+ amounts.
There has been some overlap in the slight risk area of precip over
the past 48 hours and potential for additional heavy rains in this
axis.
...Central Appalachians, Mid-Atlantic into NY State...
Much above average PW values, 1 to 2.5+ standard deviations above
the mean also expected to push northeast from the Central
Appalachians, across the Mid-Atlantic and into NY State. There is
a lot of qpf spread in the latest model suite, but consensus for
potential for widespread moderate to locally heavy totals. HREF
neighborhood probabilities are high for 1"+ amounts, but drop off
significantly for 2"+ amounts. The HREF EAS for 2"+ amounts is
showing probabilities of less than 5%, illustrating poor overlap in
HREF heavy rain areas. A marginal risk is depicted across these
areas, with the previous issuance slight risk along the northeast
PA/Southern Tier of NY removed.
..Northern Rockies...
Only some small changes made to the marginal risk area across
portions of the Northern Rockies from northern Idaho into southwest
Montana. Latest guidance continues to show potential for an
organized area of moderate to locally heavy precip ahead of a fast
moving vort pushing east from the Pac NW into the Northern Rockies
in the 1200 UTC Tue to 0000 UTC Wed period. The marginal risk
corresponds to where the HREF neighborhood probabilities show
spotty high values in the 1200 UTC Tue to 0000 UTC Wed period.
The overall quick movement of the vort should keep precip amounts
from being very heavy, but anomalous PW values 2 to 2.5 standard
deviations above the mean will support some short term rainfall
rates of 1 to 1.5" per hour, supporting potential for isolated
runoff issues in areas of steep terrain and over burn scars.
...Southwest...
No changes expected to the large scale flow with the upper ridge
remaining centered over the Southern Plains. This will again
support shortwaves rotating northward on the western side of this
ridge into the Southwest, supporting scattered convection,
localized heavy rains and isolated runoff issues.
...Southwest Florida...
No significant changes to the marginal risk area over Southwest
Florida. Above average PW values expected to persist across the
southern portion of the FL Peninsula day 1, along and south of the
east to west trof axis stretching from the Atlantic into South
Florida. The marginal risk continues to fit in well with the high
HREF neighborhood probabilities for 1, 2 and 3"+ totals. This
heavy rain potential will support isolated runoff issues,
especially in more urbanize regions.
Oravec
Day 2
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE UPPER MISSISSIPPI VALLEY INTO THE MID WEST AND OVER PORTIONS OF
NORTHERN NEW ENGLAND...
...Upper Mississippi Valley into the Midwest...
The progressive northern stream flow will continue into day 2 with
the next strong area of height falls expected to push eastward
across the Northern Plains and into the Upper Mississippi Valley.
There is potential for additional organized convection/heavy
rainfall to push eastward with these height falls across eastern
portions of the Northern Plains and into the Upper Mississippi
Valley as low level southerly flow strengthens into the west to
east oriented frontal boundary expected to stretch from the NE/SD
border, east across northern IA into northern IL. 850-700 mb moisture
flux anomalies into this front expected to rise to 2 to 3 standard
deviations above the mean in the 0000 to 1200 UTC Thursday period.
There are still a lot of model qpf spread with some typical
latitude differences with the max qpf axis. The day 2 slight risk
area was drawn to favor the more southern solutions of the NAM,
NAMNEST and CMC.
...Eastern New York State into western New England...
The strong height falls moving out of the Central Appalachians
into the Northern Mid-Atlantic day 1, will progress across eastern
NY State into New England day 2. There is a typical amount of qpf
spread for the day 2 period, but general consensus for potential
of moderate to locally heavy rainfall amounts in a region of much
above average PW values ahead of these height falls. A small slight
risk area was added along the VT/NH border where very heavy
rainfall amounts have occurred early Tuesday morning and where
several models show potential for additional heavy rains during day
2. Otherwise,a broad marginal risk area was maintained from the
previous issuance to capture the heavy rainfall spread in the
models across this area day 2.
...Southwest...
Similar to the day 1 period, the mid to upper level ridge will
remain centered over the Southern Plains. Scattered convection
again likely over portions of the Southwest as shortwave energy
again rotates northward on the western side of this ridge and into
the Southwest. Localized heavy rains and isolated runoff issues
again possible for southeast Arizona into southwest to central New
Mexico.
Oravec
Day 3
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF
THE UPPER MISSISSIPPI VALLEY, ACROSS THE LOWER GREAT LAKES, OHIO
VALLEY INTO THE CENTRAL APPALACHIANS AND ACROSS PORTIONS OF THE
SOUTHWEST AND SOUTHERN CALIFORNIA...
...Upper Mississippi Valley, Lower Lakes, Ohio Valley...
The strong height falls pushing through the Northern Plains into
the Upper Mississippi Valley day 2 will sink more to the southeast
into the Mid Mississippi Valley/Mid West region day 3. Additional
heavy rains likely ahead of these height falls and the associated
surface low in a region of above average PW values and anomalous
850-700 mb moisture flux. There are some large model qpf
difference's with respect to the axis of the heaviest rains. The
marginal risk was drawn to encompass this range form the northern
solutions of the UKMET/GEM from southern WI into the southern L.P.
of MI, to the southern solution of the NAM and EC from southern IL,
southern IN, southern OH, into northern KY. Given the spread and
low confidence at the moment with which qpf axes is the best, the
risk level was kept at marginal.
...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest during day 3. This will help to push the axis of above
average PW values farther west into southern to central California
and southern Nevada. This will support an expanding area of
scattered diurnal convection into portions of central and southern
CA/southern NV from days 1 and 2. The marginal risk area was also
expanded westward from the previous issuance to include western AZ,
southern NV into southern to south central CA.
From Mike Powell@618:250/1 to All on Wed Jul 31 08:56:00 2024
FOUS30 KWBC 310828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Wed Jul 31 2024
Day 1
Valid 12Z Wed Jul 31 2024 - 12Z Thu Aug 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM THE NORTHERN
PLAINS TO THE MIDWEST AS WELL AS PORTIONS THE SOUTHWEST US AND NEW ENGLAND...
...NORTHERN PLAINS THE MID-WEST...
Convection should be on-going across portions of Iowa at the start
of the Day 1 period at 12Z...with some risk of excessive rainfall
lingering early on. Renewed convection is expected later in the day
as stronger forcing moves across the Plains later today. WPC
favored the more southerly deterministic QPF solution offered by
the HREF...and that was reflected in expanding the previously-
issued Slight Risk area a bit southward in portions of Minnesota
and Iowa. Farther south...a large anomaly of 850 to 700 mb
moisture flux remains in place. With the HREF depicting an axis of
1 to nearly 1.5 inches of rain possible falling over lowered flash
flood guidance values in southeast Kentucky...felt confident enough
to agree extending the Slight Risk south and east into a region
where the deterministic QPF from GFS/ECMWF/UKMET were meager.
There is still a lot of model qpf spread with some typical latitude
differences with the max qpf axis.
...Northeast US...
Height falls with a wave approaching from the west will be
spreading over portions of New York into New England during the day...accompanied by a risk of moderate to locally heavy rainfall
given the abundant moisture already in place. Maintained the Slight
Risk without change given its placement over an area soaked by 4
to 8 inch amounts within the past 24 hours...making the area
especially vulnerable to any additional rainfall and locally
significant flash flooding remains a possibility.
...Southwest US...
Maintained the Slight Risk across portions of Arizona that was
introduced on Tuesday. Models continue to show an uptick in both
moisture and instability compared with the past couple of days,
with satellite supporting the model indications of a weak wave
moving in from the southeast. This combination of increased forcing
and better thermodynamic environment should support greater
convective coverage with locally heavy rainfall rates. HREF
probabilities are elevated as well...with 2"+ neighborhood
probabilities of 40-60% and 1" EAS probabilities of 5-15"
indicating at least some ensemble agreement on location/coverage
of convection.
Bann
Day 2
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE OHIO VALLEY...
The suite of numerical guidance from 31/00Z continued to support a
Slight risk area over portions of the Ohio Valley from Thursday
into Thursday night as fairly impressive forcing approaches the
region. Low level moisture should be place as the closed mid-level
low drops in from the north...helping to focus moisture flux
convergence along a quasi-stationary front in the area at the same
time that divergence aloft increases in response to the presence of
an upper level speed maximum. Given uncertainty of where the low
will be based on the model spread and where the strongest forcing
gets directed introduces plenty of uncertainty with regard to
placement of heavier rainfall. As stated earlier...the ingredients
still suggest a Slight risk area is warranted and the WPC placement
remains south of global QPF consensus...in fairly close proximity
to the UFVS machine learning ERO.
...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest during day 2...helping push the axis of above average
precipitable water values farther west into southern to central
California and southern Nevada. This will foster an environment in
which scattered diurnal convection will be expanding into portions
of central and southern California/southern Nevada.
Bann
Day 3
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO/TENNESSEE VALLEYS AND IN THE SOUTHWEST US...
...Ohio and Tennessee Valleys...
A closed low initially near the Great Lakes at the start of the Day
3 period will continue to sage south and eastward into the Great
Lakes region. Given plentiful moisture in place and increasing
cyclonic circulation aloft...localized downpours are in the realm
of possibility. One camp of thought in the models is that higher
amounts will be tied to the upper dynamics early on in the period
while the NAM seems to champion the idea of higher amounts along
the leading edge of the height falls closer to the better
instability. With the spread shown...went with a broad and
unfocused Marginal risk area for the time being.
...Southwest US...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
international border...another round of late day/evening showers
and thunderstorms is expected. Some northward/eastward expansion
compared with the previous couple of days is expected.
From Mike Powell@618:250/1 to All on Thu Aug 1 07:56:00 2024
FOUS30 KWBC 010710
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
310 AM EDT Thu Aug 1 2024
Day 1
Valid 12Z Thu Aug 01 2024 - 12Z Fri Aug 02 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST AND OHIO VALLEY...
Midwest/Great Lakes/Southern Appalachians...
The suite of numerical guidance continued to support a Slight Risk area
over portions of the Ohio Valley from Thursday into Thursday night as
fairly impressive forcing approaches the region. Low-level moisture
should be place (precipitable water values up to 2") as the closed
mid- level low drops in from the north...helping to focus moisture
flux convergence along a quasi- stationary front in the area at
the same time that divergence aloft increases in response to the
presence of an upper level speed maximum. The low-level inflow and
effective bulk shear should be high enough in magnitude (25-30 kts)
to lead to convective organization. Hourly rain totals to 2.5", and
local amounts to 5", are possible. The ingredients still suggest
a Slight risk area is warranted. Changes to continuity were
minimal.
...Southwest into southern California...
The upper ridge over the Southern Plains is forecast to build to
the west and northwest across the Rockies, Great Basin and
Southwest. An easterly wave under its base moves south of CA, helping
push the axis of above average precipitable water values (as high
as 1.75-2") farther west into southern to central California and
southern Nevada. This will foster an environment in which scattered
diurnal convection will be expanding into portions of central and
southern California/southern Nevada. Hourly rain totals to 2.5" are
possible, given the above. This would be problematic in area burn
scars and dry washes/arroyos.
Roth/Bann
Day 2
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND AROUND
PORTIONS OF CENTRAL & EASTERN KENTUCKY...
....Ohio & Tennessee Valleys...
The upper pattern agrees on the approaching trough and focused
ascent over the Ohio Valley into the Mid Atlantic. Precipitable
water values up to 2" are advertised with this system. The low-
level inflow and effective bulk shear should be high enough in
magnitude (25-30 kts) to lead to convective organization. The
available ingredients suggest hourly rain totals to 2.5" and local
amounts to 5" would be possible. However, there is much spread in
scattered convective signals underneath the mean trough. The
environment is conducive for locally heavy rains anywhere from the
Central Midwest through the Central and Northern Mid Atlantic with
the primary likely falling under the center of the upper
circulation and mean trough. This pinpoints areas like Ohio and
Western PA down into WV/KY as the best chance for convective
coverage capable of more flash flooding capabilities. Much is
contingent on the pattern evolution and with time leading into the
setup, but the mass fields along with the GFS/NAM suggest that
portions of central and eastern KY should be most favored; added a
Slight Risk in this area to address that concern which was
coordinated with the JKL/Jackson KY and LMK/Louisville KY forecast
offices. The previous Marginal Risk east of the Mississippi was
expanded slightly, but remains fairly close to continuity.
...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to
1.75")...another round of late day/evening showers and
thunderstorms is expected. Continuity was generally maintained.
Hourly rain totals to 2" should be possible. Locally heavy
rainfall would be most problematic in any area burn scars and dry washes/arroyos.
Roth/Kleebauer/Bann
Day 3
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
SOUTHWEST, OREGON, MINNESOTA, FLORIDA, & IN AND NEAR THE MID-
ATLANTIC STATES...
...In and near the Mid-Atlantic States...
A generally wet and unsettled pattern is likely with ample
moisture (precipitable water values up to 2") and instability in
place as a slow moving upper trough and surface/frontal systems all
support areas of showers and thunderstorms. Inflow at 850 hPa of 25
kts implies some level of convective organization is expected.
Hourly rain totals up to 2.5" should be possible here, and local
amounts in the 2.5-5" range are advertised by the guidance, which
could easily fall within an hour or two. As there remains
uncertainty in placement of the higher amounts, and a decent
section of the Richmond/Washington D.C./Philadelphia megalopolis
has descended into drought, raising flash flood guidance values
fairly highly, believe the Marginal Risk continues best for the
time being. Urban areas are at the highest risk for flash flooding
in this region.
...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos. Expanded the area
westward to account for the signal seen in the 00z Canadian
Regional which implies a risk as far west of the Peninsular Ranges
of CA.
...Florida Peninsula...
Moisture and instability increase as a tropical disturbance moves
in the vicinity of the FL Peninsula. Precipitable water values rise
to 2.25"+, and plenty of instability resides offshore over the warm
waters. There has been a westward shift in the guidance over the
past 24 hours more into the Gulf of Mexico due to its slower than
anticipated (in model land) development, and the best indication as
to the center of any low- to mid-level vorticity has been near
Puerto Rico. With the potential for 3" an hour totals and a
possibly better organized system to enhance rainfall potential,
went ahead and broadened the existing Marginal Risk area to
encompass the FL Peninsula for the time being to account for the
uncertainty.
...Minnesota...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.
...Oregon...
Moisture advecting northward through the West gets drawn into the
leading edge of an upper level trough between southwest OR and
northeast WA. Both the GFS and NAM get the precipitable water
values up to 1-1.5", which would be most anomalous in terrain.
There is some reflection of the trough in the 850-700 hPa wind
pattern (the NAM appears more bullish on the low-level convergence
prospects). Considering the weakness of the upper level feature,
it shouldn't preclude daytime heating/diurnal insolation, so there
should be decent instability as well. The concern is highest in
the terrain, particularly near any burn scars. This led to a new
Marginal Risk area across portions of OR, which have the best QPF
signal, albeit not that high. Hourly rain totals to 2" are
considered possible.
From Mike Powell@618:250/1 to All on Fri Aug 2 07:49:00 2024
FOUS30 KWBC 020827
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Fri Aug 2 2024
Day 1
Valid 12Z Fri Aug 02 2024 - 12Z Sat Aug 03 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MIDWEST, OHIO VALLEY, AND NORTHEAST, ALONG WITH THE EASTERN
TENNESSEE VALLEY INTO THE SOUTHERN APPALACHIANS...
...Eastern U.S....
Slowly advancing upper trough and surface front will focus broad-
scale ascent for scattered-numerous convective elements across
much of the eastern CONUS. Two areas of particular concern include
a W-E corridor from SE MI and the OH Valley eastward to NJ and
southern NY, where the deep-layer forcing (DPVA) ahead of the
opening mid-level low will be a bit more robust. The combination of
favorable deep-layer instability, especially during peak heating
hours (MLCAPEs 1000-2000+ J/Kg) along with TPW values of 1.75-2.00+
inches will make hourly rainfall rates of 1-2+ inches likely
underneath the strongest cells, with potentially higher hourly rates
(2-3") farther east toward eastern PA, NJ, and southern NY given
the closer proximity to the max TPW and low-level moisture
transport axes.
Farther south, the guidance is also favoring eastern portions of
the TN Valley into the southern Appalachians of northern GA-western
NC for more widespread convection and an enhanced risk of flash
flooding (i.e. Slight). This given the upslope low-level component
along with a more vigorous vort lobe traversing the area.
Elsewhere, any flash flooding is expected to be more
isolated/localized, thus the Marginal Risk areas.
...West...
With the upper level ridge continuing to build across the
Intermountain region and monsoonal moisture lingering near the
International Border (precipitable water values up to 1.75"),
another round of late day/evening showers and thunderstorms is
expected. Continuity was generally maintained, though we did expand
the Marginal Risk across the Sierra Nevada, Transverse Ranges, and
adjacent foothills. Hourly rain totals to 2" should be possible.
Locally heavy rainfall would be most problematic in any area burn
scars and dry washes/arroyos.
Hurley
Day 2
Valid 12Z Sat Aug 03 2024 - 12Z Sun Aug 04 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTH FLORIDA ALONG WITH PARTS OF THE MID ATLANTIC AND SOUTHEAST
COASTAL PLAIN...
...Eastern U.S...
A generally wet and unsettled pattern is likely, with ample
moisture (precipitable water values up to 2" for most, 2.25-2.50
across FL along/ahead of the tropical disturbance) and instability
in place as a slow moving upper trough and surface/frontal systems
all support numerous concentrated areas of convection.
Across FL, per collaboration we have hoisted a Slight Risk across
parts of South FL given the uptick in QPF ahead of the tropical
disturbance. While most of the models and ensemble means keep the
max areal-average totals off the SW FL coast (3-5+ inches), TPWs
reaching 2.4-2.6 inches along with the possibility of more
destabilization east of the tropical distrubance's center will make
for the potential of very intense rainfall rates (2.5-3.0+ within
an hour) underneath the more isolated stronger cells.
Farther north into the Mid Atlantic, inflow at 850 hPa of 25 kts
implies some level of convective organization is expected. Hourly
rain totals up to 2.5" should be possible here, and local amounts
in the 2.5-5" range are advertised by the guidance, which could
easily fall within an hour or two. The guidance has come in better
alignment in advertising an axis of heavier rainfall across western
portions of the coastal plain and eastern Piedmont over the
Carolinas into parts of SE VA. This given the infusion of deep-layer
moisture surging north of the tropical disturbance toward the mid-
upper level trough and surface front. Per collaboration, have
included a Slight across this region as well.
Elsewhere to the north (north of Richmond VA), there remains
uncertainty in placement of the higher amounts, and a decent
section of the Washington D.C./Philadelphia megalopolis has
descended into drought, raising flash flood guidance values fairly
highly. Therefore still believe the Marginal Risk continues best
over these areas for the time being. Urban areas are at the
highest risk for flash flooding in this region.
...Southwest...
Monsoonal convection over the Southwest (precipitable water values
up to 1.75" near the International Border) continues to support a
Marginal Risk area. Locally heavy rainfall would be most problematic
in any area burn scars and dry washes/arroyos.
...Sierra Nevada into Oregon Cascades, Blue Mountains, and parts
of the Northern Rockies to western portions of the Great Basin...
Anomalous moisture (TPW anomalies topping 3-3.5 standard deviations
above normal per the GEFS) will advect northward through the West
on the backside of the upper ridge and ahead of the shortwave
trough off the PAC NW coast. The guidance gets TPW values up to
1-1.25", which would be most anomalous in terrain. Given these
moisture profiles, along with the diurnal heating (MLCAPEs peaking
at least ~500 J/Km), spotty hourly or sub-hourly rainfall rates aoa
1" will make for localized runoff issues, especially over burn
scares.
...Upper Midwest...
The guidance is generally showing a modest signal for heavy
rainfall in and near Minnesota this period. Precipitable water
values rise about 1.25" with a warm advection pattern. This led to
an introduction of a Marginal Risk here, as flash flood guidance
values are modest after recent heavy rainfall.
Hurley
Day 3
Valid 12Z Sun Aug 04 2024 - 12Z Mon Aug 05 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF THE
FLORIDA PENINSULA AND PARTS OF THE SOUTHEAST COASTAL PLAIN...
...Eastern U.S....
The Slight Risk across FL and the Southeast was expanded a bit,
based on the growing consensus in terms of the forecast track and
forward speed of the tropical disturbance. Areal-average QPF per
the smart blends/ensembles (including the NBM) is up to 3-5" over
parts of the FL peninsula. This given the highly favorable
environment (deep layer moisture, tall/skinny CAPE profiles) along
and ahead of the tropical disturbance. Per collaboration with the
WFOs, have expanded the Slight Risk area, but for now given
uncertainty by Day 3 in terms of the eventual track/speed/strength
of the potential tropical system, have held off in hoisting a
Moderate Risk.
...Montana into the Northern Plains and Upper Mississippi
Valley... Aforementioned axis of anomalous TPW out West during Day
2 will ultimately get absorbed in the upper level jet streak and
mid- level vort lobe that will cross MT and the Dakotas on Day 3.
Dynamical forcing along with the thermodynamical profile will favor
more widespread showers and thunderstorms, though the shear
profiles will certainly favor more progressive/forward-propagating
cell motions and thus make for more of an isolated flash flood
threat. Areas across northwest MT, including Glacier NP, would be
the most susceptible given the lower FFG (1" or less within 3 hours
for most areas). An upgrade to Slight may be needed over this
region in later outlooks, especially if we start seeing a consensus
of heavier QPF from the models.
From Mike Powell@618:250/1 to All on Fri Aug 9 10:01:00 2024
FOUS30 KWBC 090758
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Fri Aug 9 2024
Day 1
Valid 12Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC AND NORTHEAST...
...Mid-Atlantic to Northeast...
Debby and it's remnant plume of tropical moisture are forecast to
lift northward today from the central Appalachians to the Interior
Northeast, while producing a swath of heavy rainfall and the
potential for numerous flash floods from near the West Virginia
Panhandle early this morning to Upstate New York this afternoon.
Precipitable water values of 2-2.5" stream along and just east of
the system will equate to above the 95th climatological percentile
and +3.0 standard deviations above the climatological mean.
Additionally, ample shear surrounding the low center and MUCAPE
values up to 1000 J/kg nearby along the Atlantic Seaboard being
advected northward will allow for efficient tropical downpours
within scattered to numerous showers and thunderstorms. A focus in
the heaviest rainfall is likely on the northern and northeastern
quadrant of the low pressure system as it transitions to an
extratropical phase, becomes frontal, and begins to increase
forward speed to the northeast by Friday night. A warm front
lifting northward will add increasing frontogenetic forcing to the
environment and lead to a more widespread precipitation shield
today across parts of PA and Upstate NY, with embedded rainfall
rates likely up to 2-3"/hr at times. The trend for this axis of
heavy rainfall has been for it to occur slightly westward than
previously forecast, now across far northwest NY. Additionally, the
trend is for faster northward progression with this system. 12z
NAEFS IVT valid 18z Fri is forecast to max all values in the
1979-2009 climo period, amplifying the moisture surge fueling
today's heavy rainfall event. 00z HREF neighborhood probs for at
least 3" in 6-hrs are high (60-80%) from central PA to the Finger
Lakes region and northwest portions of NY State, which would also
exceed local FFG for much of this region. As a result, the Moderate
Risk was maintained from the previous forecast with only minor
cosmetic changes. Numerous instances of flash flooding are expected
in this area, with locally significant flash flooding possible.
Elsewhere in the Northeast, convection developing along and ahead
of a cold front should have enough forward motion to lead to only
scattered to localized flash flooding concerns closer to the I-95
corridor, but with rainfall rates of 2"/hr possible. Areas that
received recent rainfall and poor drainage locations could have a
relatively greater flash flood risk, which supports the SLGT Risk
extending to parts of New England and the Mid-Atlantic.
...Eastern Carolinas...
A separate area along the East Coast at risk for flash flooding
today will be associated with a convergence axis along
coastal/eastern sections of North and South Carolina this afternoon
and evening. PWs elevated and above 2.25", along with ample
instability and mean winds parallel to the boundary/coastline will
support slow-moving and/or training thunderstorms across saturated
terrain. CAMs and the associated 00z HREF suite depict the
potential for 5"+ amounts throughout eastern North Carolina and
coastal South Carolina, but the expectation remains that these
totals would be pretty localized. The SLGT Risk was expanded from
the previous forecast across South Carolina given recent CAM output
and very saturated antecedent conditions in place. Will monitor
this region for upgrades in the future outlooks.
...Four Corners, Central Rockies, Central High Plains, and Southern
Plains...
Showers and thunderstorms are expected to fire up across the
Southwest and Rockies as shortwave energy passes through and
encounters the ongoing monsoonal moisture in place. There will be
increased focus for more organization and higher amounts
concentrating near the higher terrain of southern Utah, south/central
Colorado, and northern New Mexico, where a SLGT Risk area remains
in effect. Accumulations of 1-2" will be possible in the strongest
cells, a stronger correlation to flash flooding concerns compared
to historical precedence. The rest of the Southwest will still see
periods of storms capable of localized flash flood concerns with
the best threat located more into the Intermountain West and the
Mogollon Rim in Arizona. Convection (possibly organized) will also
spread eastward into western Kansas and the panhandles of Oklahoma
and Texas, as well as northeast NM. Here, 00z HREF neighborhood
probs are modest (20-40%) for at least 3" in 6-hrs. A broad
MRGL Risk covers much of the High Plains.
...Central Texas...
A MRGL Risk was introduced across parts of central Texas as
convection develops along the tail-end of a lingering cold front
and ahead of a stronger frontal boundary dropping south from the
central Plains. Shear will be weak, but steep low-level lapse rates
and sufficient moisture (PWs around 2") will allow for intense
rainfall rates over relatively wet ground conditions and a
historically flash flood prone region. 00z HREF neighborhood
probs have increased and now show elevated chances (30-50%) for
rainfall amounts exceeding 3" this evening.
Snell
Day 2
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS, AND THE COASTAL CAROLINAS...
...Southwest, Central Rockies, and Plains...
A rather broad MRGL Risk was maintained extending from the
Southwest to the central/southern Plains as diurnal heating and
monsoonal moisture will keep showers and thunderstorms developing
during the afternoon and evening hours for much of the Four Corners
region. Meanwhile, an area of more organized convection triggered
by shortwave energy from the D1 period will be making its way
eastward into the foothills and spreading out into the high
Central/Southern Plains. There is a fair amount of uncertainty with
how far east into the Plains the convection reaches, location of
heaviest amounts and the overall accumulations. Most of the
consensus is suggesting 1-3 inches however a couple models have
maxes near/over 4 inches. A Marginal Risk covers much of the Four
Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma,
northern Texas, southwest Missouri and northwest Arkansas. If
amounts trend up then a Slight Risk area may be need with further
updates.
...Coastal Carolinas...
A lingering frontal boundary and diurnal sea breeze convection will
continue chances for scattered thunderstorms across the Southeast
and more specifically the eastern Carolinas on Saturday. Soils
should remain saturated after prior heavy rainfall, increasing the
flash flood threat across an area that isn't usually as prone to
flash flooding. CAMs depict localized totals from pulse
thunderstorms could reach 1-3", which is supported by PWs over 2".
Snell
Day 3
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, FOUR CORNERS, CENTRAL ROCKIES INTO THE PLAINS, AND
THE EASTERN CAROLINAS...
...Four Corners, Southwest, and Central Rockies...
A deepening western U.S. trough by D3 will allow for another round
of diurnal monsoonal convection across the Southwest and Four
Corners region, with GEFS and ECENS depicting PWs above the 75th
climatological percentile. Opted to maintain the MRGL Risk for now
given the limited QPF signal in available guidance and the
possibility for fast storm motions.
...Plains and Mid-Mississippi Valley...
Northwest flow aloft and a gradually lifting warm front over the
Southern Plains will support MCS potential into the overnight
period on D3 along with increasing mid-level flow, which could
produce training and intense rainfall rates within convection.
Relatively high uncertainty remains with the exact placement of
heaviest rainfall (greatest potential currently near the western
Ozarks into northeast OK and southeast KS), which prompted the MRGL
Risk as opposed to a SLGT for now. Additional convection is
possible over SD and NE, with similar uncertainty in placement.
...Eastern Carolinas...
Another day of diurnal convection along a nearby frontal boundary
could be enhanced by an approaching shortwave diving over the Ohio
Valley around a deep upper-level low over SE Canada. Similar to D2,
rainfall amounts of 1-3" appear possible and are likely to occur
over saturated soils.
From Mike Powell@618:250/1 to All on Fri Aug 9 12:47:00 2024
FOUS30 KWBC 091546
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1146 AM EDT Fri Aug 9 2024
Day 1
Valid 16Z Fri Aug 09 2024 - 12Z Sat Aug 10 2024
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID ATLANTIC AND NORTHEAST...
...16Z Update...
Debby's remnants continue to press rapidly to the north as the
remaining core analyzed at 15z is rushing northward at just over 30
kts (35 mph). Bands of heavier rainfall continue to push north out
of the Central Mid Atlantic with the heaviest rain focused near the
remnant center of now Post-Tropical Cyclone Debby. The current
forecast remains on track with little variance in the QPF and
associated probabilities as rainfall rates of 1-2"/hr will be seen
within the heaviest cores as the storm drives northward out of
Northern New England by late this evening. The previous MDT risk
was trimmed on the southern edge of the previous forecast to
reflect the radar trends of the heaviest precip shifting north with
more of a focus across Northern PA through Upstate NY with the max
for the period likely situated across the North Country of NY
state. A trailing cold front will allow for pockets of heavy rain
to develop this afternoon across portions of the Central Mid
Atlantic, but will quickly vacate eastward as low-level convergence
along the front will sweep east with a degrading threat of heavy
precip in wake of the boundary. Some of the SLGT risk across the
interior Central Mid Atlantic was also adjusted given the
environment improving with much of the heavier precip likely
falling along and east of the Blue Ridge.
The Eastern Carolinas exhibited very little in the way of variance
from the previous forecast issuance with the afternoon and evening
time frame remaining the period of interest with the convective
development anticipated. Hi-res was in strong agreement on the
placement of a line of convection extending from coastal SC up
through Eastern NC with much of the heavier cores likely to fall
east of the I-95 corridor. Embedded heavy thunderstorms within the
main convective axis will likely produce a few cells that can reach
upwards of 5" with a modest signature on the 12z HREF neighborhood
probability for the threshold settling between 30-50% over a large
area encompassing most of Eastern NC. HREF EAS signals for at least
2" is relatively high as well (20-30%) over the same areas, so the
threat is likely on the higher-end of the SLGT risk threshold,
especially given the antecedent saturated grounds after a multi-day
onslaught from Debby. This led to an easy decision to keep
continuity with locally significant wording reserved for any
urbanized areas within the above region.
Finally, out west the pattern remains on track with scattered
thunderstorm development during peak diurnal max with a complex
initiating over the High Plains of Southeast CO and Northeast NM,
shifting east with the mean flow focused along the northern
periphery of the mid-level ridge. Some pretty good signals for at
least 2" given a solid EAS probability between 30-50% over a small
zone between the NM/OK/TX borders. Neighborhood probability was
bullish for the >3" signal running between 60-70% over the
aforementioned area. There's little signal for >5", however so the
threat is likely capped between 3-5" at peak QPF. That's still
fairly prevalent to a localized flash flood concern within that
portion of the Southern High Plains, so the risk is likely within
the higher-end of the SLGT comparatively to what will occur over
NM. Much of the impact will be focused in those burn scar areas
where heightened sensitivity drives much of the risk. This was
sufficient to maintain what was forecast previously with only some
minor adjustments based on the latest HREF mean QPF footprint.
Kleebauer
...Previous Forecast...
...Mid-Atlantic to Northeast...
Debby and it's remnant plume of tropical moisture are forecast to
lift northward today from the central Appalachians to the Interior
Northeast, while producing a swath of heavy rainfall and the
potential for numerous flash floods from near the West Virginia
Panhandle early this morning to Upstate New York this afternoon.
Precipitable water values of 2-2.5" stream along and just east of
the system will equate to above the 95th climatological percentile
and +3.0 standard deviations above the climatological mean.
Additionally, ample shear surrounding the low center and MUCAPE
values up to 1000 J/kg nearby along the Atlantic Seaboard being
advected northward will allow for efficient tropical downpours
within scattered to numerous showers and thunderstorms. A focus in
the heaviest rainfall is likely on the northern and northeastern
quadrant of the low pressure system as it transitions to an
extratropical phase, becomes frontal, and begins to increase
forward speed to the northeast by Friday night. A warm front
lifting northward will add increasing frontogenetic forcing to the
environment and lead to a more widespread precipitation shield
today across parts of PA and Upstate NY, with embedded rainfall
rates likely up to 2-3"/hr at times. The trend for this axis of
heavy rainfall has been for it to occur slightly westward than
previously forecast, now across far northwest NY. Additionally, the
trend is for faster northward progression with this system. 12z
NAEFS IVT valid 18z Fri is forecast to max all values in the
1979-2009 climo period, amplifying the moisture surge fueling
today's heavy rainfall event. 00z HREF neighborhood probs for at
least 3" in 6-hrs are high (60-80%) from central PA to the Finger
Lakes region and northwest portions of NY State, which would also
exceed local FFG for much of this region. As a result, the Moderate
Risk was maintained from the previous forecast with only minor
cosmetic changes. Numerous instances of flash flooding are expected
in this area, with locally significant flash flooding possible.
Elsewhere in the Northeast, convection developing along and ahead
of a cold front should have enough forward motion to lead to only
scattered to localized flash flooding concerns closer to the I-95
corridor, but with rainfall rates of 2"/hr possible. Areas that
received recent rainfall and poor drainage locations could have a
relatively greater flash flood risk, which supports the SLGT Risk
extending to parts of New England and the Mid-Atlantic.
...Eastern Carolinas...
A separate area along the East Coast at risk for flash flooding
today will be associated with a convergence axis along
coastal/eastern sections of North and South Carolina this afternoon
and evening. PWs elevated and above 2.25", along with ample
instability and mean winds parallel to the boundary/coastline will
support slow-moving and/or training thunderstorms across saturated
terrain. CAMs and the associated 00z HREF suite depict the
potential for 5"+ amounts throughout eastern North Carolina and
coastal South Carolina, but the expectation remains that these
totals would be pretty localized. The SLGT Risk was expanded from
the previous forecast across South Carolina given recent CAM output
and very saturated antecedent conditions in place. Will monitor
this region for upgrades in the future outlooks.
...Four Corners, Central Rockies, Central High Plains, and Southern Plains...
Showers and thunderstorms are expected to fire up across the
Southwest and Rockies as shortwave energy passes through and
encounters the ongoing monsoonal moisture in place. There will be
increased focus for more organization and higher amounts
concentrating near the higher terrain of southern Utah, south/central
Colorado, and northern New Mexico, where a SLGT Risk area remains
in effect. Accumulations of 1-2" will be possible in the strongest
cells, a stronger correlation to flash flooding concerns compared
to historical precedence. The rest of the Southwest will still see
periods of storms capable of localized flash flood concerns with
the best threat located more into the Intermountain West and the
Mogollon Rim in Arizona. Convection (possibly organized) will also
spread eastward into western Kansas and the panhandles of Oklahoma
and Texas, as well as northeast NM. Here, 00z HREF neighborhood
probs are modest (20-40%) for at least 3" in 6-hrs. A broad
MRGL Risk covers much of the High Plains.
...Central Texas...
A MRGL Risk was introduced across parts of central Texas as
convection develops along the tail-end of a lingering cold front
and ahead of a stronger frontal boundary dropping south from the
central Plains. Shear will be weak, but steep low-level lapse rates
and sufficient moisture (PWs around 2") will allow for intense
rainfall rates over relatively wet ground conditions and a
historically flash flood prone region. 00z HREF neighborhood
probs have increased and now show elevated chances (30-50%) for
rainfall amounts exceeding 3" this evening.
Snell
Day 2
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, CENTRAL ROCKIES INTO THE PLAINS, AND THE COASTAL CAROLINAS...
...Southwest, Central Rockies, and Plains...
A rather broad MRGL Risk was maintained extending from the
Southwest to the central/southern Plains as diurnal heating and
monsoonal moisture will keep showers and thunderstorms developing
during the afternoon and evening hours for much of the Four Corners
region. Meanwhile, an area of more organized convection triggered
by shortwave energy from the D1 period will be making its way
eastward into the foothills and spreading out into the high
Central/Southern Plains. There is a fair amount of uncertainty with
how far east into the Plains the convection reaches, location of
heaviest amounts and the overall accumulations. Most of the
consensus is suggesting 1-3 inches however a couple models have
maxes near/over 4 inches. A Marginal Risk covers much of the Four
Corners, eastern Wyoming, western Nebraska and Kansas, Oklahoma,
northern Texas, southwest Missouri and northwest Arkansas. If
amounts trend up then a Slight Risk area may be need with further
updates.
...Coastal Carolinas...
A lingering frontal boundary and diurnal sea breeze convection will
continue chances for scattered thunderstorms across the Southeast
and more specifically the eastern Carolinas on Saturday. Soils
should remain saturated after prior heavy rainfall, increasing the
flash flood threat across an area that isn't usually as prone to
flash flooding. CAMs depict localized totals from pulse
thunderstorms could reach 1-3", which is supported by PWs over 2".
From Mike Powell@618:250/1 to All on Sat Aug 10 09:51:00 2024
FOUS30 KWBC 100746
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
346 AM EDT Sat Aug 10 2024
Day 1
Valid 12Z Sat Aug 10 2024 - 12Z Sun Aug 11 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CAROLINAS INTO SOUTHEASTERN VIRGINIA...
...Carolinas & Virginia...
A stalled out front over the western Carolinas and central Virginia
will act as a focus for storms to develop this afternoon on the
warm/moist southeastern side of the front. The atmosphere will be
characterized with plentiful deep moisture with PWATs over 2 inches
across much of the region. HRRR guidance shows several clusters of
storms forming across southeast Virginia and the central Carolinas
late this afternoon. The storms will then track southeastward
towards the coast into an area where instability could exceed 4,000
J/kg along the coast. This level of instability and moisture will
support potential for storms to produce rainfall rates to 3 inches
per hour. Fortunately the guidance suggests the storms will keep
moving into the evening, but those heavy rainfall rates would have
the potential to cause flash flooding even if antecedent conditions
weren't so favorable.
Debby's remnants tracking across this region over the last few days
has led to very saturated soils all across Virginia and the
Carolinas. With numerous streams, creeks, and rivers already well
into flood stage, additional rainfall from storms Saturday
afternoon will struggle to drain, resulting in additional flooding
and prolonging ongoing flooding. In coordination with all of the
impacted offices, a Slight Risk area was introduced with this
update.
00Z HREF guidance peaked at over 50% chance of exceeding 6 hour
FFGs in the Slight Risk area, with an over 70% chance of exceeding
3 inches of rain across much of eastern North Carolina. These
values added to the confidence to upgrade the ERO risk.
...Rockies into the Plains...
An approaching positively tilted shortwave trough moving into
California will increase lift to its east across the Four Corners
region. Storms are likely to be widely scattered across this area,
which would support an isolated flash flooding instance or 2,
currently covered with the large Marginal Risk. Additional moisture
in southern Arizona and lift into the Peninsula Ranges of southern
California may cause local flash flooding issues, but the guidance
is inconclusive at best as to where any potential Slight risk area
could be drawn. In coordination with the impacted offices, have
opted to hold at a Marginal for now, but will reevaluate each day's
chances for flooding based on the previous day's convective
behavior.
MCS development with potential repeating storms in the Plains will
be a concern across Kansas through Oklahoma late Saturday night.
Antecedent conditions are quite dry for soil moisture across this
area, and the CAMs are not in great agreement on the location and
behavior of the storms, despite ample moisture available. Here too
a Slight Risk is close, and may need to be considered with future
updates.
Wegman
Day 2
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE CAROLINAS
AND FOR PORTIONS OF THE OZARKS AND CENTRAL PLAINS...
...Carolinas...
The stalled out front over the Carolinas will continue to draw deep
tropical moisture northward up the Southeast coast. The front will
have made enough progress to likely keep the rainfall down into
southeastern Virginia, but there will likely be more storms into
South Carolina. Thus, the Slight risk is just a bit south of the
Day 1/Saturday Slight Risk. Nonetheless with ongoing flooding from
Debby, additional rainfall, even if more widely scattered, will
have an outsized potential for impacts from additional flooding.
00Z HREF guidance through 00Z Monday suggests additional elevated
potential for exceeding FFGs in NC, and plentiful moisture and
instability with a stalled out front providing the forcing will
likely help organize the storms, increasing their flooding
potential.
...Central Plains/Ozarks...
MCS development on the leading edge of a low level jet (LLJ) is
expected from southeast Kansas through northwest Arkansas. Despite
recent dry weather, training convection with PWATs increasing to
around 2 inches will support storms capable of rates to 3 inches
per hour at times. This very heavy rainfall combined with terrain
interactions in the Ozarks will favor widely scattered instances of
flash flooding. In coordination with the impacted offices, a
Slight Risk upgrade was introduced with this update. Given CAMs
frequent trouble with correctly simulating storms with MCSs, there
are likely to be both temporal and intensity adjustments with the
expected rainfall. Despite this potential, there was good enough
agreement in the guidance to draw in the Slight risk, which for now
remains low-end given the recent dry weather and dry soils.
...Southwest...
More widespread convection is likely to impact the Four Corners
region Sunday afternoon. The shortwave that will start the day in
northern California will continue eastward across the Intermountain
West. Thus, it appears the best potential for storms capable of
flash flooding will be across central UT and western CO, generally
north of the UT National Parks. Nonetheless with better moisture
further south, AZ and northwestern NM will not be out of the woods,
and isolated flash flooding will remain possible there. Storms are
also likely along the Peninsular Ranges of southern California
again Sunday afternoon, where a small Marginal risk was introduced.
Wegman
Day 3
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION, CENTRAL PLAINS, OZARKS, AND THE CAROLINAS...
Ongoing rainfall from Sunday night may continue into Monday morning
across southern Missouri. With only a few hours of rain in the
morning, the Marginal across Missouri was maintained. However,
should rainfall from Day 2/Sunday overperform, then a future Slight
may be needed. Once again another round of storms Monday afternoon
across the Four Corners region is likely to result in isolated
flash flooding. However, there is greater uncertainty with
coverage by Monday in this region with lesser forcing than on
Sunday.
For the Carolinas, lingering moisture with the same front may cause
yet another round of storms Monday afternoon into the evening. With
greater potential that the bulk of the activity will be offshore by
Monday afternoon, the previously inherited Marginal risk remains
largely unchanged.
From Mike Powell@618:250/1 to All on Sun Aug 11 07:40:00 2024
FOUS30 KWBC 110815
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun Aug 11 2024
Day 1
Valid 12Z Sun Aug 11 2024 - 12Z Mon Aug 12 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR CENTRAL UTAH
THROUGH NORTHWESTERN ARIZONA, PORTIONS OF THE OZARKS AND CENTRAL
PLAINS, AND FOR THE COASTAL CAROLINAS...
...Central Utah through Northwestern Arizona...
Monsoonal moisture will continue to draw northward across the Four
Corners region today. The jet stream and an upper level jet streak
will be set up across northern Utah today. PWATs increasing above
0.75 inches will boost portions of southern Utah to 2.5 sigma above
normal. With moisture and forcing in place, expect a renewed round
of afternoon convection across this area. With previous days'
storms having soaked the soils in some areas, the flash flooding
threat will be higher today as compared to previous days across
Utah and portions of northwestern Arizona. The Slight risk area was
expanded southward with this update to include much of the Grand
Canyon and points north, including Glen Canyon.
...Portions of the Ozarks and Central Plains...
Convection currently ongoing across central Oklahoma will continue southeastward into southeast Oklahoma and Missouri this morning.
This MCS will likely dissipate as often happens late this morning
with daytime heating, but the primary event is expected tonight,
and perhaps in the form of 2 separate areas. Gulf moisture racing
north with the LLJ will interact both with an upper level
shortwave, a stalled front over the area, and countering flow from
an area of high pressure over the Midwest helping hold the surface
front in place despite the deep moisture moving in from the
southwest on the other side of the front. Training convection is
likely to develop, especially over Oklahoma where the Slight risk
area remains in place with few changes. 00Z HREF neighborhood probabilities remain over 50% for 3 inches of rain in the 24 hrs ending at 12Z,
but much of that rain is likely during the overnight hours.
...Coastal Carolinas...
Guidance has been gradually shifting south and east/closer to the
coast with the axis of the heaviest rain with the afternoon
convection expected tonight. Thus, the Slight and surrounding
Marginal risk areas have been shrunk to the southeast accordingly.
Nonetheless antecedent conditions along the coast remain very
favorable in the recovery post-Debby. Expect similar coverage of
flash flood warnings as today's storms, but are likely to occur
further east. Given the uncertainty as to where the storms will
form, the Marginal risk still extends into central NC and western
SC for the potential for more widely scattered storms, as well as
across eastern Georgia.
Wegman
Day 2
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE FOUR
CORNERS REGION EAST TO THE MID-MISSISSIPPI VALLEY AS WELL AS FOR
PORTIONS OF THE COASTAL CAROLINAS...
...Four Corners Region to the Mid-Mississippi Valley...
Concern continues to increase with potential heavy rain and
subsequent flash flooding in various areas. For the Four Corners,
no major changes are expected, but with the greatest moisture
having shifted east, convective coverage should be lesser on Monday
as compared to today. Meanwhile, the signal for heavy rain remains
strong from eastern Colorado into western Kansas. Easterly flow
north of an ejecting low will draw plentiful Gulf moisture into the
High Plains of Colorado. Meanwhile, a stalled front to the north
will supply lower level forcing. The result will be an area of
heavy rain caused by training storms moving east along the front,
as moisture flowing northwestward on the warm side of the front
supports backbuilding. There remains some uncertainty both with the
placement of the heaviest rains, and for Kansas, the development of
drought conditions potentially resulting in less flooding. However,
it appears likely a Slight will be needed as plentiful moisture
will support storms capable of rates to 2 inches per hour, which in
areas of training storms should overcome otherwise high FFGs.
Further east into Missouri, lingering storms from tonight will
continue into Monday morning. Then, the stalled out front over the
area will begin to reorient, but pivoting over Missouri. This
should reduce the overall coverage of rainfall over Missouri for
the day, but will keep the area in the crosshairs for heavy
rainfall going forward.
...Coastal Carolinas...
Some lingering storms are possible along the coast Monday afternoon as
the front over the area dissipates. With much reduced storm
coverage, the Marginal remains over the area for the potential for
isolated instances of flash flooding. This would likely be the
result of sea breeze interactions.
Wegman
Day 3
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE FOUR
CORNERS REGION TO THE MID-MISSISSIPPI VALLEY, AS WELL AS FOR
EASTERN NORTH CAROLINA...
...Missouri...
A renewed push of Gulf moisture with the LLJ will impact a
persistent front across Missouri today. This will likely result in
an area of training thunderstorms capable of localized flash
flooding. The area is likely to be upgraded to a Slight with future
updates and higher confidence, but given the counter flow out of
the northeast on the dry side of the front over the Midwest, it's
likely that training storms are likely to impact a narrow corridor
over Missouri along the front. There is potential that the St.
Louis metro may be part of the heavy rain corridor.
...Eastern North Carolina...
A bit stronger moisture push off the Atlantic may result in more
widespread showers and storms a bit further inland into North
Carolina. With little to organize the convection however, the flash
flooding threat remains isolated.
From Mike Powell@618:250/1 to All on Mon Aug 12 10:01:00 2024
FOUS30 KWBC 120833
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 12 2024
Day 1
Valid 12Z Mon Aug 12 2024 - 12Z Tue Aug 13 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN COLORADO AND WESTERN KANSAS, EASTERN OKLAHOMA AND FAR
WESTERN ARKANSAS, AND THE COASTAL CAROLINAS...
...Northeast OK and Western AR...
Training convection is expected to be ongoing across this corridor
at 12z Monday. As of 07z the coverage and intensity of convection
over northeast OK is generally greater than most of the 00z high
res guidance. With the low level jet only intensifying, expect this
trend to continue, and thus would expect the convective training
threat into Monday morning to be more significant than the 00z high
res guidance would suggest. At least scattered flash flooding is
anticipated across portions of eastern OK, possibly pushing into
portions of far western AR as well. This convection should be on a
downward trend by late morning into the early afternoon hours.
...Eastern CO into KS...
Convection is expected to develop over portions of eastern CO by
later this afternoon. Models indicate that a weak shortwave will
eject eastward originating from the ongoing convective activity
over UT...with an upper jet to the north supporting at least weak
divergence aloft in its right entrance region. Meanwhile, easterly
flow in the low levels will advect in higher moisture, with PWs
forecast to increase towards 1.5", above the climatological 90th
percentile for mid August. Given the forcing and moisture in
place, the forecast ~2000 j/kg will allow for robust convective
development. With deep layer mean flow from the west, and low
level flow from the east, cells may initially end up slow moving,
with cell mergers a possibility. Given the environment in place,
tend to think some of more aggressive high res models may end up
closer to reality for this event. This would support localized
rainfall upwards of 3-5" over portions of eastern CO and far
western KS. HREF QPF, environmental ingredients in place, and the
CSU machine learning ERO are all supportive of an isolated to
scattered flash flood threat. Thus we will go ahead and upgrade
this region to a Slight risk.
This activity is likely to grow upscale overnight and then move
eastward across KS. Convection will likely be quicker moving by
this time, although can not rule out a west to east training axis
setting up. If this were to occur then Slight risk level impacts
may continue overnight across KS. However given some lingering
uncertainty on these convective details, and the fact that soil
saturation and streamflow conditions across KS are running below
average...will keep the risk level at Marginal for now.
...Carolinas...
Upgraded portions of the eastern Carolinas to a Slight risk for
today. We should see another round of convection near the stalled
front this afternoon, with HREF 3" neighborhood probabilities in
the 40-70% range. Totals of this magnitude will likely stay
isolated to scattered in nature...however given the increased
sensitivity over the area after Debby and post Debby
rainfall...expect another day of isolated to scattered flash
flooding. Today is a bit trickier of a forecast with a weak low
along the front. While the higher probabilities of excessive
rainfall are along the coast this afternoon, we could see at least
a localized flash flood risk a bit further inland this morning
into the early afternoon as well.
...Southwest...
A broad Marginal risk remains across much of the Southwest, with
isolated flash flooding possible over this large geographic region.
The coverage/organization of intense rainfall may be a bit less
than what we saw on Sunday, but nonetheless an isolated risk
likely continues. Do see some more elevated probabilities over
southern AZ, so may end up with a bit better concentration of
convection there, but still seems to fall shy of Slight risk
levels. Portions of central/northern UT also stand out as
potentially seeing greater convective coverage...but this signal is
generally north of the more sensitive basins in UT...and thus not
thinking a Slight risk is needed at this time.
Chenard
Day 2
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS INTO MISSOURI AND NORTHERN ARKANSAS...
...Portions of KS/MO/AR...
Likely to have some ongoing convection at 12z Tuesday across
portions of southeast KS and southwest MO. This will be what
remains of the Monday night organized convection that is expected
to move across KS. Most indications are that this activity should
be on a weakening trend after 12z, so while an isolated flash flood
threat may persist, not currently anticipating a widespread
threat. The greater threat is the likelihood of training
convection Tuesday night across this corridor.
This appears to be evolving into a potentially significant training
convective setup. Robust instability will be advecting in from the
southwest, likely resulting in a tight instability gradient
somewhere in the vicinity of eastern KS into western to central MO.
Meanwhile the low level jet will increase, resulting in strong and
persistent moisture transport into the region. Corfidi vectors end
up becoming quite weak, and what movement they do indicate is
generally parallel to the expected moisture convergence axis and
instability gradient. Thus we very well may end up with repetitive
backbuilding upscale convective growth over this area Tuesday
night. There remains some uncertainty on the exact details of
convective evolution and placement of highest totals...but
confidence is increasing in an excessive rainfall threat, with
potentially significant localized impacts. The coverage of excessive
rainfall will likely be quite a bit narrower than the Slight risk area...however it remains broader to account for uncertainty in the
maximum rainfall axis.
...Southwest...
Scattered convection is again likely from portions of AZ/NM
northward into ID/WY. There will be a bit more in the way of
westerly flow compared to earlier days, which may allow cells to
move along at a faster pace. This should keep the flash flood risk
isolated in nature...however heavy rainfall rates will again be
probable in the more intense cells...which could result in a
localized flash flood within the broad Marginal risk area.
...Southeast...
Convection is again likely Tuesday across portions of GA/SC/NC.
Overall it is an interesting setup, with model guidance suggesting
a more well defined low over SC by this time, with stronger
forcing and flow aloft moving in as well. This should seemingly
allow for some strong convective development...with the stronger
flow aloft supporting quicker cell motions off to the southeast.
These cell motions would typically suggest a lowered flash flood
risk...however we will need to keep an eye on how the convergence
axis east of the low plays out. This boundary could act as a focus
for some training convection, which would offset the quicker cell
motions and still produce a flash flood risk. Overall think enough
uncertainty on these details remains to keep the risk level at
Marginal for now. However will continue to monitor trends and
would not rule out an eventual Slight risk upgrade over portions of
the area.
Chenard
Day 3
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE MISSISSIPPI VALLEY...
The threat of excessive rainfall shifts into Plains and MS Valley
on Wednesday. Likely to have some ongoing risk to start the day
across portions of central MO...with the threat likely shifting to
the north by late in the day into the overnight hours. It again
looks like a favorable environment for training/backbuilding
convection with an increasing low level jet into a warm front.
Strong and persistent moisture transport into this boundary is
typically supportive of backbuilding convection and a flash flood
risk.
There is some model QPF spread, with convection and locally heavy
rainfall likely all the way from the Dakotas and MN into MO.
However it looks like the best ingredients for excessive rainfall
are over MO to start the day, and then across portions of IA and
western IL later in the day/overnight. This is where the better
instability and most persistent moisture transport is currently
forecast. And while the location of greatest QPF differs from model
to model and run to run...this axis is favored by the majority of
solutions. Thus will place the Slight risk here for now, with the
understanding that some adjustments are likely as the event nears.
So while confidence on the exact location is only average...do
think we are getting towards above average confidence in the
impacts from this event...with at least isolated to scattered flash
flooding expected, some of which could be locally significant.
From Mike Powell@618:250/1 to All on Tue Aug 13 09:19:00 2024
FOUS30 KWBC 130825
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 AM EDT Tue Aug 13 2024
Day 1
Valid 12Z Tue Aug 13 2024 - 12Z Wed Aug 14 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EASTERN KANSAS INTO MISSOURI, NORTHEAST COLORADO, AND SOUTH CAROLINA...
...Portions of KS/MO/AR...
As of 07z organized convection is moving eastward across KS. The
00z HREF guidance has a decent handle on this activity and
indicates it should gradually take on more of a southeastward
motion and be impacting portions of southeast KS at 12z this
morning, moving into southwest MO through the morning hours. At
least some flash flood risk will probably continue into the
morning hours with some training potential persisting. Although
the overall expected progressive nature of convection by this time
should limit the magnitude of the threat, and convection should
see a weakening trend by afternoon as it moves into northern AR.
The greater flash flood risk is likely to evolve tonight into
Wednesday morning across portions of eastern KS into MO. The
ingredients remain favorable for a potentially significant
training convective axis. Robust instability will be advecting in
from the southwest, likely resulting in a tight instability
gradient somewhere in the vicinity of eastern KS into western to
central MO. Meanwhile the low level jet will increase, resulting in
strong and persistent moisture transport and convergence over the
region. Corfidi vectors end up becoming quite weak, and what
movement they do indicate is generally parallel to the expected
moisture convergence axis and instability gradient. Thus we very
well may end up with backbuilding upscale convective growth over
this area by Tuesday night into Wednesday morning.
Confidence in there being flash flood impacts continues to grow,
although the axis of heaviest rainfall will likely be quite
narrow...thus there remains uncertainty on where exactly this ends
up. HREF data supports a narrow axis of 3-5"+ rainfall, and given
the setup this seems reasonable. This is likely a higher end
Slight risk, with a narrow corridor of locally significant impacts
possible. If confidence in the location increases later today, can
not rule out the need for a focused MDT risk upgrade.
...Southeast...
Convection is again likely today across portions of GA/SC/NC. At
18z today the model consensus is for an area of low pressure to be
positioned near the GA/SC border. HREF guidance indicates a
relatively narrow axis of higher instability within the easterly
flow just north the stationary front extending from the low. Weak
mid level shortwave energy and stronger northwesterly flow aloft
moving over top this axis of convergence should aid in convective
development today. Quicker deep layer flow should mean faster cell
motions off to the southeast...however the stationary front could
act as a focus for some training. And easterly low level flow
countering the deeper layer westerly flow may also support some
cell merger activity. Absent wet antecedent conditions this is
probably more of a Marginal risk level threat. However with soil
saturation and streamflows continuing to run well above average,
and HREF 3" neighborhood probabilities of 40-70% and 5"
probabilities of 15-30%, think that scattered flash flooding could
evolve today. Thus will go ahead and upgrade to a Slight risk.
...Northeast CO..
Opted to introduce a small Slight risk area across portions of
northeast CO and adjacent areas of far southeast WY and southwest
NE. Frontal forecasts this afternoon/evening are for a stationary
front to be draped across this area potentially acting as a focus
for convective development. Upslope easterly flow north of this
front should aid in locally enhancing lift/convergence, and in
many ways this setup is similar to Monday. With several flash
flood warnings Monday, and some of these same areas expected to
see this round of storms, think isolated to scattered flash
flooding is again probable. Easterly flow may be a bit weaker
today, and guidance does suggest quicker forward propagation off to
the east than Monday...so it is possible that rainfall magnitudes
stay a bit lower. However the setup appears favorable enough for
some slower cell motions and mergers near the front before the
faster propagation takes hold...that think Slight risk level
impacts are possible.
..Western U.S...
Scattered convection is again likely from portions of AZ/NM
northward into ID/WY/MT. There will be a bit more in the way of
westerly flow compared to earlier days, which may allow cells to
move along at a faster pace. This should keep the flash flood risk
isolated in nature...however heavy rainfall rates will again be
probable in the more intense cells...which could result in a
localized flash flood within the broad Marginal risk area.
Day 2
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MIDDLE AND UPPER MISSISSIPPI VALLEY...
Training convection will likely be ongoing at 12z Wednesday across
portions of MO. See the day 1 discussion for more details on this
expected evolution...but in summary the ingredients will be there
for a significant training threat to continue into Wednesday
morning. Just like on day 1, there remains some uncertainty on the
exact axis of maximum training, but this is likely a higher end
Slight risk at the moment, with some flash flooding likely
wherever the axis sets up. By late morning into the afternoon the
convection should become more progressive and/or weaken as it
shifts southeastward.
Thereafter the risk of excessive rainfall shifts northward into
portions of IA and southern MN. Convection here will likely
develop by afternoon and continue into the overnight hours, with
the focus expected to be near and just north of a warm front
slowly lifting north across the region. Stronger mid/upper forcing
will move into the region, which combined with strengthening low
level moisture transport, should allow for organized convective
development. With the front not moving much and the moisture
transport axis rather persistent, expect we will see some
backbuilding/training convection near the front. Where exactly this
sets up remains a bit of a question. The 00z GFS/NAM/UKMET have
trended northward into more of MN, however the GEM reg, AIFS and
GFS Graphcast all still support IA into far southern MN as the
preferred location. With models often too far north with convective
QPF, hesitant to expand the Slight risk too far north. Thus still
think keeping it centered over IA and only into far southern MN is
the way to go for now...generally favoring the the further south
model solutions mentioned above.
A secondary rainfall maximum may occur over portions of ND into far
northern SD along another slow moving convergence axis. The 00z
HREF data is supportive of Slight risk level impacts across this
region. However HREF skill at these longer lead times is sometimes questionable, and global model solutions show a bit more spread on
the convective evolution over this area. Given this uncertainty
and the fact that this area is typically not as flash flood prone,
decided to keep the risk level at Marginal for now and continue to
monitor.
Day 3
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
ILLINOIS, INDIANA AND SOUTHERN WISCONSIN...
The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by
this time, but the overall ingredients remain in place for areas
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but with broad troughing
moving through, this will likely not be the end of the convective
risk. Instead we should be able to reload with instability and get
additional round(s) of convection into Thursday night. Exactly
where the best risk of multiple heavy convective rounds exists is
still uncertain, but the most likely axis appears to be across
portions of IL/IN. This is a bit south and east of the inherited
Slight risk, and so the risk was adjusted accordingly. There is
still some excessive rainfall threat further north over portions of
WI and MI, but at the moment think the greater training risk is
further south and east where the favorable ingredients should
linger longer.
From Mike Powell@618:250/1 to All on Wed Aug 14 08:51:00 2024
FOUS30 KWBC 140830
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
430 AM EDT Wed Aug 14 2024
Day 1
Valid 12Z Wed Aug 14 2024 - 12Z Thu Aug 15 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTH DAKOTA AND MIDDLE MISSISSIPPI VALLEY...
A bit greater uncertainty than normal with how convection evolves
this morning across portions of IA/MO/AR, although things are
becoming a bit clearer as of 08z. The convection moving across NE
(as of 08z) will likely be over portions of western IA and
northwest MO by 12z this morning. It will likely align itself more
northwest to southeast by this time, allowing for a greater
training potential. However the longevity of this training is a bit
uncertain as the low level jet gradually weakens. Suspect that an
isolated to scattered flash flood risk will exist, but not
confident in anything more significant than that at this time.
Another axis of training convection is likely this morning across
portions of southern MO into northeast AR. As of 08z starting to
see the beginning of this scenario starting to evolve, and expect
to see an uptick in organization and training between now and 12z.
NAM Nest runs have indicated localized 5-10" rainfall with this
event, and while not impossible, it does seem probable that
amounts will stay below that level. However still thinking several
inches of rainfall is likely, locally higher, and thus do expect a
corridor of isolated to scattered flash flood risk to develop.
This activity will likely continue into the morning hours before
dissipating by later in the morning
By this afternoon into evening expect convection to develop across
far southeast NE into western IA and northwest MO, with the focus
expected to be near a warm front slowly lifting north across the
region. Stronger mid/upper forcing will move into the region, which
combined with strengthening low level moisture transport, should
allow for organized convective development. Suspect that the
convection this morning across NE/IA may help keep the warm
frontal position a bit further south today. Given the heavier
rainfall rates should generally be near and just south of the warm
front, we were able to trim some of the northern areas (MN and
northern IA) out of the Slight risk with this update...and this is
generally supported by 00z HREF QPF probabilities and FFG
exceedance probabilities. Area of heavy rainfall are still likely
over northern IA into MN, but rates should stay low enough to keep
any flash flood risk more localized in nature. Even over northern
MO into central IA this late day and overnight convection will
probably be rather quick moving, limiting rainfall total
potential. However we could see some brief training near the warm
front, and some of these areas will have also seen heavy rain from
the morning round of convection. Thus do think at least some flash
flood risk will exist with this later convection as well.
Overall the broad Slight risk stretches from central IA to
northeast AR, and covers the risk from both the convection this
morning and also later today/tonight. The southern part of this
Slight risk is primarily for this mornings activity, with the
northern portion of the risk potentially getting both rounds of
storms.
A secondary rainfall maximum should occur over portions of ND this
morning into early afternoon along another slow moving convergence
axis. Hard to ignore the impressive 00z HREF signal over this
area, with 3" neighborhood probabilities in the 50-80" range, and
6hr FFG exceedance probabilities over 40%. There was a localized
training cell that resulted in a flash flood warning last night
over ND, and the pattern does appear favorable for that potential
this morning as well, except probably a bit more convective
coverage today. Thus think adding a Slight risk is the way to go,
with isolated to scattered flash flooding a possibility.
Chenard
Day 2
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
MISSOURI, ILLINOIS, INDIANA AND KENTUCKY...
The heavy rainfall threat shifts east into the Great Lakes and OH
Valley by Thursday. A bit more uncertainty with the details by
this time, but the overall ingredients remain in place for areas
of excessive rainfall. Expect one round of convection to be moving
across the region Thursday morning, but currently anticipating
this activity to be on a downward trend by this time and thus
should not be posing much of a flash flood risk. However with
deeper troughing still upstream at this point, would expect to see
additional convective development by the afternoon hours. This
convection is likely from portions of eastern MN and WI southward
into portions of MO/IL/IN/KY. Convection across the northern half
of this area will probably stay pretty quick moving off to the
east, likely limiting the extent of the flash flood risk, keeping
the threat more at Marginal risk levels.
The greater instability and moisture axis will likely end up
further south from southeast MO into portions of IL/IN and KY. Even
here cell motions should be rather quick, however with this area
further displaced from the forcing to the north and mean flow more
parallel to the front, do expect that we could at least see some
brief training of convection. Given the favorable thermodynamic
environment for heavy rainfall rates, any training would likely
result in at least some flash flood potential. This appears most
likely along this corridor, and thus will carry a Slight risk here.
Chenard
Day 3
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST INTO THE GREAT LAKES...
On Friday the likelihood of convection shifts eastward stretching
from portions of the Southeast up into the Great Lakes region. The
probability of flash flooding by this time appears to decrease,
but at least some risk likely persists. Stronger deep layer flow
supports quicker cell motions and not really seeing much of a low
level focus for training. However, the troughing moving overhead is
quite broad in nature, likely meaning the duration of lift will be
extended, potentially supporting a few rounds of convection. Also
deep layer flow, while quick, is fairly unidirectional, which can
support some brief backbuilding of convection at times. Thus while
the overall setup for flash flooding and model QPF output are not
all that impressive, there are enough favorable ingredients in
place to suggest at least an isolated flash flood risk will exist.
This warrants the continuation of the Marginal risk.
From Mike Powell@618:250/1 to All on Thu Aug 15 08:28:00 2024
FOUS30 KWBC 150800
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Thu Aug 15 2024
Day 1
Valid 12Z Thu Aug 15 2024 - 12Z Fri Aug 16 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID MISSISSIPPI VALLEY AND LOWER OHIO VALLEY...
...Mid MS Valley to Lower Ohio Valley...
The active weather associated with the large troughing over the
Northern Plains and UPper Midwest this morning will move eastward
today. At the start of the period, convection should be ongoing
across portions of Missouri into Illinois and portions of Iowa,
tied with the overnight convective line segments. This activity
should be both relatively progressive and also weakening with the
loss of the nocturnal low level jet and less favorable influx of
moisture. However, a few pockets of heavier rainfall and rain rates
will be possible that could total a couple/few inches through mid
afternoon.
Further south/southwest, outflow boundaries and the approaching
cold front from the west/northwest will help spark an additional
round of showers and thunderstorms late this afternoon into the
late evening/overnight hours, most likely centered over portions of southern/central Missouri to southern Illinois, far southwest
Indiana, and portions of western Kentucky. Here, the setup is much
more favorable for 1) robust deep convection, 2)
training/backbuilding storms with the flow becoming more parallel
to the storm motions, and 3) an impressive evening/night low level
jet of 30-35 kts impinging on the boundary in place.
THe 00Z HREF neighborhood probabilities indicate potential for
some intense rain rates with the storms, where the 2" hourly total
probs reach 30 percent for several hours this evening/overnight
with a slight signal for 3" hourly totals at times. Overall,
isolated totals of 3-5" are possible with some localized higher
amounts not out of the question based on the 00Z hi-res guidance
and the HREF showing a few areas of 25 percent for 5" over the 24
hour period and a near 15 percent signal for 8" totals.
For the Day 1 Excessive Rainfall Outlook, still messaging a Slight
Risk for this area for the potential of isolated to scattered
instances of flash flooding, especially if the heaviest amounts
fall over portions of southern Missouri where soil moisture is
already elevated due to recent heavy rainfall events.
Taylor
Day 2
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE GREAT LAKES TO LOWER OHIO VALLEY...
...Great Lakes to Ohio Valley...
By Friday, the cold front and associated upper level troughing will
shift eastward toward the Great Lakes and Ohio Valley. Expect
another day of scattered to numerous showers and thunderstorms as a
result of the higher moisture ahead of the front along with
favorable instability developing during the peak of the daytime
heating. The set up does favor faster/progressive storm motions but
a few repeating rounds or backbuilding storms will be possible so a
few instances of flash flooding can't be ruled out so the Marginal
Risk remains in place for the area.
...Mid MS Valley...
Late in the forecast period, the latest guidance is keying on a
subtle shortwave trough embedded in the northwest flow between the
upper ridge over Texas and the large troughing over the Great
Lakes. With the nocturnal low level jet reaching 25 to 30 kts
impinging on the surface boundary expected to be draped across
portions of Kansas and Missouri, there is increased likelihood of
convection developing early Saturday morning. While confidence in
exact placement is a bit higher than normal, the setup and
environmental ingredients suggest potential for some localized
heavy rainfall and this is showing up in the various deterministic
and ensemble probabilities this cycle. Isolated/localized flash
flooding will be possible.
Taylor
Day 3
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S. AND THE EASTERN U.S...
...Southwest U.S...
The return of deeper monsoonal moisture is expected across the
region, initially late Friday night across portions of southern
Arizona, but by Saturday a plume of higher moisture surges
north/northeast through the region. This is in response to the
anomalous upper ridge axis becoming centered over eastern New
Mexico and western Texas while a strong upper trough approaches the
coast of California. This will set up a favorable flow of this
higher moisture in between across much of Four Corners region
during the period. PW anomalies reach +2 to +2.5 standard
deviations above normal during the period while the latest model
guidance is suggesting a subtle shortwave trough rippling through
the flow, which will act to enhance the forcing for ascent across
the region. Combined this with the expected instability during
peak heating and the set up will become quite conducive for
scattered daytime to evening showers and thunderstorms. The latest
guidance is favoring northern Arizona and much of Utah for the
greatest QPF and potential for higher rain rates, but the threat
will exist across the larger region. Isolated to widely scattered
instances of flash flooding will be possible, particularly for the
more vulnerable locations and a Marginal Risk covers the region.
...Mid Atlantic to Northeast...
A pronounced upper trough is forecast to move from the Great Lakes
region at the start of the period to the Upper Ohio Valley and
Central Appalachians by 12Z on Sunday. At the surface, a cold front
is expected to pass through with a ribbon of higher moisture
surging ahead of it. The combination of the favorable forcing for
ascent and higher moisture, along with the daytime heating
instability, should be sufficient to bring scattered to numerous
showers and thunderstorms to the region. While the setup should
favor relatively progressive storm motions, some areas could see a
few repeating rounds or backbuilding storms on the
southern/southwest flank of any line segments that may bring
locally higher rainfall totals. Based on the latest deterministic
and ensemble guidance plus ML first guess fields, a broad Marginal
Risk is advertised for the region for localized/isolated flash
flooding.
...Southeast...
The advancing cold front will become more west to east oriented
across the Southeast during the period. The moisture profiles in
the region will remain elevated (around 1.5 std deviations above
normal) and with robust daytime heating and instability, strong
thunderstorms are forecast to develop along/ahead of the frontal
boundary then quickly move to the east/southeast. A
localized/isolated flash flood threat exists and the Marginal Risk
was extended southward to account for pockets of heavy rainfall
totaling a few inches in places over a relatively short period of
time.
Taylor
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From Mike Powell@618:250/1 to All on Fri Aug 16 08:45:00 2024
FOUS30 KWBC 161235
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 AM EDT Fri Aug 16 2024
Day 1
Valid 12Z Fri Aug 16 2024 - 12Z Sat Aug 17 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
SOUTHEAST U.S...
...Southeast U.S...
Convection across portions of the lower Ohio Valley early this
morning is forecast to reach portions of southern/eastern Kentucky
and Tennessee later today and eventually into northern
Alabama/Georgia. There is lower confidence on how the activity will
hold together or what additional line segments will develop but
with plentiful moisture and sufficient instability in place this
afternoon, several areas of heavier rainfall will be possible.
Based on the latest guidance and 00Z HREF probabilities, pockets
of 1-2" hourly totals will be possible, most likely across portions
of middle/eastern TN, far southeast KY, and into northern
AL/northwest GA where the Slight Risk in the Excessive Rainfall
Outlook exists. Some localized totals of 2-4" will be possible
within that area and could lead to scattered instances of flash
flooding.
...Ohio Valley to Great Lakes...
For today and tonight, a cold front is expected to move eastward
from the Midwest through the Ohio Valley as the main upper level
shortwave energy digs further into the Great Lakes region. This
combination should provide plenty of support for scattered to
numerous showers and thunderstorms to develop along the advancing
cold front across the Ohio Valley into the Central Appalachians.
The activity should be fairly progressive and quick moving,
limiting the overall duration of any pockets of heavy rainfall at
any one location, but a few spots of merging storm cells or
training could develop that pose a flash flood risk. Further north
into the Great Lakes (Wisconsin/Michigan), storm motions under the
core of the upper trough are likely to be slower and when combined
with the higher PWs and somewhat elevated soil moisture percentiles (Wisconsin), a few instances of flash flooding will be possible.
...Southwest U.S...
Increasing monsoonal moisture will begin to lift northward into
portions of the region today, the highest PW anomalies will likely
be tied to far southern Arizona but the 00Z guidance is keying on
enough moisture and sufficient instability developing today
combined with the subtle shortwave lifting through to spark at
least isolated thunderstorms into portions of southern NV and
southern/central New Mexico. Some of the convection will be
capable of producing intense rain rates and could lead to flash
flooding over the typical vulnerable locations.
...South Florida...
A trailing frontal boundary and trough over the region combined
with a very high moisture axis will bring another round of
scattered to numerous thunderstorms to South Florida later today
through the evening hours. The combination of high PWs and
sufficient instability will lead to very intense and efficient rain
producing thunderstorms, capable of producing 2-3" hourly totals at
times. The 00Z guidance is quite bullish for southeast FLorida
coastal areas with potential for some locations to see several
inches today (00Z HREF probs are moderate for 5" and show a slight
signal for 8"). The most concerning area is for the highly
urbanized corridor of southeast Florida but also the southwest
coast from near Naples/Fort Myers southward could also see some of
the excessive rainfall from thunderstorms. A Marginal Risk was
introduced this cycle.
Taylor
Day 2
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN U.S...SOUTHWEST U.S...SOUTH FLORIDA...AND PACIFIC NORTHWEST...
...Eastern U.S...
The anomalous upper trough and associated cold front will make its
slow progression eastward into the Eastern U.S. Saturday into
Saturday night. Another day of mostly loosely organized convective
segments are expected in the modest axis of higher moisture and
available instability. THe latest guidance points toward areas of
the OHio Valley, Central APpalachians, and Mid Atlantic being the
focus for higher rainfall totals which could total a couple inches.
Based on the environmental ingredients, pockets of 1-2" hourly
totals will be possible where any boundary collisions or storm
mergers allow for a slightly longer duration in rainfall at any one
location but the storm motions overall should be fairly
progressive. However given some of the terrain sensitivities and
urban areas in the risk areas, a few instances of flash flooding
will be possible.
Further south, the lingering boundary across portions of the
Southeast will again be the focus for another round of mainly
afternoon/evening thunderstorms. Storm motions may be slightly less
and with more robust moisture/instability, some of the rain rates
may be rather intense but short-lived. This may lead to isolated
instances of flash flooding.
...Southwest U.S...
The return of deeper monsoonal moisture is expected across the
region, initially late tonight across portions of southern
Arizona, but by Saturday a plume of higher moisture surges
north/northeast through the region. This is in response to the
anomalous upper ridge axis becoming centered over eastern New
Mexico and western Texas while a strong upper trough approaches the
coast of California. This will set up a favorable flow of this
higher moisture in between across much of Four Corners region
during the period. PW anomalies reach +2 to +2.5 standard
deviations above normal during the period while the latest model
guidance is suggesting a subtle shortwave trough rippling through
the flow, which will act to enhance the forcing for ascent across
the region. Combined this with the expected instability during peak
heating and the set up will become quite conducive for scattered
daytime to evening showers and thunderstorms. The latest guidance
is favoring northern Arizona and much of Utah for the greatest QPF
and potential for higher rain rates, but the threat will exist
across the larger region. Isolated to widely scattered instances of
flash flooding will be possible, particularly for the more
vulnerable locations and a Marginal Risk covers the region.
...Pacific Northwest...
There is a increasing signal for heavy downpours across portions
of the Washington and Oregon Cascades. During this period there
will be and approaching trough along with a steady influx of PW
values of 1+ inches. Some of the hires guidance project convection
to spread/track northward along the terrain with hourly rain rates
upwards of 0.75 inch/hour. In collaboration with the local forecast
office a Marginal Risk area was raised to highlight the elevated
threat for excessive rainfall and the potential impacts leading to
debris flows on the recent burn scar areas from active wildfires.
...South Florida...
Another round of deep convection capable of producing flash
flooding is expected on Saturday, thanks to the lingering trough
over the region. While the 00Z guidance isn't as robust for Day 2
as it is for Day 1 (and perhaps a bit to the south of the major
urban areas), the repeating rounds / cumulative effect may bring a
few round of flash flooding to the area again on Saturday/Saturday
evening.
Taylor
Day 3
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...
...Mid Atlantic and Northeast...
By Sunday, the timing of the cold front and upper trough is
expected to allow for another round of scattered to numerous
showers and thunderstorms across the Mid Atlantic and portions of
the Northeast. Based on the latest guidance, a focus area of
locally heavy rainfall due to the combination of favorable
dynamics, moisture (PWs near 2 std deviation above normal), and
potential for training/backbuilding is setting up across portions
of urban corridor from near Baltimore through southern New York.
This is where the potential exists for a few rounds of heavier
rainfall and given the anomalous moisture in place, pockets of
intense rain rates will be possible over the more urban areas. Some
areas may pick up a quick couple of inches. With this in mind, a
Slight Risk was introduced for portions of the area where
scattered instances of flash flooding will be possible.
...Southwest and Intermountain West...
A continuation of the deep monsoonal moisture and daytime to
evening showers and thunderstorms are expected across much of the
Southwest U.S. and into portions of the Intermountain West Sunday
into Sunday evening. Convection will be capable of producing hourly
totals up to 0.75" at times, particularly from northern Arizona
through Utah where the better instability is forecast to setup.
This will bring another day of localized to widely scattered
instances of flash flooding for the more vulnerable locations in
the region.
Taylor
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From Mike Powell@618:250/1 to All on Sat Aug 17 09:35:00 2024
FOUS30 KWBC 170745
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sat Aug 17 2024
Day 1
Valid 12Z Sat Aug 17 2024 - 12Z Sun Aug 18 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF THE
SOUTHWEST INCLUDING THE SLOT CANYON REGION OF UTAH AND PORTIONS OF
THE MID ATLANTIC...
...Southwest U.S...
An active convective day is in store for portions of the Southwest
U.S. as the deep monsoon moisture surges northward through the
region. PWs are forecast to reach nearly +2 to +3 standard
deviations above normal, with values as high as 1.25" into Idaho
and locally 1.5"+ across Arizona and southern Utah. Aloft,
shortwave energy will be moving through the flow, providing greater
forcing for ascent. Finally, with daytime heating and favorable
lapse rates, upwards of 1500-2000 J/kg of instability is expected
to develop. All of this points to robust thunderstorms capable of
producing intense rain rates (localized up to 1" in an hour) over
the more sensitive regions of the Southwest U.S. including the
Slot Canyon region of Utah. HREF neighborhood probabilities are
moderate (50%) for localized 2" totals over southern Utah and even
show a slight signal (15-20%) for 3" totals.
No significant changes were needed to the Slight Risk area and the
rest of the risk areas were adjusted minimally to the latest
guidance. The most vulnerable locations will be the slot canyons,
normally dry washes, and areas near recent burn scars for
potentially localized significant flash flooding.
...Eastern Great Lakes, Mid-Atlantic, and into the Mid-Mississippi
Valley...
The slow moving upper trough will continue to make a progression to
the east today, primarily affecting areas from the eastern Great
Lakes through the central Appalachians and Mid Atlantic region.
Fairly decent forcing for ascent provided by the approaching height
falls and upper divergence, which will provide the best support for
convection across the Mid Atlantic later today. Further west,
convection along the Appalachians/Ohio Valley will be tied to the
advancing cold front. Moisture profiles are sufficient but not
overly impressive and instability is expected to be enough to
produce scattered to numerous showers and thunderstorms, though
storm organization is generally expected to be weak and overall
progressive as well. This should limit the scope of any flash
flooding concerns.
With that said, there are signals for some pockets of higher rain
totals, one of which is across the Mid Atlantic where some of the
00Z hi-res guidance paints some hefty QPF totals, although spotty
in places. This drives the HREF probs for totals for 3" above 50
percent across northern VA through eastern/east-central PA.
Confidence in this is lower with the various scenarios is
conditional based on how earlier/morning activity plays out.
However, given the run to run continuity of the HREF signals and
potential for at least a couple rounds of heavier rains, a Slight
Risk for mainly urban flash flooding was introduced from portions
of MD (near Washington, DC) through eastern PA.
Farther south along the stalled boundary, some training of
convection is possible as the mean flow becomes more parallel to
the expected storm motions, particularly across portions of the
Tennessee Valley southward. Despite the decent HREF signal for localized/pockets of higher rain totals, generally high FFG and
drier soils should keep the flash flood threat at the Marginal
level.
...Pacific Northwest...
An impressive upper level trough will amplify into a closed low off
the northern CA coast, dropping 500mb heights to less than -3 sigma
this afternoon. Downstream of this low, pinched flow will
transition into broad divergence over the Pacific and Interior
Northwest, surging moisture and ascent into the region. PWs are
progged to reach above 1 inch, and possibly above 1.25" in parts of
the Cascades, reaching nearly +3 sigma from climatology. As the
upper low pivots slowly northeast, this moisture fetch will be
persistent into the region, and IVT values peak around 350 kg/m/s,
reflective of a rare but weak August AR into WA/OR/northern CA.
During this time, ascent increases within the RRQ of a
strengthening upper jet streak, and mesoscale upslope ascent
occurs into the Cascades. This will result in widespread showers
and even scattered thunderstorms in response to MUCAPE reaching
500-1000 J/kg. The HREF neighborhood probabilities for 1"/hr reach
as high as 30-40%, so despite generally rapid storm motions, brief
torrential rainfall is likely, with total rainfall being enhanced
by repeating rounds of cells. August ARs usually do not result in
anything more than isolated flood/flash flood instances, but the
MRGL risk was extended along the Cascades into northern CA, and
rapid runoff/instances of flash flooding are possible, with
impactful flash flooding, including debris flows, possible across
recent burn scars.
...South Florida...
Another day of enhanced coverage of convection is expected across
South Florida and the Keys in the vicinity of the lingering
surface trough draped across the area. This feature combined with
fairly anomalous moisture should lead to another day of scattered
to numerous showers and thunderstorms, some of which will be
capable of producing intense rain rates (2-3" hourly totals) and
potentially produce a quick 2-4"+ over some areas. The latest
guidance points to the greatest focus area to be south of the most
urbanized corridor but nonetheless, some isolated/localized
flooding concerns will be possible and the Marginal Risk remains in
place.
Taylor
Day 2
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...
...Mid Atlantic and Northeast...
A cold front will advance eastward into the region as the main
upper trough slowly advances through the Great Lakes. This will
bring another day of scattered to numerous showers and
thunderstorms to much of the region. In the low level southwesterly
flow ahead of the front, with enhanced training likely on S-N
oriented mean flow parallel to the from from near Washington, D.C.
through Upstate NY. Here, PWs will be nearly +2 sigma according to
NAEFS, which will be acted upon by the robust synoptic lift to
deepen convection. Rainfall rates of 1-2"/hr are likely, which
through training could produce locally more than 3" of rainfall
based on the latest ensemble probabilities.
Where this training occurs across urban areas or sensitive soils,
scattered instances of flash flooding are possible. The recent CSU
FG fields depict an enhanced risk for flash flooding along I-95 and
into the Tri-State area, leading to higher confidence in the Slight
Risk.
Further south, the Marginal Risk extends into the Piedmont and
coastal Carolinas where the combination of heavy rainfall over more
sensitive soils due to recent heavy rains may result in isolated
instances of flash flooding.
...Southwest and Intermountain West...
Impressive and deep monsoonal moisture will continue to stream
around the western periphery of the monsoon ridge and combine with
daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will
be capable of producing hourly totals up to 0.75" at times,
particularly across portions of Utah where the better instability
and highest PW anomalies are forecast. It's possible a Slight Risk
may be needed in future updates across portions of northern AZ into
southern UT but there is some uncertainty on cloud cover
potentially limiting the available instability and thus the rain
rates may end up lower.
...South Florida...
Another day of scattered, slow-moving thunderstorms are expected
across the region where rain rates may approach 2-3"/hr at times.
This is due to the lingering surface trough in the vicinity
focusing the higher PWs. Given the potential for this to occur over
the more urban areas, the inherited Marginal Risk was minimally
changed to highlight the continued isolated flash flood risk.
Taylor
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)
From Mike Powell@618:250/1 to All on Sun Aug 18 08:50:00 2024
FOUS30 KWBC 180812
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
412 AM EDT Sun Aug 18 2024
Day 1
Valid 12Z Sun Aug 18 2024 - 12Z Mon Aug 19 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID ATLANTIC...
...Mid Atlantic and Northeast...
An approaching cold front will bring another day of scattered to
numerous showers and thunderstorms to portions of the Mid Atlantic
and Northeast today into tonight. Aloft, the broad upper trough
will slowly advance eastward into the eastern Great Lakes and
central Appalachians. Ahead of it, southwesterly flow will allow
for warm and moist air to continue lifting into the region and will
be characterized by dewpoints well into the 70s and PWs above 1.5"
with locally higher values exceeding 1.75", which is around 2-2.5
standard deviations above normal. With peak heating, instability of
between 1500-2000 J/kg is forecast to develop.
The 00Z guidance continues to key on the urban corridor from near
Washington, DC to near NYC metro for the greatest QPF and
probabilities for intense rain rates today into tonight. Several
clusters of thunderstorms are likely to develop (some this morning) and
with the mean flow out of the southwest near parallel to the storm
motions, some enhanced training or backbuilding will be possible,
particularly over eastern PA, northern New Jersey, and into
southern New York. Here, the HREF neighborhood probabilities are
high for 3" totals (above 60 percent) and show a slight signal for
5" (15-20 percent). Rain rates of 2-3"/hr will be possible given
the PW environment. Given the proximity and likelihood for this to
occur over the more urbanized corridor, scattered instances of
flash flooding will be possible, some of which could be locally
significant.
Elsewhere in the region outside of the Slight Risk, thunderstorm
activity is expected to be more isolated in nature, but given the
environmental ingredients could still result in localized flash
flooding. This includes much of PA and NY states where there is
still a decent signal in the HREF probabilities for 2-3" isolated
totals. Another area of concern is further south from eastern VA into
the Piedmont and coastal regions of the Carolinas where the
combination of locally heavy rainfall over more sensitive/saturated
ground conditions could lead to additional flash flooding
concerns.
...Southwest and Intermountain West...
Impressive and deep monsoonal moisture will continue to stream
around the western periphery of the monsoon ridge and combine with
daytime instability to produce scattered to widespread showers and thunderstorms from AZ as far north as WY and ID. Convection will
be capable of producing hourly totals up to 0.75" at times,
particularly across portions of northern AZ and southern Utah where
guidance continues to show potential for locally higher rainfall
amounts.
...South Florida...
Another day of scattered, slow-moving thunderstorms are expected
across the region where rain rates may approach 2-3"/hr at times.
This is due to the lingering surface trough in the vicinity
focusing the higher PWs. The consensus among the 00Z guidance is
that this should largely stay south of the major metro areas, but
will be close enough to continue the Marginal Risk for portions of
South Florida and the upper Keys.
...Ozarks Region...
Shortwave energy rounding the top of the upper ridge will quickly
drop through the Ozarks region late in the period
(this evening/overnight). With a modest low level jet forming in
response and a decent moisture influx, a narrow strip of locally
heavy rainfall is likely to develop in the region. Given the
northwest flow aloft and storm motions, some backbuilding is
possible. While confidence in these scenarios isn't very high,
some of the outlook area has seen recent heavy rainfall and the
potential for a narrow/localized area of 1-3" is possible based on
the 00Z hi-res guidance and HREF neighborhood probabilities. This
could lead to a few instances of flash flooding, mainly in the
06Z-12Z period early Monday morning.
Taylor
Day 2
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE NORTHEAST U.S. AND SOUTHWEST INTO THE INTERMOUNTAIN WEST...
...Northeast...
The upper trough will make a final pass across the Mid Atlantic to
Northeast on Monday with the cold front expected to move across
the coastal areas during the late morning into the evening hours.
With peak heating and sufficient destabilization, vigorous
convective elements are expected to develop again, mainly from the
coastal Mid Atlantic through much of eastern New York and portions
of New England. Storm motions should be fairly progressive, but a
few storms could repeat/train due to merging/colliding cells and
storm totals could approach 1-2" in spots leading to flash
flooding.
...Southwest/Intermountain West...
By Monday, the bulk of the anomalous moisture is expected to have
lifted northward through the Four Corners region and into the
Intermountain West/Rockies. With another shortwave trough embedded
in the flow moving through, this should be sufficient to produce
another day of isolated to scattered thunderstorms, most likely
over portions of northern Utah, Colorado, and into western WY, far
eastern Idaho. Flash flooding will be possible, particularly for
the most sensitive locations like areas near recent burn scars.
Taylor
Day 3
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...
The upper ridge over the Southern U.S. is expected to build/grow
and expand westward into more of the Southwest U.S. on Tuesday.
This will likely limit the extent and coverage of monsoon
thunderstorm activity across the region, but with PWs still
elevated across Arizona and the expectation for at least a modest
amount of instability, isolated thunderstorms are possible through
the late morning into evening hours. If they do develop, these
storms would pose an isolated flash flood risk for the typical
vulnerable locations like areas near burn scars, dry washes, and
slot canyons.
Taylor
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)
From Mike Powell@618:250/1 to All on Mon Aug 19 09:39:00 2024
FOUS30 KWBC 190738
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
338 AM EDT Mon Aug 19 2024
Day 1
Valid 12Z Mon Aug 19 2024 - 12Z Tue Aug 20 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHEAST U.S...
...Northeast...
Broad meridional flow across the Eastern CONUS will continue with a
dual surface low structure progged to move northeast through the
period with a cold front swinging eastward out of the adjacent Ohio
Valley up through Quebec, clearing the eastern seaboard later in
the evening. The deep cyclonic flow ahead of the trough will
continue advecting warm, moist unstable air poleward with
increasing theta-E's and accompanying instability along and ahead
of the cold front situated across Southern New England up through
Interior Northern New England by the late-morning and afternoon
time frame. The combination of increased mid-level forcing from the
trough and surface convergence along the cold front will aid in
developing a line of convection in-of the above area with a primary
focus north of I-80 with the best chance for heavy rainfall likely
north of the LI-NYC-Hudson Valley corridor. This puts areas that
have seen significant rainfall within the crosshairs of another
locally enhanced rainfall prospect before the cold front moves past
and drier air sweeps through in wake of the front.
In terms of potential, the upper threshold will be less than what
occurred today due to a shorter time frame of impact due to the
frontal progression, as well as a more narrow sector of favorable
instability that will play a significant role in the heavy rain
footprint, and overall flash flood prospects. 00z HREF neighborhood probabilities paint some of that story with a focal point of 20-40% probabilities for >3" located within the confines of Southern CT up
through Interior New England, including parts of the complex
terrain within MA/VT/NH. The alignment of these higher
probabilities coincide with the better MUCAPE field forecast during
the morning and afternoon hours before the cold front passes the
area. A favorable footprint of at least 1"/hr rates off the
ensemble (HREF) also give credence to the potential with an area
of 50-90% encompassing the above region with low-end probabilities
for at least 2"/hr showing up across Northern VT and NH. These
areas have been impacted a few times in recent memory with the area
across CT very fresh from today's barrage that brought locally 8+"
to parts of the state. Area FFG's across New England are relatively
low with soil moisture averages falling just above the 50th
percentile, or near normal. CT is not so lucky with the top soil
layer completely inundated from this past event leading to very
sensitive grounds for flooding in this area of interest.
Considering the above factors, a moderately favorable environment
for heavy rainfall, and with a strong agreement from the UFVS First
Guess Fields denoting a recommended high-end SLGT risk, a Slight
Risk was added across much of CT, West-Central MA, up through much
of VT and NH. A MRGL risk was maintained further south for isolated
flash flood prospects from scattered convection migrating eastward
during the frontal passage that could lend a few chances to those
within the urban corridor from Hampton Roads up through the NYC
metro.
...Southwest U.S...
The region will continue to lie within the western flanks of mid-
level ridging situated over the Southern Plains into the
Continental Divide. Relatively moist airmass with differential
heating during the afternoon will yield scattered thunderstorms
across the Desert Southwest with locally enhanced rainfall
plausible within the terrain areas of AZ up through the Inter
Mountain West. A few shortwaves embedded in the flow will aid in
some better organized convective clusters capable of providing 1+"
rainfall amounts in a short time, enough to cause some issues if
they fall within complex terrain, remnant burn scars, or slot
canyons located across the region. The threat remains on the lower
end of MRGL, but still within the threshold leading to a general
maintenance of the risk from previous forecast.
...High Plains...
Multiple stronger shortwaves will eject eastward along the northern
periphery of the ridge, encountering a formidably unstable airmass
as they enter into the High Plains of the Northern and West-Central
U.S this afternoon and evening. Consensus has grown on a few areas
within the High Plains of MT into SD, as well as the CO/WY Front
Range for a cluster of thunderstorms capable of locally heavy
rainfall developing later this afternoon, carrying through the
evening as they ridge along the theta-E gradient in place across
the center of the CONUS. HREF neighborhood probabilities were
favorable for at least 1-2" of rainfall (60-90%) in either of these
areas with some lower-end probabilities for upwards of 3" in any
cells that propagate through the High Plains. The setup should be
on the progressive side overall, however some backbuilding
potential behind any MCS development over the Northern Plains could
produce a more locally significant rainfall footprint where if/when
it develops. The zones of potential include the Southeast WY and
Northeast CO corridor along with Southeast MT into Western SD as
the secondary focal points in the setup. ML output was targeting
this area as the latest runs with an axis of higher QPF positioned
within the same zones as above. First Guess Fields are also
pin-pointing a broader MRGL risk extended into the above zones with
even a some signals for a potentially higher risk (SLGT) embedded.
With the top soil layers trending drier than normal, the prospects
for a more widespread significant rainfall are not anticipated,
thus an additional MRGL expansion into the High Plains was enough
to cover for the potential.
Kleebauer
Day 2
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...
Mid-level ridge across the Southern U.S is still on track to
maintain a stronghold on the overall pattern for much of the
Central and Western U.S by Tuesday. Ridge axis will expand and
strengthen according to the model consensus leading to unfavorable
conditions for convective development over under the ridge itself.
Arizona will continue to lie right on that western fringe of the
ridge placement with enough lingering instability and moisture
promote a threat of isolated thunderstorms capable of producing
locally heavy rainfall. The threat is certainly on the low-end of
the potential, but non-zero, especially along the International
border between Santa Cruz and Pima counties in AZ. The previous
MRGL was maintained with no change in the alignment.
A small note to add that lingering convection across the Northern
and Central Plains stemming from activity on D1 will have the
ability to maintain enough intensity to promote an isolated threat
for flash flooding the very beginning of the period before
diminishing. The global model consensus is all over the place on
exactly where the heaviest rain could occur, so there was not
enough agreement to institute a MRGL risk. There is a chance for a
small addition in later updates if there is more agreement within
the model suite, but there's still time to hash out those details
in the next succession of updates. CAMs guidance will hopefully
shed more light on the necessity, if any for an addition.
Kleebauer
Day 3
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...
A more robust moisture advection pattern will ensue across the
Southwest U.S with some Pacific moisture being pulled northward
thanks to the combination of the Southern ridge shifting eastward
and becoming more elongated north-south, along with a shortwave
trough quickly moving inland through CA leading to a pathway for
elevated moisture and embedded shortwaves pivoting out of Sonora
and nearby Baja to move northward. Ensemble QPF is much greater
compared to the recent D2 time frame thanks to the poleward
advancement of anomalous moisture (+2 to +3 deviation PWATs based
off the latest NAEFS) and subsequent instability increase. The
ascent pattern across the Great Basin is the driver for a better
signal further north of AZ thanks to diffluent flow out ahead of
the shortwave. A widespread signal of 0.5-1" of rainfall at this
lead is indicative to the potential for more widespread convective
impacts across the Southwest and parts of the Inter Mountain West.
As of now, the MRGL in place will suffice, but a targeted upgrade
is plausible considering the expected evolution of the pattern
heading into mid-week.
From Mike Powell@618:250/1 to All on Tue Aug 20 07:36:00 2024
FOUS30 KWBC 200752
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Tue Aug 20 2024
Day 1
Valid 12Z Tue Aug 20 2024 - 12Z Wed Aug 21 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...
Current upper air analysis across the CONUS depicts a sprawling
mid-level ridge positioned over much of the Southern Plains into
the Southwest. The ridge itself will maintain its orientation with
some modest strengthening heading through the afternoon with the
center of the ridge positioned across the Southern Rockies. Enough
diurnal destabilization and moisture trapped under the ridge will
offer a threat of widely scattered convection across the terrain in
AZ, extending into Southern CO and Northern NM. The overall QPF
footprint is fairly lackluster in the areal average, but a few
pockets of heavier precip can be found within the CAMs indicating a
low-end potential for heavy rain and potential flash flooding. The
primary concerns will remain tied to the complex terrain and
adjacent towns that could be impacted by rapid onset flooding. Slot
canyons and remnant burn scars located across the Southwest will be
under the threat as well, one of the main reasons for continuity
from the previous forecast, as well as an expansion eastward into
Southern CO and Northern NM where multiple burn scars are located.
This remains the primary area of interest for the period.
A small area across east-central SD will also experience a threat
of locally heavy rainfall as remnants of a complex of thunderstorms
migrates eastward along the leading edge of favorable mid-level
ascent as a shortwave ejects out of the High Plains. Favorable
instability axis across the Northern Plains favors points further
west with the storm complex likely to enter a less favorable
convective regime to maintain a strong enough intensity to induce
flooding prospects. LLJ will also be on the decline leaving behind
a relatively mundane setup for prolonged convective enhancement
that would be necessary for flash flood prospects. Considering the
above variables, and the addition of drier soil moisture located
within the forecasted zone of impact, a nil risk area was
maintained, however the threat is non-zero (<5%).
Kleebauer
Day 2
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE NORTHERN ARIZONA...
A more robust moisture advection pattern will ensue across the
Southwest U.S with some Pacific moisture being pulled northward
thanks to the combination of the Southern ridge shifting eastward
and weakening, along with a shortwave trough quickly moving inland
through CA leading to a pathway for elevated moisture and embedded
shortwaves pivoting out of Sonora and nearby Baja to move
northward. Ensemble QPF is much greater compared to the D1 time
frame thanks to the poleward advancement of anomalous moisture (+2
to +3 deviation PWATs based off the latest NAEFS) and subsequent
instability increase. The ascent pattern across the Great Basin is
the driver for a better signal further north in-of AZ and UT
thanks to diffluent flow out ahead of the shortwave. Ensemble QPF
has increased since the last series of runs with a more pronounced
convective depiction signaling some totals between 1-1.5" now
introduced in the means. The back end of the 00z HREF is already
showing the increased convective pattern at the beginning of the
diurnal period with the remainder of the evening still to go.
Ensemble bias corrected QPF is running between 0.75-1.25" for an
areal average over a good portion of AZ with the focal points
situated over the terrain in Southern AZ and across the Mogollon
Rim and Grand Canyon areas of Central and Northern AZ. The
prospects are certainly favorable for more localized flash flood
concerns just based off the anomalous PWAT advection anticipated.
After collaboration with offices out west, a targeted SLGT risk was
added across Northern AZ encompassing places like Flagstaff to the
Grand Canyon, representative of the strongest heavy precip signals
expected during the D2 period. Additional upgrades to a SLGT risk
are also possible within the next series of updates, especially
within Southern AZ and Southern UT pending the run-to-run
consistency within guidance.
Kleebauer
Day 3
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST U.S...
A continuation of the elevated moisture pattern across the
Southwest will transpire in D3 with an axis of scattered to
widespread convection likely across much of AZ through the Inter
Mountain West. Small mid-level perturbations are still forecast to
rotate northward around the western flank of the ridge positioned
across the Southern Plains. A digging upper low off the PAC
Northwest coast will allow the flow to become even more meridional
across the West allowing for the more anomalous PWAT advection to
occur further in the interior leading to the best heavy rain axis
to likely situate over Northern AZ through much of UT and Western
CO. The growing consensus on the specifics of where the heaviest
precip will focus will likely correlate to an upgrade at some point
in the future, but timing of the mid and upper pattern still has
some points to hash out before going into more detail. As of now,
the MRGL from the previous forecast was maintained but expanded on
all sides as the 00z ensemble QPF footprint expands with agreement
from the recent ML output on the precip placement.
Across Florida, a stalled frontal boundary will become the focal
for scattered convection on Thursday afternoon with the areal QPF
average relatively elevated, but still not depicting a substantial
enough signal to warrant a risk addition to the area. This will be
a period to monitor for perhaps a targeted risk along either coast,
pending sea breeze pattern evolution where heavy rain could affect
the the urban corridors either side of the Peninsula, or an area
inland like Orlando proper.
From Mike Powell@618:250/1 to All on Wed Aug 21 08:47:00 2024
FOUS30 KWBC 210752
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Wed Aug 21 2024
Day 1
Valid 12Z Wed Aug 21 2024 - 12Z Thu Aug 22 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
ARIZONA AND SOUTHWESTERN UTAH...
...Southwest...
A relatively strong mid-level ridge will be broken down on the
western flank as a series of mid-level shortwaves begin ejecting
off the Pacific with a more amplified, closed-low pattern evolving
towards the back end of the forecast period off the Pacific
Northwest. The weakening force of the pattern will allow for a more
north-south alignment of streaming mid-level energy to ride out of
Sonora and traverse the Desert Southwest to the Great Basin. In
tandem, a pretty formidable moisture flux will begin advecting into
the region, highlighted by a well-forecast IVT pulse (500-700
kg/ms) pivoting through much of AZ with roots from the tropical
Eastern Pacific. This will allow for PWATs to surge closer to +2 to
+3 standard deviations, a factor that typically enhances the
convective pattern across the West and has a history of more
widespread thunderstorms with heavy rainfall. Latest 00z HREF is
privy to the enhancement with the blended mean QPF signal depicting
a more robust convective threat with local maxima bordering upwards
of 2" in spots confined to the terrain. The greatest threat(s)
will be located across Southern to Southeast AZ where the initial
surge will be taking place, intersecting the terrain situated near
the border. The second areas will be within the Mogollon Rim and
Grand Canyon areas of AZ into the Southwestern portion of UT. Those
areas will see their best potential the second half of the period
as the surge of elevated moisture reaches the region and continues
the convective threat into the evening. HREF probability fields for
1" of precip are high (60-90%) across much of the above areas with
2" markers a respectable 25-40% within the neighborhood
probability field. These are textbook signatures for elevated
convective flash flood threats leading to a general maintenance of
the SLGT risk from previous forecast with only minor modifications
to the risk area.
Interior areas across UT and CO will also see an uptick in
convective coverage, but the deeper moisture plume will still be
making progress around the northern periphery of the ridge, so the
threat isn't expected to be as robust as further southwest. The
environment is favorable for some isolated impacts, especially
within any stronger mid-level perturbations that rotate overhead
allowing for better organized cell structures in their presence.
The MRGL risk was modified only to outline the trends within the
ensemble QPF where locally heavy rainfall could produce some flash
flood prospects.
...New York and Northern New England...
Upper low across Quebec will slowly migrate south into Upstate NY,
eventually drifting eastward through VT/NH during the second half
of the period. Large scale ascent pattern under the low will
generate periods of showers with embedded thunderstorms as the
disturbance pivots through the region. Some of the signals across
the Green/White Mountains of New England are sufficient for
potential locally heavy rain cores that become tied to the terrain
thanks to a slower mean storm motion. The lack of surface
instability will limit the threat to a more elevated convective
regime which typically doesn't allow for pronounced rainfall rates.
The antecedent moist soils still lingering after what transpired
the past week and the slow mean storm motions are still enough to
sway to the side of a MRGL risk which was added the previous
update. There was little deviation in the setup from that point, so
decided to maintain continuity and keep the MRGL risk over portions
of NY state and Northern New England.
Kleebauer
Day 2
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S...
An increasingly amplified pattern out west will take shape as the
digging upper trough and closed-low positioned off the Pacific
coast will begin interacting with the anomalous moisture feed
advecting northward around the western flank of the ridge to the
southeast. PWATs will remain settled between +2 to +3 deviations
above normal across much of the Southwestern U.S with advancement
deeper into the Inter Mountain West leading to an enhanced
convective regime extending from Northern AZ through UT and Western
CO. Areal average MUCAPE across the Southwest will range from
500-1000 J/kg with a maximum of 1500-2000 J/kg situated within the
core of the IVT pulse bisecting portions of Southern UT down
through Northern AZ. This area in question will be the primary
region of focus for heavy rainfall and relevant flash flooding,
especially given the ongoing nature from the D1 time frame, as well
as the complexity of the terrain with numerous slot canyons and
remnant burn scars still situated over the area.
Ensemble QPF footprint between 1-1.5" on average is a testament to
the increasingly bullish outlook for heavy rainfall as these
signals are usually reserved for the better flash flood setups in
recent past. The back end of the 00z HREF for reference has a
sharp increase in the 1"/hr rainfall rate probabilities between
18-00z in the period with a blossoming convective pattern likely
extending through the following evening when correlating to global deterministic. The pattern will evolve with convective lingering
late into the period with even some nocturnal storm prospects
further inland into UT and Western CO. The previous SLGT risk
forecast was maintained with the higher-end SLGT potential located
within that zone between Flagstaff up through Zion National Park,
including the Grand Canyon. The SLGT extends further inland through
the interior of UT into Western CO with a secondary maxima being
depicted within the western slopes of the Rockies, east of Grand
Junction.
Elsewhere:
The setup across Maine and Florida will lend to non- zero chances
for flash flooding, but the overall signal is still just below the
5% risk threshold to necessitate a MRGL risk. The previous MRGL
across Maine was removed due to timing of the heaviest convection
occurring mainly within the D1 period with an overall decay in the
heavy rainfall prospects during the front half of the period.
Across Florida, the setup is conducive for locally heavy rainfall
thanks to a stalled front and pooling moisture near the boundary.
Right now, the threat is borderline with the area of interest
mainly over the Northeast FL coastal plain between Jacksonville to
Daytona. Area FFGs are very high in this area leading to the
primary threat being almost pure urban flash flooding with less
concern for flooding outside those local zones. With some time
remaining leading up to the period, will continue to assess the
signal to see if a MRGL addition is prudent.
From Mike Powell@618:250/1 to All on Thu Aug 22 08:38:00 2024
FOUS30 KWBC 220749
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
349 AM EDT Thu Aug 22 2024
Day 1
Valid 12Z Thu Aug 22 2024 - 12Z Fri Aug 23 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE GREAT BASIN AND FOUR CORNERS AREA IN THE WESTERN U.S...
...Western U.S...
A continued amplified pattern across the west will evolve further
with a retaining deep moisture fetch arcing through the Desert
Southwest and further into the Inter Mountain territories within UT
and CO. A general +2 to +3 deviation PWAT anomaly will bisect the
region of Northern AZ through Central UT and Western CO by the
afternoon hours today leading to an enhanced convective output
during peak diurnal destabilization. A growing consensus amongst
CAMs members for widespread thunderstorm development with stronger
cores able to produce 1"/hr rates or better aligns well with the
pattern and will induce a greater potential for flash flooding
along the aforementioned areas.
A strong HREF EAS signal for 1" of rainfall exists within the
Southwest portion of UT, including some of the busier National and
State Parks where slot canyon positioning and stronger flash flood
prospects overall create a heightened prospect for impacts during
the expected convective time frame. HREF neighborhood probabilities
for >2" of rainfall are also very high (50-80%) within the local
area of Southwest UT down into the Northern tier of AZ, a signal
generally befitting for flash flood scenarios in recent history.
The threat is not as robust further south into the Mogollon Rim as
the deeper moisture advection regime will be located further north,
but the environment will still be primed for scattered heavy rain
signatures within that zone, so continuity on the southern end of
the SLGT risk was maintained from previous forecast. Further north,
the SLGT risk remains, but the threat is on the higher-end of
potential for the areas near Flagstaff up through that area of
Southwest UT thanks to the alignment of the best instability, deep
layer moisture, and correlated IVT pulse traversing the area.
Pending the evolution of hi-res guidance and radar trends leading
into the afternoon, a targeted Moderate Risk is plausible,
especially within the slot canyon corridor that has the highest
flash flood susceptibility. The areas further north across UT and
CO will see their fair share of convective impacts, especially late
in the afternoon and evening when instability reaches peak and any
mid-level perturbations stemming from convection to the Southwest
advect northeastward allowing enhanced ascent locally as they
maneuver overhead.
Another area of interest will occur across the CO Front Range as
combination of elevated moisture, instability, and a stronger
shortwave ejecting out of the Central Rockies will ignite another
round of heavy thunderstorms to the east of Denver, an area that
has seen impacts as of late with regards to flash flooding. A
secondary QPF max located across the area allowed for a pretty
elevated signal within the 3hr FFG exceedance probabilities
(40-60%), mainly after 00z when LLJ influence peaks the intensity
of the forecasted convection. A MRGL risk is currently located over
the area, but that corridor across Eastern CO, away from the divide
is the focal point for that time frame.
...Florida...
Two separate zones for flash flood potential will exist across FL
this period. The first of which will be situated across the western
coast of the Peninsula with the Clearwater/Tampa/Sarasota corridor
as the primary focus. A quasi-stationary front will meander over
the northern half of FL with smaller mid-level impulses wandering
eastward off the Gulf, creating a multi-wave convective scenario
during the entirety of the forecast. 00z sounding out of KTBW
indicated a sufficiently deep warm cloud layer (>15.5k ft) with
areal PWATs running over 2" with expected continuity during the
period. Mean flow running parallel to the front will create an,
"Express lane" for multiple mid-level shortwaves off the adjacent
Gulf to bisect the area, each carrying their own convective
conglomeration as they move ashore across the region. Rainfall
rates of 2-3"/hr will be plausible in the scenario, impacting an
already water logged area of FL that is still dealing with the
aftermath of Debby. The lower FFG intervals within the urban
corridors, and now surrounding areas with the slow receding waters
create a higher likelihood of flash flooding to occur, especially
with the multitude of convective waves anticipated during the
forecast period. A MRGL risk was added to account for the threat.
The second area of interest lies further northeast, closer to the
Jacksonville metro and surrounding coastal portions of Northeast
FL south of Jacksonville, as well as far Southeast GA where the
recent CAMs output is the most aggressive with regards to heavy
rain prospects. A strong HREF neighborhood probability for >5"
(50-80%) exists within that zone between GA down to Jacksonville
proper with a low-end probability for >8" also depicted within the
prob fields. This all stems from the front location and expected
instability axis to develop during diurnal heating leading to
development along the boundary with slow storm motions anticipated.
In coordination with the Jacksonville WFO, a MRGL risk was added
for that area encompassing Southeastern GA down through Northeast
FL with emphasis on the urban areas along the coastal plain.
Kleebauer
Day 2
Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION AND INTERMOUNTAIN WEST...
The anomalous moisture feed that allowed for the active periods
prior will vacate to the northeast with a skinnier axis of elevated
PWATs generally focused from the AZ/NM state line up through the
Inter Mountain West of UT/CO. This positions the the Four Corners
area up through the Wasatch as the the main targets for heavy
convection during the afternoon and evening Friday before the setup
dwindles the back end of the period. A secondary focus across the
terrain in Southeast AZ over into the NM Bootheel will also be an
area that could see some scattered heavy rain signals with some
support from global deterministic and ensemble members. Current QPF
signal is modest with areal averages closer to 0.25-0.5" with a
maxima focused near the Four Corners over Southwestern CO (>1").
The secondary max is across the Wasatch through the terrain in
Western CO where isolated totals >1" are plausible, but more
sporadic in coverage compared to the Four Corners region. Overall,
the setup is more benign in the grand scheme, especially
considering the previous active periods.
From Mike Powell@618:250/1 to All on Fri Aug 23 10:25:00 2024
FOUS30 KWBC 230829
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Fri Aug 23 2024
Day 1
Valid 12Z Fri Aug 23 2024 - 12Z Sat Aug 24 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS THE FOUR
CORNERS REGION/INTERMOUNTAIN WEST AND PARTS OF THE SOUTHEAST US...
...Southeast US...
Maintained the Marginal risk area that was introduced on Thursday
over portions of northeast Florida...and expanded the area
northeastward along the Georgia coast line. 00Z runs of the HREF
increased the neighborhood probabilities of 1 to 2 inch per hour
rates in response to a frontal boundary..and associated
convection...being a bit closer to the coast than shown by earlier
runs.
...Southwest US...
Moisture feeding northward into the Southwest US will begin to be
channeled into a more narrow corridor disrupted today...leading to
a somewhat smaller coverage of late day and evening convection
capable of producing heavy rainfall. A secondary focus across the
terrain in Southeast AZ over into the NM Bootheel will also be an
area that could see some scattered heavy rain signals with some
support from global deterministic and ensemble members. Current QPF
signal is modest with areal averages and the setup is more benign
in the grand scheme, especially considering the previous active
periods.
Bann
Day 2
Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST/FOUR CORNERS AND SOUTH FLORIDA...
...Southwest and Four Corners...
Lingering moisture and relative instability coupled with
progressive shortwaves cutting through the Interior Mountain West
will set up another round of isolated convection during the
afternoon and early evening hours on Saturday before a gradual
dissipation with the loss of diurnal heating. Pockets of heavy
rainfall are still forecast across a similar corridor as on Day 1,
so there is some concern that the antecedent conditions could be
favorable for greater impacts pending what occurs leading up to the
start of the forecast period on Saturday morning. Overall ensemble
QPF footprint still indicates a max of 1-2", at best, within the
strongest cores. This is still enough for flash flooding within
complex terrain and especially around any burn scars. The previous
MRGL risk was relatively unchanged.
...South Florida..
Incoming moisture flux with roots from the Caribbean will make
headway into South Florida on Saturday with area precipitable water
anomalies approaching 2 deviations above normal. The increasing
moisture will be in proximity to a quasi-stationary front to the
north...with a sharp delineation point of where the moisture
pooling will reside. A Marginal risk area was maintained across
the Southern FL Peninsula.
Bann
Day 3
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST/FOUR CORNERS AND SOUTH FLORIDA...
As an upper high over the Southern Plains begins to move eastward
and away from the Southwest...another push of moisture and
shortwave energy will move northward. This will sett up another
round of isolated to widely scattered thunderstorms capable of
producing flash flooding or run off problems within complex
terrain and especially around any burn scars. Given the fetch of
moisture from the southeast...the areal coverage was expanded
somewhat compared with the size of the Marginal risk area on Dy 2.
From Mike Powell@618:250/1 to All on Sat Aug 24 10:11:00 2024
FOUS30 KWBC 240846
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
446 AM EDT Sat Aug 24 2024
Day 1
Valid 12Z Sat Aug 24 2024 - 12Z Sun Aug 25 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST...MIDWEST...AND SOUTH FLORIDA...
...Southwest...
Another round of daytime convection is expected in the Southwest
again on Saturday, although the primary threat area should be
relatively narrow, focused in a plume of deeper moisture along the
periphery of an advancing deep trough. The Marginal Risk was
generally maintained in the same area as the previous outlook
issuance with only minor adjustments around the southern periphery.
Localized heavy downpours will remain possible given the plume of
moisture still being drawn northward out of Mexico and northward
around the western side of a sprawling upper level ridge. However,
the mid-level flow will be much stronger than usual for this time
of year helping to mitigate the flash flooding concern by providing
a strong and persistent steering flow. Nevertheless, around an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.
...Midwest.
A complicated forecast exists with several conditional threat
areas that still need to be sorted out. Models are generally in
good agreement in showing several rounds of elevated convection
along the periphery of a pronounced elevated mixed layer, with
700mb temperatures approaching record highs in the sounding archive
and ERA reanalysis dataset. One of the rounds is focused early in
the Day 1 period in or near Missouri. Satellite and radar imagery
from the overnight hours was showing thunderstorm development along
the Kansas/Nebraska border moving westward around the 1000-700 mb
thickness gradient and being fed by south to southwesterly winds of
35 to 50 kts per area VAD Wind profilers upstream over Kansas and
Oklahoma. The 06Z run of the HRRR continued to generate a narrow
stripe of moderate to heavy rainfall in western Kansas...its areal
coverage apparently being in check from the warm mid level
temperatures noted above but providing support to maintain the
Marginal risk area without much need to adjust the placement. The
other round looks to be focused after 00Z farther to the north from
southeast Iowa into northwestern Illinois. Ingredients favoring
heavy to excessive rainfall extend to the north and northwest in
so those solutions extending as far north and northwest into
Minnesota or southeast South Dakota cannot be ruled out yet.
Multiple hi- res models also show much of the northern sections of
the region remaining almost entirely dry. However, the risk should
an inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.be assumed to
be non- zero in the areas described above, and above 5 percent in
the Marginal Risk areas. Further updates and expansion of the
Marginal Risk is possible.
...South Florida..
Incoming deep moisture with roots from the Caribbean will make
headway into South Florida today with area precipitable water
around the 90th percentile, which is well above 2 inches. The
increasing moisture will be in proximity to a quasi-stationary
front to the north...with a sharp delineation point of where the
moisture pooling will reside. A Marginal risk area was maintained
across the Southern FL Peninsula.
Bann
Day 2
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF NEW MEXICO AND COLORADO...AND THE MIDWEST...
...New Mexico and Colorado...
As the western periphery of the Southern Plains upper level high
begins to erode, the heavy rain threat area should expand eastward
on Sunday into more of New Mexico and Colorado. Scattered daytime
convection should develop in a plume of deeper moisture and
moderate instability, and the thunderstorms will be capable of producing
flash flooding within complex terrain and especially around any
burn scars. Hourly rain rates may locally exceed 1 inch. The
Marginal Risk was mostly maintained in the same areas.
...Midwest...
The latest guidance shows potential for for organized convection
and an associated heavy rainfall threat to persist into the Day 2
period over portions of southeast Iowa into western Illinois and
northeast Missouri. The latest CSU machine learning Excessive an
inch of rain falling in a short period of time may still lead to
flash flooding, and the Marginal Risk was maintained.
Rainfall Outlook still appeared to be displaced north and west of
where the ensembles favored heaviest rainfall...and the 00Z runs
did little to resolve the considerable degree of mesoscale
uncertainty both from the overall setup across the region. The
model consensus was best on the idea that heavy rainfall was
possible but they varied widely on placement. Therefore, the
Marginal Risk should still be viewed as the most likely placement
for heavy rain on Sunday morning, but future updates are possible.
Lamers/Bann
Day 3
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST ON MONDAY...
...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but strengthening steering flow should
help limit residence time to mitigate some of the flash flooding
concern. As mentioned earlier...though...even brief heavy rainfall
rates in a short period of time may still lead to flash flooding,
and a Marginal Risk was maintained in the region.
...Northern Plains...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains helping to sharpen a front
across parts of the Upper Midwest/Northern Plains. Precipitable
water values are forecast to approach 1.75 inches over portions of
the eastern Dakotas prior to the arrival of the surface front and
increasing divergence/difluence associated with the upper level
system. The expectation is that moisture flux convergence along the
front will focus convection moving into an unstable environment
driven by diurnal heating that leads to locally heavy downpours.
Farther west where instability is more driven by cold mid level temperatures...rainfall should be less focused and less intense.
From Mike Powell@618:250/1 to All on Sun Aug 25 08:57:00 2024
FOUS30 KWBC 250838
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Aug 25 2024
Day 1
Valid 12Z Sun Aug 25 2024 - 12Z Mon Aug 26 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST ...
...Arizona/New Mexico and Colorado...
The axis of highest precipitable water values start the period
extending from eastern Arizona and western New Mexico into
southwest Colorado...with maximum values at or slightly above an
inch near the international border. This should help fuel
additional late day and evening convection...with at least a low-
end chance for rainfall rates that approach an inch per hour.
Flow increases across the region as the western portion of the
upper high erodes and a closed low over Nevada this morning makes
its way eastward...which should help convection be progressive
enough to mitigate at least some of the excessive rainfall threat.
Even so...even brief heavy rainfall can produce flash flooding
within complex terrain and especially around any burn scars.
...Midwest...
With the upper high/warm temperatures firmly in place and
apparently sufficiently strong to suppress buoyancy...and high
resolution models now struggling to generate more than light
showers...have removed the Marginal Risk area over portions of Iowa
and Illinois.
Bann
Day 2
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE NORTHERN PLAINS/UPPER MIDWEST...
...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow
should help limit residence time to mitigate some of the flash
flooding concern. As mentioned earlier...though...even brief heavy
rainfall rates in a short period of time may still lead to flash
flooding, and a Marginal Risk was maintained in the region.
...Northern Plains...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains on Monday...helping to
sharpen a front across parts of the Upper Midwest/Northern Plains.
Precipitable water values are forecast to approach 1.75 inches
over portions of the eastern Dakotas prior to the arrival of the
surface front and increasing divergence/difluence associated with
the upper level system. The expectation is that moisture flux
convergence along the front will focus convection moving into an
unstable environment driven by diurnal heating that leads to
locally heavy downpours. Spread lingers in the guidance with
respect to how progressive the system will be and how much of
excessive rainfall threat will be associated with the upper system
compared to the threat associated with the convection pushing into
the better instability. Kept the westward extension introduced on Saturday...but there was a consensus that some territory could be
trimmed out near the international border.
Bann
Day 3
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST US AND THE UPPER GREAT LAKES...
...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon and
evening across portions of the Southwest US as a moisture plume
lingers across the area and winds aloft/steering flow weakens in
response to the a mid level wave moves away over the Northern
Plains as it begins to fill. Model based ingredients suggest the
potential for locally heavy downpours may extend a bit west of the
area shown by the deterministic runs...so made a minor expansion
that way.
...Upper Great Lakes...
There is a Marginal risk of excessive rainfall over portions of the
western Great Lakes as the upper trough initially over the Upper
Midwest continues to move northeastward during the day.
Precipitable water values of 1.75 to 2 inches should already be in place
before the flow aloft becomes increasingly difluent along and
north of a quasi-stationary boundary helping enhance rainfall rates
locally. The operational guidance tended to the potential for a
couple of inches of rain...although there was considerable north-
south spread in the guidance limiting the confidence that excessive
rainfall will occur at any spot.
From Mike Powell@618:250/1 to All on Mon Aug 26 09:36:00 2024
FOUS30 KWBC 260833
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Mon Aug 26 2024
Day 1
Valid 12Z Mon Aug 26 2024 - 12Z Tue Aug 27 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS INTO THE UPPER MISSISSIPPI VALLEY...
...Summary...
Per collaboration with WFOs ABR, FSD, MPX, and ARX, have hoisted a
Slight Risk across eastern SD into southern MN, western WI, and a
small portion of northeast IA. Have also added Marginal Risk areas
across the Mid-Upper TX Coast to eastern portions of the Hill
Country, much of the west coast of FL, and across parts of New
England.
...Northern Plains into the Upper Mississippi Valley...
An upper trough over the Intermountain region will begin lifting
northeastward across the High Plains on Monday, becoming less
amplified with time as it encounters the central U.S. ridge.
Precipitable water ahead of the upper trough and associated surface
cold front area expected to peak between 1.75-2.00 inches over the
outlook areas. Pre-frontal surface low tracking slowly from northeast NE-southwest SD early Mon to the eastern MN-western WI border by
12Z Tue will coincide with a slower eastward frontal progression
later Monday and especially Monday night. As a result expect
multiple rounds of organized convection across the outlook areas,
with multicellular upscale growth potential along with some
supercells. MLCAPEs of 3000-4000+ J/Kg this afternoon-evening within
the very moist environment will support intense instantaneous
rainfall rates; however, given the relatively weak llvl flow
compared to the mid-upper levels, expect the convection to be
forward propagating at a fairly decent clip through this evening.
This will cut down on the rainfall potential during the daylight
hours. However, given the slow moving surface low and front, again
expect another round of convection Mon night as PWs remain between
1.75-2.00 inches with MUCAPEs still between 1000-2000 J/Kg. The
aggregate of both events will likely enhance the flash flood risk
within the Slight Risk area, given the growing support from the
high-res CAMs.
Elsewhere, the Marginal Risk area remains, though did pull the
southern edge south a bit into much of southern WI (including the
MKE area) towards the max instability gradient.
...Mid-Upper TX Coast to Eastern Portions of the Hill Country...
Well defined low-mid level circulation near the TX Gulf Coast early
this morning will traverse slowly into South-Central TX this
evening and overnight. Instability will remain greatest offshore,
though SBCAPEs near 1000 J/Kg distributed in a tall/skinny profile,
with WBZ levels over 13 kft and PWs of 2.00-2.25 inches, will
produces areas of heavy rain. Given the weak deep-layer flow (0-6km
bulk shear 20kts or less), expect the more intense rainfall rates
to be short lived before the more intense updrafts collapse as the
cells become outflow dominated. Therefore have opted for a Marginal
Risk over this region, with the anticipation that any flash
flooding/short term runoff concerns will be isolated.
...Western FL Peninsula...
The guidance shows a more enhanced ECSB (East Coast Sea Breeze)
given the added synoptic low-level easterly component. This will
focus peak diurnal convective coverage along the west coast of the
FL Peninsula, which is supported by the model QPFs (especially
CAMs). PWs between 2.1-2.3 inches, along with SB CAPEs peaking
between 1500-2500 during the afternoon and early evening will
support intense short term rainfall rates. In fact, the 00Z HREF
40km neighborhood probability of 2"/hr rainfall rates climbs to
40-60% over/near the Tampa Bay area between 20-00Z (while 3"/hr
rain rates climb to around 25%). Per the CAMs, localized totals of
3-5" can be expected, much of which would be within a 3 hour
period. Cannot rule out localized flash flooding as a result,
particularly in urban areas in and around the Tampa Bay area.
...Parts of New England...
Vigorous shortwave energy and compact LFQ forcing pivoting south-
southeast on the western periphery of the upper low will set the
stage for fairly robust deep-layer forcing over a relatively narrow
corridor across parts of New England Today. The mid/upper ridge
impinging from the west is likely causing the uptick in upper level
flow (localized 90-100kt 250mb jet streak) across the Hudson
Valley/eastern NY. Steepening mid level lapse rates along with
MLCAPEs ~1000 J/Kg will help fuel numerous showers/tstms during the
peak heating hours this afternoon, with storm motions south-
southeast. The 00Z HREF depicts high probabilities of >1"/hr rates
between 18-00Z, peaking between 60-70% across eastern
CT/RI/southeast MA. Localized storm totals of 3+ inches may lead to
isolated instances of flash flooding.
...Southwest US...
The threat of excessive rainfall will linger into Monday across
portions of the Southwest US as precipitable water values of 0.75
to 1.00 inches remain in place from extreme southeast Arizona into
New Mexico. Scattered daytime storms remain possible within the
plume if deeper moisture...but anomalously strong steering flow
should help limit residence time to mitigate some of the flash
flooding concern. As mentioned earlier, although even brief heavy
rainfall rates in a short period of time may still lead to flash
flooding, and a Marginal Risk was maintained in the region.
Hurley
Day 2
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...
...Summary...
Have expanded the Marginal Risk southward across the Upper Midwest,
to account for the potential of locally heavy rainfall before the
frontal passage.
...Upper Midwest-Upper Great Lakes...
Upper shortwave will continue to dampen somewhat by Day 2, which
will weaken the deep-layer Qs forcing. Prior to the surface frontal
passage, PWs will remain between 1.75-2.00 inches (around 2
standard deviations above normal). Instability will be much greater
over the southern portion of the outlook area (down towards the
I-80 corridor, which was a big reason to nudge the ERO Marginal
Risk farther south (matching up with SPC's Day 2 Severe Slight
Risk). Expect the activity to move rather swiftly before dropping
south of the outlook area Tue night. Still appears to be a
localized flash flood risk though, given the weakening deep-layer
forcing and degree of downwind propagation. Therefore, have
continued with the Marginal Risk.
...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest US as a moisture plume
lingers across the area and winds aloft/steering flow weakens in
response to the a mid level wave moves away over the Northern
Plains as it begins to fill. Model based ingredients suggest the
potential for locally heavy downpours may extend a bit west of the
area shown by the deterministic runs...so made a minor expansion
that way.
From Mike Powell@618:250/1 to All on Tue Aug 27 09:03:00 2024
FOUS30 KWBC 270841
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
441 AM EDT Tue Aug 27 2024
Day 1
Valid 12Z Tue Aug 27 2024 - 12Z Wed Aug 28 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PARTS OF
THE SOUTHWEST AND UPPER MIDWEST...
...Summary...
Compared to yesterday's Day 2 ERO, we have expanded the Marginal
Risk area to include much of the Upper to Mid MS Valley, along with
parts of the Lower MO Valley. Only minor tweaks were made to the
Marginal Risk area across the Southwest.
...Lower Missouri Valley-Mid to Upper Mississippi Valley-Upper
Great Lakes...
Upper shortwave over the Northern Plains early this morning will
continue to dampen somewhat during the day, which in term will
weaken the deep-layer Qs forcing. By the same token, as the
shortwave becomes sheared, the confluence aloft will sharpen a
compact upper level jet streak (90-100 kts) in the lee of the
flattening shortwave, thereby increasing the Qn forcing within the
right entrance region (strengthening the low-level frontogenesis).
The guidance, including the CAMs, appears to be having a tough time
resolving this transition of favorable dynamical forcing. The
ensemble of model QPFs show a bimodal distribution with the max
rainfall axes -- one to the north (central MN-northern WI-Upper
MI), behind Monday night MCS, where the DPVA/Qs forcing is most
favorable, and another farther south (northeast KS/southeast NE,
southern IA, central-northern MO, much of IL-IN), where the uptick
in right entrance region forcing (upper divergence and low-level
frontogenesis) coincides with a more moist and unstable
environment. TPWs climbing between 1.8-2.1 inches over this
southern portion, along with ML CAPEs of 1500-2500+ J/Kg, will
support locally intense shorter-term rainfall rates. Moreover, as
the front flattens (becomes more W-E oriented), more parallel to
the mean 850-300 mb flow, suspect cell training will become a
greater risk. What will likely keep this event from needing a
Slight Risk is the relatively weak low-level inflow. The mean flow
is fairly weak as well (850-300 mb wind 15-20 kts), however there
is sufficient shear to keep any organized segments forward
propagating.
Farther north, the instability won't be as robust, however the
soils will have already been primed from the MCS Monday night.
Therefore with this secondary max QPF axis that the 00Z CAMs are
depicting, another round of heavy rainfall could lead to isolated
flash flooding given the antecedent wet soils and reduced FFG.
...Southwest US...
The risk of excessive rainfall will linger on Tuesday afternoon
and evening across portions of the Southwest U.S., as the moisture
plume lingers across the area and winds aloft/steering flow
weakens in the wake of the departing Northern Plains shortwave. PWs
peaking around 1.3 to 1.4 inches (1.5 to 2 standard deviations
above normal for late August), along with MLCAPEs of 500-1000 J/Kg,
will support isolated hourly rain rates of 0.75-1.00" underneath
the strongest convective cores. The 00Z HREF probabilistic
guidance depicts this, with the risk spreading a bit farther west
(into eastern AZ) compared to yesterday. Have maintained the
Marginal Risk in the Day 1 ERO given the continued localized flash
flood threat during the peak heating hours (afternoon/evening).
evening.
Hurley
Day 2
Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND
PARTS OF THE MIDWEST INTO THE OHIO VALLEY AND NORTHERN MID-ATLANTIC...
...Summary...
Compared to yesterday's Day 3 ERO, the Southwest and northern-tier
Marginal Risk areas were largely unchanged. However, based on the
00Z guidance suite, the Marginal Risk across the northern Mid
Atlantic Region was expanded westward to include much of the OH
Valley and parts of the Mid MS Valley.
...Southwest...
Guidance mid-level vorticity progs continue to support a more
north-south outlook area compared to Day 1, though nonetheless
still quite similar. Instability and moisture parameters (afternoon
and evening CAPE and PW profiles) remain similar, and thus the
anticipation of more isolated/localized instances of flash flooding
than otherwise, particularly after 18Z coinciding with peak
diurnal heating.
...Upper Midwest...
Strong, deep-layer warm and moisture advection ahead of the next
vigorous mid-upper level trough (closed 500-300 mb low) and
associated surface fronts will favor more organized/widespread
elevated convection by Wed night ahead of the advancing low-level
warm front. Models are still indicating spread with the QPFs,
enough to keep the ERO risk level at Marginal for now. Later
shifts will need to assess the need to upgrade at least a portion
of the outlook area to Slight however, as the heavy rainfall signal
exists given the degree of dynamical forcing and thermodynamical
profiles. 00Z ECMWF and regional GEM in fact indicates localized
totals of 3+ inches between 00-12Z Thu.
...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Northern Mid Atlantic...
W-E frontal boundary will sink southward on Wednesday, then remain quasi-stationary Wed night as the mid- upper trough traverses the central-northern Rockies and High Plains. Deep-layer moisture and
instability will be sufficient for areas of heavy rainfall along
this boundary, while the easterly storm motions (parallel to the
low-mid level fronts) would support the risk of cell training.
Outside of the 00Z Regional GEM, the guidance at this point isn't
overly robust with the QPF within the outlook area, at least not
yet, despite some uptick in right- entrance region forcing from the
upper level jet streak over northern NY-New England. Thus for now,
have kept the ERO risk at Marginal.
Hurley
Day 3
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
CENTRAL PLAINS INTO THE UPPER MIDWEST, ALONG WITH THE MID ATLANTIC REGION...
...Summary...
Few changes were made to yesterday's Day 4 ERO Marginal Risk areas
across the Central Plains to Upper Midwest and over the Mid
Atlantic.
...Eastern Portions of the Central Plains into the Upper Midwest... Aforementioned upper trough and associated surface cold front will
continue eastward on Thu-Thu night, though with the mid-upper low
lifting north into western Ontario by 12Z Fri. This will keep the
bulk of the deep-layer forcing north of the U.S., however
sufficient low-level frontogenesis will persist ahead of the mid-
upper shortwave well south of the main mid-upper low. Much
cooler/drier post-frontal airmass will lead to a fairly tight
moisture gradient along the front, with strengthening negative PW
advection behind it. Given the anticipated swift movement of the
front (strong deep-layer shear), for now it would appear any
organized, more widespread linear convective segments ahead of the
front will too be rather progressive. Therefore, have maintained
the Marginal Risk area.
...Mid Atlantic Region...
Lingering quasi-stationary boundary across the Mid Atlantic will
couple with increasing right entrance region forcing via an upper
level jet streak traversing NY-New England. Over time, the CAD
signature becomes evident from the guidance -- i.e. cooler/more
stable low-level air and a south-southwest dip in the surface
boundary occur along and near the Mid Atlantic Coast. However
plenty of deep-layer instability should linger west of the
DelMarVa, which along with the forcing, should allow for more widespread/organized convection and a localized flash flood threat.
From Mike Powell@618:250/1 to All on Wed Aug 28 09:25:00 2024
FOUS30 KWBC 280829
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
429 AM EDT Wed Aug 28 2024
Day 1
Valid 12Z Wed Aug 28 2024 - 12Z Thu Aug 29 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHWEST, NORTHERN PLAINS TO UPPER MISSISSIPPI VALLEY, AND
PARTS OF THE MIDWEST INTO THE OHIO VALLEY AND NORTHERN MID-ATLANTIC...
...Summary...
Few changes were made to yesterday's Day 2 ERO Marginal Risk
areas.
...Southwest...
Guidance mid-level vorticity progs continue to support a more
north-south outlook area compared to Day 1, though nonetheless
still quite similar. Instability and moisture parameters (afternoon
and evening CAPE and PW profiles) remain similar, and thus the
anticipation of more isolated/localized instances of flash flooding
than otherwise, particularly after 18Z coinciding with peak
diurnal heating.
...Eastern Dakotas-Western Minnesota...
Strong, deep-layer warm and moisture advection ahead of the next
vigorous mid-upper level trough (closed 500-300 mb low) and
associated surface fronts will favor more organized/widespread
elevated convection by Wed night ahead of the advancing low-level
warm front. After 00Z, 850 mb southerly flow increases to 40-50 kts
from west to east across the outlook area, which is around 4
standard deviations above normal per the SREF and GEFS. Not
surprisingly, 850-700 mb moisture flux anomalies also climb to +3
to +4. Given the strength of the low-mid layer moisture transport
and uptick in IVT, isolated QPF totals of 3-5" per some of the 00Z
CAM runs should not come as a surprise. However, at least for now,
reasons for keeping the ERO below the Slight threshold over this
region Wed night include (a) the continued high spread in guidance
QPFs (including CAMs), along with (b) progressive southeast motion
of the convection growing upscale (MCS) per the forecast Corfidi
Vectors, given the shear between the low-level flow and mean
850-300 mb wind.
...Eastern Portions of the Mid Mississippi Valley-Ohio Valley-
Northern Mid Atlantic...
W-E frontal boundary will sink southward on Wednesday, then remain quasi-stationary Wed night as the mid-upper trough traverses the central-northern Rockies and High Plains. Deep-layer moisture and
instability will be sufficient for areas of heavy rainfall along
this boundary, while the easterly storm motions (parallel to the
low-mid level fronts) would support the risk of cell training,
especially when the Corfidi Vectors drop to 5 kt or less). WPC QPF
and smart blends (NBM and HREF) have come up from the previous
forecast, with the D1 period now fully within the high-res CAM
window. Uptick in right-entrance region forcing (upper divergence...
low-level FGEN) from the upper level jet streak over northern NY-
New England will favor more widespread convection, especially when
coinciding with peak diurnal heating. However, lack of robust low-
level moist inflow/transport will likely not sustain/prolong strong
updrafts and intense hourly/sub-hourly rainfall rates. As a
result, have maintained the Marginal Risk from yesterday's Day 2
ERO, albeit with a few tweaks based on the latest guidance trends.
Hurley
Day 2
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
LOWER MISSOURI VALLEY INTO THE UPPER MISSISSIPPI VALLEY...
...Summary...
The biggest change from yesterday's Day 3 ERO was to upgrade a
portion of the Marginal Risk to Slight across parts of the Lower MO
Valley into the Upper MS Valley. Have also added a Marginal Risk
across the central-western LA and Middle-Upper TX Gulf Coast.
...Lower MO Valley and Corn Belt into the Upper MS Valley...
Aforementioned upper trough and associated surface cold front will
continue eastward Thu-Thu night as the mid-upper low lifting north
into western Ontario by 12Z Fri. Given the progression of synoptic
features, the bulk of the deep-layer forcing and more anomalous
low-level inflow/moisture flux will pivot across the Upper MS
Valley and southern Canada. Compared to the southern portions of
the Slight Risk area, generally speaking instability will not be
as high over the northern portion (i.e. along the path of the
triple point). However, owing largely to the SSW LLJ, especially as
it becomes more aligned with the mean 850-300 mb flow, the
eastward progression of the quasi-linear convective segments will
slow during the period as per the weakening Corfidi Vectors
(decreasing forward propagation). Therefore the risk for cell
training will increase along and ahead of the front.
Farther south (IA and eastern NE), the pre-frontal activity should
be more forward-propagating, however much higher deep-layer
instability south of the triple point, especially by late Thu-Thu
night, will maintain the risk of more intense hourly rainfall
rates (1.75-2.00+ inches) a little while longer (beyond 00Z Fri)
per the 00Z HREF exceedance probabilities.
...Mid Atlantic Region...
Lingering quasi-stationary boundary across the Mid Atlantic will
couple with increasing right entrance region forcing via an upper
level jet streak traversing NY-New England. Over time, the CAD
signature becomes evident from the guidance -- i.e. cooler/more
stable low-level air and a south-southwest dip in the surface
across MD-VA. However plenty of deep-layer instability should
linger west of the DelMarVa, especially west of I-95, which along
with the forcing, should allow for more widespread/organized
convection and a localized flash flood threat.
...Louisiana and Middle-Upper Texas Coast...
Underneath the broad upper ridge, models show a subtle mid-upper
level shortwave lifting gradually northward toward the central-
western LA Coast on Thu. Subtle upper-level forcing (DPVA/upper
divergence) along with persistent (albeit not overly robust) low-
level moisture transport from the GOMEX will make for a localized
flash food risk, especially coinciding with peak diurnal heating
Thu afternoon. PW values increasing to 2.3-2.4 inches along with
MLCAPEs between 1000-2000 J/Kg (tall/skinny distribution) and WBZ
heights climbing to aoa 15,000 feet will establish an optimal
thermodynamic profile for heavy rainfall. Sub-hourly rainfall rates
of 2-3" will be possible underneath the strongest cores. By the
same token, 0-6km bulk shear generally 20kts or less will lead to
more pulse-type convection and thus more short-lived intense
rainfall rates. Still enough of a flash flood risk to support a
Marginal Outlook area in the ERO, especially given the uptick in
southeasterly low-level flow from the Gulf on Thursday, which
would be of equal or greater magnitude compared to the 850-300 mb
wind. Corfidi Vectors aligning in the opposite direction than the
inflow would support increased upwind propagation and cell
training.
From Mike Powell@618:250/1 to All on Thu Aug 29 10:00:00 2024
FOUS30 KWBC 290823
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Thu Aug 29 2024
Day 1
Valid 12Z Thu Aug 29 2024 - 12Z Fri Aug 30 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE UPPER MIDWEST AND THE MID ATLANTIC AS WELL AS ALONG THE GULF COAST...
Lower Missouri Valley to the Upper Midwest...
Height falls associated with an upper trough and surface cold
front will continue eastward today and tonight as a deep mid-
upper low lifts north into western Ontario by 12Z Fri. Given the
progression of synoptic features, the bulk of the deep- layer
forcing and more anomalous low-level inflow/moisture flux will
pivot across the Upper MS Valley and southern Canada. HREF
neighborhood probabilities for greater than 2 inch and greater than
3 inch amounts remain fairly aggressive (40-70%) within the
previously issued Slight risk area with the core of the potential
being located over parts of Iowa and southern Minnesota. As
mentioned before...this area has been dry as of late, so some of
the precip will be beneficial, but anomalous moisture and strong
mid- level ascent could generate rainfall rates capable of
flooding, especially within any urban footprint.
Mid Atlantic Region...
Maintained the Slight risk area across the VA Piedmont down into
portions of Southeast VA and as far north as the WV/MD/PA line with
only minor modifications. There will be a fairly well-depicted
axis of low- level instability and anticipated surface convergence
in proximity to a backdoor front settling in to the area. A core
of elevated theta-E will align with the front from northwest to
southeast during the afternoon hours while a jet streak to the
northeast catches the Central Mid Atlantic just within the right-
entrance region of the jet leading to appreciable upper forcing
during peak destabilization. Convective activity is expected to initiate
across the Blue Ridge and adjacent Appalachian Front then drift
eastward. various CAMs and global guidance. MLCAPE between
2500-3500 J/kg will be common within the vicinity of the front, so
ample surface based instability will be present to maximize
potential of any thunderstorm development.
Louisiana and Middle-Upper Texas Coast...
Models show a subtle mid-upper level shortwave lifting gradually
northward toward the central- western LA Coast today and tonight.
Subtle upper- level forcing along with persistent (albeit not
overly robust) low- level moisture transport from the Gulf of
Mexico will result in a localized flash food risk, especially
coinciding with peak diurnal heating . Precipitable water values
increase to 2.3-2.4 inches along with MLCAPEs between 1000-2000
J/Kg and WBZ heights climbing to aoa 15,000 feet will establish an
optimal thermodynamic profile for heavy rainfall as shown in the
NAM and ARW guidance. This should result in rainfall rates of 2-3
inch on a sub-hourly time scale will be possible underneath the
strongest cores. Given that the overall forecast reasoning has not
changed and the latest and the latest guidance continues to show 3
to 5+ inch amounts hugging the coastline...saw little reason to
make more than minor nudges to the Slight risk and Marginal risk
areas.
Bann
Day 2
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE
GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA ...
Gulf Coast...
There is a Slight Risk of excessive rainfall along the immediate
Gulf coast region of southeast Texas into a small portion of
adjacent Louisiana...with a Marginal risk area extending as far
north as the upper Great Lakes in response to gulf moisture
streaming northward ahead of a well-defined cold front advancing
eastward. In addition...a well defined circulation that has
persisted a couple of days over the Gulf of Mexico and Texas will
meander eastward into Arkansas. While its impact will be waning as
the system fills...it still could allow for additional convective
flare-ups along and near the Gulf of Mexico. Precipitable water
values of 2 to 2.25 inches are widespread from eastern Texas into
Louisiana and Arkansas with an axis then extending north and east
ahead of the front being some 2 to 2.5 standard anomalies above
climatology being an indication of the moisture-laden atmosphere
still in place. Discrepancies linger in the guidance on placement
and amounts in where the deepest moisture gets funneled...with the
above- mentioned upper level feature playing a key role. Between
the NBM still showing 2 to 4+ inch amounts along the southeast
Texas coast on Day 2 alone and a more than 6 inches for a 2-day
total in proximity to the 28/12Z guidance and already covered by
yesterday's Day 3 outlook...saw little reason to make any changes
to the Slight Risk area.
Great Lakes to Southern Rockies/Southern High Plains...
Convection will continue to be driven by deep gulf moisture that is
initially drawn northward and then northeastward towards the Great
Lakes ahead of a well defined cold front pushing southward and
eastward. By 00Z Saturday...moisture anomalies of 2 standardized
anomalies will be in place. The overall upper forcing still does
not appear to be terribly robust but the diurnal instability
supports at least the potential for locally excessive rainfall.
Bann
Day 3
Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG A SMALL
PORTION OF THE UPPER TEXAS COAST AND ADJACENT COASTAL LOUISIANA...
There is a Slight risk of excessive rainfall once again over
portions of the Upper Texas coast and adjacent portions of
Louisiana with a broad Marginal risk are extending northward and
eastward as gulf moisture streams northward ahead of...and
eventually intersects with...a cold front. The 29/00Z suite of
guidance focuses highest rainfall amounts from Saturday into early
Sunday over portions of the Upper Texas coast and nearby Louisiana
where a moisture plume with precipitable water values in excess of
2.25 inches should be positioned at the start of the period (a
nearly 2.5 standardized anomaly greater than climatology for this
time of year and above the 90th climatological percentile along the
gulf coastline shown by both the ECENS and GEFS). Two inch
precipitable water values are forecast to extend from eastern Texas northeastward in a corridor to far southern Indiana. During the
ensuing 24 hours...the northern portion of the front reaches
portions of New England and the Mid-Atlantic region but the
southern portion of the front makes little progress into Texas and
Arkansas. Expectations are that downpours will be possible along
and ahead of the front. Given the amount of moisture and weaker
steering flow, though, the southern portion of the Marginal area
has a better chance for excessive rainfall while storms to the
north should be more progressive leading to a nominally low/more
isolated excessive rainfall risk. The question remains whether or
ot the stronger dynamics farther north will enhance amounts/rates.
Will leave as a Marginal risk area for the time being in deference
to the NBM amounts and guidance from the CSU ML EROs.
From Mike Powell@618:250/1 to All on Fri Aug 30 10:06:00 2024
FOUS30 KWBC 300826
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Fri Aug 30 2024
Day 1
Valid 12Z Fri Aug 30 2024 - 12Z Sat Aug 31 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG THE IMMEDIATE
GULF COASTAL AREAS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...
...Gulf Coast...
Moderate to heavy rainfall expected along the Gulf coast as deep
moisture advects northwards as a tropical low drifts in the
northwest portions of the Gulf of Mexico. Repeated rounds of slow-
moving heavy rain cells from the Gulf into the Texas/Louisiana
border area east of Houston. Guidance depict 1 to 4 inches for
areal average along the coast with isolated higher amounts
possible. Some of the hi-res guidance have very isolated maximums
of 6 to 11 inches. Areal averages of The focus for any potential
flooding in this small area will be mainly in the urban areas
around Beaumont/Port Arthur and points east. A Slight Risk is in
effect from the Houston area east to Vermillion Bay. The
surrounding Marginal risk was expanded to include areas near/west
of Mobile.
...Great Lakes to Southern Rockies/Southern High Plains...
Convection will continue to be driven by deep gulf moisture that
is streaming into the north/northeast towards the Great Lakes
ahead of a well defined cold front pushing southward and eastward.
Precipitable water values of 2 to 2.25 inches are widespread from
eastern Texas into Louisiana and Arkansas with an axis then
extending north and east ahead of the front being some 2 to 2.5
standard anomalies above climatology being an indication of the
moisture-laden atmosphere still in place. There continues to be
some variance on where the deepest moisture gets funneled... with
the above- mentioned upper level feature playing a key role.
Although the overall upper forcing still does not appear to be
terribly robust, the diurnal instability supports at least the
potential for locally excessive rainfall. A broad Marginal Risk was
maintained for this period from New Mexico/Texas to Wisconsin/New
York.
...Mid-Atlantic...
Portions of the region will have PW values greater than 2 inches
along with slow moving storms near the front. This environment
looks to be quite favorable for localized heavy rain with some of
the guidance showing probabilities of 3 inches/3 hour 30-50%
across Northeast North Carolina. A Marginal Risk area was raised
for portions of southeast Virginia and northeast North Carolina.
Campbell
Day 2
Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ALONG A SMALL
PORTION OF THE UPPER TEXAS COAST AND ADJACENT COASTAL LOUISIANA...
...New Mexico and Texas...
Showers and thunderstorms are expected to fire across portions of
New Mexico and Texas as the trailing cold front slows its
progression over the region. Some of these storms will be capable
of producing hourly rain rates greater than 0.50/1 inch per hour.
A Marginal Risk area was maintained for this part of the country.
...Gulf Coast...
Showers and thunderstorms will persist along the Gulf coast along
the nearly stationary tropical low and frontal boundary. The Slight
Risk area that was in effect has bee maintained as well as expanded
eastward to just east of Vermillion Bay. A broad Marginal risk area
extends northward and eastward as gulf moisture streams northward
ahead of...and eventually intersects with...a cold front. Guidance
shows the highest amounts occurring from Saturday into early
Sunday over portions of the Upper Texas coast and nearby Louisiana
where a moisture plume with precipitable water values in excess of
2.25 inches should be positioned at the start of the period (a
nearly 2.5 standardized anomaly greater than climatology for this
time of year and above the 90th climatological percentile along the
gulf coastline shown by both the ECENS and GEFS).
...Lower Mississippi Valley to New England...
Two inch precipitable water values are forecast to extend from
eastern Texas northeastward in a corridor to far southern Indiana.
During the ensuing 24 hours...the northern portion of the front
reaches portions of New England and the Mid-Atlantic region but the
southern portion of the front makes little progress into Texas and
Arkansas. Moderate to heavy rain will be possible for areas along
and ahead of the advecting boundary.
Given the amount of moisture and weaker steering flow, though, the
southern portion of the Marginal area has a better chance for
excessive rainfall while storms to the north should be more
progressive leading to a nominally low/more isolated excessive
rainfall risk. The question remains whether or ot the stronger
dynamics farther north will enhance amounts/rates. Some of the
guidance do signal a concentration of QPF and increasing threat for
excessive rainfall over portions of central pennsylvania.
Antecedent conditions will have lowered FFGs across this part of
the region thus becoming more favorable for growing flooding
concerns. After coordination with the local forecast office decided
to hold off for another cycle to reassess the need for a possible
upgrade to a Slight Risk for central portions of Pennsylvania.
Campbell/Bann
Day 3
Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024
...New Mexico and Texas...
Showers and thunderstorm activity is expected to linger with the
capability of producing excessive rainfall. The threat for
localized flood concerns will remain elevated thus a Marginal Risk
was maintained for this period.
...Gulf Coast...
The highest rainfall in this multi-day heavy rain event will have
likely already occurred however showers and thunderstorms are
expected to linger over the region near the coastline. A Marginal
Risk remains in affect from the southeast Texas coastline to
eastern Louisiana coastline.
...Eastern Tennessee Valley and Mid-Atlantic region...
Moderate to locally heavy rainfall will persist across portions of
the Mid-Atlantic as a cold front approaches the region. Some of
these areas will have lowered FFGs by this period and may be more
susceptible for additional rainfall to increase the threat for
local flooding concerns. A Marginal Risk area was maintained from
eastern Tennessee Valley to the Mid-Atlantic region.
From Mike Powell@618:250/1 to All on Sat Aug 31 09:44:00 2024
FOUS30 KWBC 310823
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Sat Aug 31 2024
Day 1 Valid 12Z Sat Aug 31 2024 - 12Z Sun Sep 01 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE GULF COAST
FROM GALVESTON TEXAS THROUGH THE MISSISSIPPI COAST AS WELL AS FOR
THE CENTRAL APPALACHIANS...
...Texas to the Mid-Atlantic and New England...
A slow-moving cold front over the Midwest will move into the
central Appalachians this afternoon. Surface dew points in the 60s
and rising precipitable water values to around 1.75 inches (+2
sigma or 98th percentile) will support both a severe weather threat
(see SPC outlook) and a heavy rain threat. Though convection
should be progressive with the front (or pre-frontal trough), FFG
values are around 1-2"/hr and 00Z HREF neighborhood probs show
values rising to 40-80% for >1"/hr and 10-40% for >2"/hr. Focus of
highest threat will lie from eastern WV northward through central
PA and into the Southern Tier of NYS where the complex terrain can
add to the flash flooding sensitivity coincident with slightly
higher than average soil moisture. Maintained the Slight Risk from
the previous forecast with little change in shape. Along the front
in general, a Marginal risk was maintained from New England back
through the mid-MS Valley/Mid-South into TX where convection will
be a bit less coherent and the threat for any flash flooding will
be lower, but non-zero.
...Central Gulf Coast...
Decaying mid-level vort max will drift eastward along the coast,
detached from any surface trough just offshore. However, in-situ
moisture remains highly anomalous -- precipitable water values
around 2.25 inches which is about +2 to +3 sigma. Most of the hi-
res guidance keeps the heaviest rainfall today along and just off
the coast but hint at drawing some heavier cores inland a bit
toward the I-10 corridor in LA. Maintained the Slight Risk outline
for southern Louisiana given the continued threat of heavier rain
today.
...Florida...
Above normal moisture (PW > 2") will linger over southeast FL
where afternoon convection could support some local downpours
(2-3"/hr rates) that may induce some flooding over the urban areas
from ~PBI to MIA/HST and perhaps into the Keys.
...Arizona...
Added a Marginal Risk for the Mogollon Rim as a weak mid-level
vort center pushes through the region. Though precipitable water
values are around normal, sufficient instability this afternoon
could support some isolated heavier rain cores with 1"/hr rates.
Fracasso
Day 2 Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC...
...Southeastern New Mexico and West Texas...
250mb upper trough moving out of northern Mexico and southern
AZ/NM and mid-level vort max (perhaps weakly closed low) will creep
closer to far western Texas day 2. Sufficient moisture (though
near normal) will be in place over the region to sustain some
isolated excessive rainfall via afternoon convection.
...Gulf Coast...
Meandering weakness in the mid/lower-levels will again help drive
some heavier rainfall along the Upper TX Coast into southern
Louisiana. Maintained a Marginal Risk for now given the uncertainty
in the day 1 period with respect to heavier rain placement (on or
offshore) and amounts. Again, the highest rainfall may linger just
off the coast but potential is certainly there to nudge inland.
...Eastern Tennessee Valley and Mid-Atlantic region...
Cold front will continue to push through the area to the east and
south, squeezing the best forcing into southern Virginia and into
North Carolina where afternoon convection may yield 1-2" amounts
and a localized threat of flash flooding. There and to the west
over western VA into WV, FFG guidance is lower and some lingering
rain may eclipse those values as the front works through.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS AND
THE CAROLINAS...
...Texas...
Mid-level vort and upper trough will move into West Texas on
Monday atop increased moisture from the southeast. Guidance
continues to show modest QPF of 1-5" in various locations but the
spread is quite large -- nearly covering the whole state in their
QPF max region among the deterministic, ensemble, and AI guidance.
The CSU machine learning first guess fields suggest a broad Slight
Risk over the Hill Country which is, perhaps, a best consensus
placement, but prefer to hold off an upgrade to a Slight Risk for
now until at least the ensembles come into better agreement.
...Carolinas...
Cold front will sink into the Carolinas which will nudge the
excessive rainfall threat south from day 2. Best dynamics move
offshore with the trough axis but the Marginal Risk should cover
any isolated flooding threat due to afternoon convection.
From Mike Powell@618:250/1 to All on Sun Sep 1 09:54:00 2024
FOUS30 KWBC 010838
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
438 AM EDT Sun Sep 1 2024
Day 1
Valid 12Z Sun Sep 01 2024 - 12Z Mon Sep 02 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NEW MEXICO/TEXAS, THE NORTHERN GULF COAST, AND THE MID-ATLANTIC...
...Southeastern New Mexico and West Texas...
An approaching 250 mb upper trough will move into far West Texas
today while a surface boundary lingers across the Hill Country.
Precipitable water values are forecast to be around 1.5-1.75" which
is about +1 to +2 sigma with the typical NW to SE gradient.
Afternoon convection should percolate today and could fall over
areas that have seen a few inches of rain on Saturday (with lower
FFG values as a result). Coverage should be somewhat isolated so
the Marginal Risk will suffice.
...Gulf Coast...
Meandering weakness in the mid-lower levels will again help drive
some heavier rainfall along the Upper TX Coast into southern
Louisiana. Maintained the Marginal Risk as the bulk of the guidance
keeps the heaviest rain offshore, though it will be close (again)
just like on Saturday.
...Eastern Tennessee Valley and Mid-Atlantic Region...
A cold front will approach the Appalchians this afternoon as the
lead boundary dissipates, leaving a trough over the coastal plain.
The best forcing for afternoon convection will lie there, generally
over far southeastern Virginia into central/eastern North Carolina.
00Z HREF probs of exceeding FFG values are around 10-20% (via 1-2
inch per hour rates), though there are hints at some higher rates
still. Activity should be mostly progressive, suggesting a
localized threat of flash flooding. Kept the Marginal Risk for this
area.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
HILL COUNTRY OF TEXAS...
...Texas...
Mid-level vort and upper trough will move into West Texas on
Monday atop increased moisture from the southeast. Guidance
continues to show modest QPF of 1-5 inches near and around the Hill
Country, with various foci in the deterministic models. ECMWF-AIFS
was still decidedly north compared to the other models. The CSU
machine learning first guess fields still suggest a broad Slight
Risk over the Hill Country, so at least there is some consistency
over the past 24-36 hours in this guidance vs the models (which is
the point). FFG values are lowest near/south of San Angelo to Del
Rio (due to recent rain) where the HREF probs of exceeding these
values reach ~50%.
...Carolinas...
Cold front will sink farther into the Carolinas from day 1, which
will nudge the excessive rainfall threat south as well. Best
dynamics move offshore with the trough axis so the Marginal Risk
should cover any remaining isolated flooding threat due to
afternoon convection.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TEXAS...
...Texas...
Mid-level vort will be exiting the region though precipitable
water values will remain high (+1 to +2 sigma initially, then
decreasing late Tuesday). Depending on how much (and where) rain
falls on day 2, there is room to upgrade some of this region to a
Slight Risk pending better model agreement in future runs. For now,
ensemble consensus is just east of the Hill Country but individual
models are quite varied north/south. First guess fields from the
CSU machine learning algorithm do show a Slight Risk outline,
lending credence to a future upgrade.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE HILL COUNTRY/CENTRAL TEXAS AND ALONG THE TEXAS GULF COAST...
...Texas...
An upper level trough with a mid-level vort will be advancing into
western Texas during this period. Meanwhile a surface low hugging
the South Texas Coast will maintain easterly flow from the Gulf
allowing PW values of 2+ inches to pool over much of the state. The
latest guidance continues to depict areal averages of 1 to 5 inches
of QPF across the Hill Country into east-central Texas and along
much of the Texas coastline. Recent rainfall has increased soil
saturation across the state thus lowering FFGs. The hi-res guidance
is showing training of slow moving convection capable of 1 to 2.5
inches/hour rain rates which could easily lead to scattered areas
of excessive rainfall and localized flash flooding. A Slight Risk
areas remains in effect for west-central and central Texas.
Additionally, a Slight Risk was added for the coastline today where
storms will be producing rates of 1 to 3 inches/hour.
...Carolinas...
A cold front sinking through the Mid-Atlantic region will continue
to move showers and thunderstorms southward over the Carolinas
today. Guidance keeps the bulk of the QPF offshore during this
period where there are better dynamics in place however there will
still be an isolated threat for localized excessive rainfall and
flooding. A Marginal Risk area was maintained for eastern portions
of the Carolinas.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL TEXAS...
...Texas...
The mid-level vort is expected to exit the region although
precipitable water values will remain high (+1 to +2 sigma
initially, then decreasing late Tuesday). A large portion of the
flooding potential will be contingent on a verified forecast from
Day 1/Monday, as Monday will have the heavier rain/stronger storms
of the two days. However, even the lesser amounts of rain expected
Tuesday should still cause flooding issues in the area with a
widespread 1 to 1.5 inches forecast with locally higher amounts. A
Slight Risk remains in effect for central portions of the state.
The heaviest rains in the state are expected to be along the Gulf
Coast from Galveston south and west through Corpus Christi. Locally
heavy rains to 2 inches per hour will be possible. For now the
greatest flooding threat remains confined to any urban areas, but
should Monday's rains overperform here then a Slight may be needed
for the hardest hit areas with future updates.
...Idaho/Montana...
A potent shortwave trough will interact with up to 2 sigma above
normal atmospheric moisture on Tuesday. PW values rising to near 1
inch will support storms capable of heavy rain that may produce
isolated flash flooding should the storms impact particularly flood
sensitive or more urban areas.
The influx of anomalous Gulf Moisture will persist all across the
southern tier states while the upper trough and vort exits the
plains and into the Mississippi Valley. The surface front will
continue to be draped west to east across the South continuing to
provide focus for showers and thunderstorms across the region.
During the late afternoon and evening hours there will be an
intensification of instability and forcing to boost rainfall rates
across the Lower Mississippi Valley, particularly over Louisiana,
western Mississippi and southeast Arkansas. Most of the guidance
are suggesting that 1 to 3 inches will concentrate over this part
of the region thus increasing the risk for excessive rainfall and
local flooding potential. There is the potential for a few
locations for local maximums to exceed 6 inches near northern Louisiana/Arkansas. Onshore flow across southeast Louisiana and
southern Mississippi could yield 1 to 4 inches as well. A Slight
Risk area was raised for parts of Louisiana, Mississippi and
Arkansas.
Campbell
d
--- SBBSecho 3.20-Linux
* Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)
From Mike Powell@618:250/1 to All on Tue Sep 3 08:37:00 2024
FOUS30 KWBC 031328
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
928 AM EDT Tue Sep 3 2024
....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTH TEXAS...
...13Z Outlook Update...
...Texas...
Slight risk areas were combined and expanded to include more of
south Texas (near the Del Rio area). This morning, slow-moving
convection has materialized and produced areas of 1-2.5 inch/hr
rain rates just east of the Rio Grande. These storms are in a
sufficiently moist/unstable enviornment with weak flow aloft for
continued heavy rainfall. Low FFG thresholds (near zero in spots)
were also noted. The flash flood threat should persist for most of
the D1 forecast period under this regime. Reference MPD #0968 for
additional mesoscale details.
...Southeastern Mississippi eastward to northern Florida...
Latest CAMs indicate a robust axis of convection materializing amid
weak flow aloft from the Pine Belt of Mississippi eastward to the
Jacksonville Metro area. HREF probabilities of 24-hr, 3 inch+
precipitation totals maximize near Jacksonville, where models
depict a persistent fetch of deep convection originating along a
frontal boundary over the Gulf Stream. Multiple rounds of
precipitation are expected today through the early overnight
hours especially across northeastern Florida. Localized flash
flood potential is expected where rainfall can occur over
sensitive/urbanized locales. The Marginal risk has been expanded
northwestward and eastward to include areas of isolated flash flood
potential.
The remainder of the forecast is unchanged. Refer to the previous
discussion below for more details.
Cook
...Previous Discussion...
...Texas...
The mid-level vort is expected to exit the region although
precipitable water values will remain high. Flooding issues will be
possible in the area with a widespread 1 to 1.5 inches forecast
with locally higher amounts. A Slight Risk remains in effect for
central portions of the state although it was expanded y 1 tier of
counties to the south to reflect the latest trends in the guidance.
A slow moving tropical low will continue to bring periods of heavy
rain in the form of slow- moving showers and thunderstorms from
the Gulf into the adjacent Texas Coast. Most areas will not see
steady rain the entire time, but very localized areas of training
storms resulting in flash flooding. The heaviest rains in the
state are expected to be along the Gulf Coast from Galveston south
and west through Corpus Christi. Locally heavy rains to 2 inches
per hour will be possible.
...Mississippi and Alabama Gulf Coast...
The Marginal Risk was maintained for the potential of slow moving
and training storms to the east of the low along a stalling front
into coastal Mississippi and Alabama. The abundance of atmospheric
moisture and instability, as well as, the slow-moving nature of
the synoptic systems increases confidence that an isolated flash
flood or two could develop along this stretch of coast.
...Idaho/Montana...
There is potential for isolated to scattered heavy rainfall as an
advancing shortwave encounters an anomalously moist airmass. PW
values rising to near 1 inch will support storms capable of heavy
rain that may produce isolated flash flooding should the storms
impact particularly flood sensitive or more urban areas.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI
DELTA REGION AND ADJACENT GULF COAST...
...Gulf Coast and Lower Mississippi Valley...
The surface front will continue to be draped west to east across
the South, albeit further south than previous forecast cycles. This
frontal boundary will provide focus for showers and thunderstorms
to spread across the entire Gulf Coast from south Texas all the
way to the Florida Peninsula on Wednesday. Most of the guidance
shows 1 to 3 inches will concentrate over this part of the region
thus increasing the risk for excessive rainfall and local flooding
potential.
There is the potential for a few locations for local
maximums to exceed 6 inches near northern Louisiana/Arkansas.
Onshore flow across southeast Louisiana and southern Mississippi
could yield 1 to 4 inches as well. The focus for the heaviest rains
will be in the central Gulf Coast from Louisiana east through the
Florida Panhandle. Locally heavy rains are likely, especially from
New Orleans east through Pensacola, where a higher-end Slight is in
place. With increasing amounts of rain forecast in and around New
Orleans, continued increases in rainfall there may require an
eventual targeted Moderate Risk should trends continue.
Given the trends, the back edge of the Marginal Risk was trimmed
eastward toward the Texas coast in addition to reducing the
northern boundary by 1 tier of counties across Alabama and
Mississippi.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL
GULF COAST...
...Gulf Coast and Lower Mississippi Valley...
The area of precipitation mentioned for the Day 2 period will
shift eastward across the central Gulf Coast with the heaviest QPF
still focusing across portions of southeast Louisiana, southern
Alabama, southern Mississippi and clipping western portions of the
Florida panhandle. Again, there was a notable southern shift in
placement within a majority of the guidance from previous
cycles.coastlineThe inherited Marginal Risk areas was removed
from southern Tennessee and northern portions of Alabama,
Mississippi and northwest Georgia while trimmed eastward across
central Louisiana. The Slight Risk spans along the Gulf Coast from
west-central Louisiana to eastern Choctawhatchee Bay Florida.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI
DELTA REGION AND ADJACENT GULF COAST...
...Gulf Coast and Lower Mississippi Valley...
The surface front will continue to be draped west to east across
the South provide focus for convection to spread from south Texas
to Florida Peninsula. The latest guidance has continued to favor a
less progressive eastward advancement with the precipitation
shield. A blend of CAMs/global guidance has shifted the focus for
heavier rainfall to be along the Upper Texas coast where they will
be located closest to an organizing and elongated area of low
pressure in the western Gulf of Mexico. General consensus has 2 to
5 inches along the coast between Corpus Christi and Lake Charles. Southeast
TX also still sports >80% soil moisture percentiles within the
0-40cm layer according to NASA SPoRT-LIS, suggesting soils there
would be more sensitive to 2-3"/hr rainfall rates.
Onshore flow across southeast Louisiana and southern Mississippi
could yield 1 to 2 inches as well. Locally heavy rains are likely,
especially from New Orleans east to Mobile. For the Florida
Panhandle there will be an influx of anomalous PWs and modest
instability along the stalled frontal boundary that may trigger
additional thunderstorms capable of producing torrential downpours.
Areas most at risk for possible flash flooding will be areas whose
soils are more saturated following Tuesday's thunderstorm activity
and as far east as the Jacksonville metro area.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL
GULF COAST...
...Gulf Coast and Lower Mississippi Valley...
The latest guidance keeps a slower progression of the QPF
associated with an elongated area of low pressure in the northwest
Gulf of Mexico. As such, the Slight Risk area was expanded further
west along the Gulf Coast, between Houston and Corpus Christi, to
account for the westward trend. While dispersion is on the higher
side on Day 2, there is a better consensus for heavier rainfall
along the central Gulf Coast given the bulk of PWs >99th
climatological percentile residing along the Gulf Coast. The region
is also closer to the surface-850mb southeasterly flow that will
sustain a steadfast regime of low level theta-e advection into the
nearby stationary frontal boundary. Additional forecast changes are
likely given so much of this setup lies with the development of
low pressure along the Gulf Coast.
...New Mexico and Colorado...
Convection is expected to fire up ahead of an approaching cold
front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE.
Soils across this part of the region are sensitive given recent
moisture and burn scars. The Marginal Risk area was maintained for
the localized flash flood potential for the afternoon and evening.
Campbell/Mullinax
Day 3
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND EASTERN GULF COAST...
...Gulf states and Southeast...
The frontal boundary is expected to slowly progress eastward during
this period will draped along the Gulf Coast and the Southeast.
Deep moisture will continue to override the boundary helping to
concentrate the heaviest QPF along the central and eastern Gulf
coast. A Slight Risk remains in effect from southeast Louisiana to
the Florida Panhandle while a Marginal Risk area spans from south-
central Louisiana to northeast Florida and northward to South
Carolina. Areal averages of 1 to 2 inches will be common across the
region with the possibility of very isolated maxes of 3 to 5
inches, especially along the coastline.
...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL ALONG THE
CENTRAL GULF COAST...
...Gulf Coast, Lower Mississippi Valley and Southeast...
In coordination with local forecast offices near the Mississippi
Delta region, a Moderate Risk area was hoisted for this period to
include portions of southeast Louisiana and southern Mississippi. There
is consensus for heavier rainfall along the central Gulf Coast
given the bulk of PWs >99th climatological percentile residing
along the Gulf Coast. The region is also closer to the
surface-850mb southeasterly flow that will sustain a steadfast
regime of low level theta-e advection into the nearby stationary
frontal boundary. Additionally, the latest guidance shows a
potential for locally higher amounts upwards of 10" per NBM 95th
percentile to focus in the vicinity of the New Orleans metro and
points east and south. A Slight Risk spans from the southeast Texas
coast to southern Mississippi.
Deep-layer instability across portions of the Southeast may be
lacking, however PWs of 2.2-2.4" will make for highly-efficient
rainfall rates as the quasi- stationary front shifts a bit
northward by tonight as the upper shortwave trough dips across the
western Gulf Coast. A Marginal Risk spans from extreme southern
South Carolina to central Florida and westward to the central Texas
coast.
...New Mexico and Colorado...
Convection is expected to fire up ahead of an approaching cold
front thanks to PWs topping 0.75" and up to 500 J/kg of MUCAPE.
Soils across this part of the region are sensitive given recent
moisture and burn scars. The Marginal Risk area was maintained for
the localized flash flood potential for the afternoon and evening.
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL AND EASTERN GULF COAST...
...Gulf states and Southeast...
During this period the trough will be digging across the western
Gulf Coast region and increasing (and persistent) low-level
frontogenesis along the central-eastern Gulf Coast to the Georgia
Coast. While narrowing with time, the corridor of favorable
thermodynamics (PWs of 2.2 to 2.5" with tall/skinny CAPE profile)
and low-level FGEN along the quasi-stationary surface boundary will
favor the more intense short term rainfall rates. This is
essentially where the Slight Risk was drawn, which also correlates
well with the highest 24hr QPF exceedance probabilities from the
NBM.
While there is some spread on exactly where the highest QPF will
focus there is a general consensus for 1 to 3 inches across the
region. Local maxes may be closer to with the possibility of very
isolated maxes of 4 to 7 inches, especially along the coastline.
The NAM, although hedging toward being an outlier solution for
this period, does show isolated maximums closer to 10 inches. At
this time, a Slight Risk seems to adequately cover the level of
threat for excessive rainfall and flooding concerns. There may be
the need to upgrade with a targeted Moderate Risk along the central
Gulf Coast should guidance continue to trended up.
Campbell/Hurley
Day 3
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE EASTERN GULF COAST AND SOUTHEAST ALONG WITH PORTIONS OF THE NORTHEAST...
...Gulf Coast and Southeast...
Slow moving convection will continue to focus along the draped
frontal system along the Gulf Coast and Southeast U.S. Abundant
Gulf moisture will continue to feed into the region leading to
higher rainfall rates. Multi-day rains will have increased soil
saturation and lowered flash flood guidance by this period thus
there will be a continued threat for excessive rainfall and local
flooding concerns. A Marginal Risk is in effect across northern
Florida and southern Georgia.
...Northeast...
An upper trough/low will pivot across the region during this
period sending showers and thunderstorms across the Northeast where
multiple recent rains have increased soil sensitivity. A Marginal
Risk area is in effect for eastern New York, northeast
Pennsylvania, northern New Jersey and northwest Vermont.
From Mike Powell@618:250/1 to All on Fri Sep 6 08:10:00 2024
FOUS30 KWBC 060804
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Fri Sep 6 2024
Day 1
Valid 12Z Fri Sep 06 2024 - 12Z Sat Sep 07 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR SOUTHEASTERN
LOUISIANA...
...Gulf Coast & Southeast...
A very slow moving tropical low will track east along a stationary
front draped along the Gulf Coast today. Near record amounts of
atmospheric moisture in place across the Gulf will be drawn
northward into the front ahead of the low. PWATs currently are
around 2.6 inches and will not change much through the day. This
would be over 3 sigma above normal for this time of year...which
for southeastern Louisiana is no small feat.
The southerly flow ahead of the low will keep a steady supply of
deep Gulf moisture tracking into the Gulf Coast primarily...but
will spread east across portions of the Southeast in the Slight
Risk. The big question about heavy rain will be how much
instability will be available for any showers and thunderstorms.
Since it's currently raining over much of Louisiana and will
continue into tonight, any instability will need to be advected in
to the Gulf. That said, given the near record levels of moisture
available, it will not take much to result in 2+ inch per hour
rates with the strongest convection. The area's soils are quite
water-logged from multiple days of rains, so today's rainfall
should fully convert to runoff...greatly increasing the flooding
threat.
In coordination with LIX/Slidell, LA forecast office, the Moderate
Risk area was left unchanged with this update, with the greatest
threat for the greater New Orleans and Baton Rouge areas due to
lower FFGs. The Slight was nudged a row of counties north, and
removed from most of northeast Florida. Due to recent heavy rains
across much of the Peninsula however, the Marginal risk was
expanded to include the Orlando, Tampa, and Ft. Myers metros.
...New Mexico...
A small low-confidence Marginal Risk area was added for the
Sacramento Mountains of south-central New Mexico with this update.
Afternoon showers and thunderstorms may form right over the very
flood prone burn scars near Ruidoso, which could cause local flash
flooding once again. However, it would require just that scenario
to unfold for flooding to occur which is low confidence since the
storm would have to form right over the correct spot, a microscale
challenge that's nearly impossible to know for sure well in
advance. The pattern supports one more day of area showers and
storms, prompting the localized upgrade.
Wegman
Day 2
Valid 12Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
EMERALD COAST OF THE FLORIDA PANHANDLE...
...Southeast...
The stationary front from Friday will drift southeastward across
the Southeast on Saturday. For the Central Gulf Coast, it would
probably be more accurate to call it a cold front, as the front
will make steady progress across southern Louisiana and into the
northwestern Gulf. Meanwhile across the Southeast, the front will
stall out over the Florida Panhandle, southern Georgia and up the
Carolina coast. It's in this region where training storms may cause
localized flash flooding. Moisture will not be a problem at all as
PWATs will be from 2 inches in the Carolinas to nearly 2.5 inches
in the Florida Panhandle. Thus, just as on Friday, the big limiting
factor for heavy rainfall will be instability, as very slow moving
fronts tend to be very lacking in that department due to extensive
cloud cover.
In coordination with TAE/Tallahassee, FL forecast office, a Slight
Risk area was introduced with this update. Storms are likely to be
robust rain makers out over the northern Gulf on Saturday. With
very weak steering flow and limited instability inland however,
the area with greatest likelihood for the storms to impact land
will be around the Emerald Coast of the Florida Panhandle. The most
likely hazard area was highlighted in the Slight. The expectation
is that any storms that move into the Emerald Coast will greatly
struggle to maintain the heavy rainfall rates more than a few miles
inland from the coast, so the Slight is only for the immediate
coast.
The Marginal Risk area was expanded several rows of counties north
with this update, while in the Carolinas it was more like a couple
states. The primary driving factor for these changes is the slowing
of the front in recent model runs. Thus, the rain may persist for a
bit longer than expected into Saturday morning on the northern edge
of the Marginal Risk area. Meanwhile Saturday will be another day
with potential strong thunderstorms capable of heavy rains across
the northern Florida Peninsula. This hard-hit area is likely to see
multiple days of afternoon thunderstorms capable of isolated flash
flooding.
...Northeast...
The inherited Marginal Risk area was removed in coordination with BTV/Burlington, VT and ALY/Albany, NY forecast offices. Fast-moving
storms with this fast-moving front in this part of the country,
very limited/no instability, and somewhat dry weather in recent
days should all preclude any flash flooding in this area.
Wegman
Day 3
Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE GULF COAST...
...Louisiana to the Atlantic Coast...
The stalled out front will weaken in place through the day on
Sunday. This once again should prevent most areas from getting
prodigious amounts of rain, despite abundant atmospheric moisture.
Further, there is increasing disagreement in the guidance as to
where the axis of heaviest rainfall associated with the front will
be on Sunday. Guidance trends have been towards the northwest,
resulting in a large expansion of the Marginal in that direction
due to the aforementioned uncertainty. It's likely given how wet
this entire area of the country has been that an area of Slight
Risk impacts will occur somewhere within the Marginal risk, but
where that will be is highly uncertain right now. Thus, it's likely
a Slight Risk upgrade will be needed with increased confidence.
...Deep South Texas...
A Marginal Risk area was added to the Brownsville to South Padre
Island area of Deep South Texas with this update. A tropical wave
will have moved through the Caribbean and into the western Gulf by
Sunday. Associated heavy rainfall may impact the immediate Deep
South Texas coast. Recent heavy rainfall has lowered FFGs here
which may not fully recover by the arrival of this heavy rainfall
due to potential additional shower and thunderstorm activity both
today and Saturday prior to this period.
From Mike Powell@618:250/1 to All on Sun Sep 8 08:13:00 2024
FOUS30 KWBC 080817
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Sun Sep 8 2024
Day 1
Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST...
...Summary...
Only a few very minor changes were made to the inherited Marginal
Risk areas across the CONUS.
...Southwest...
A bit more widespread convection is expected with the monsoon this
afternoon across the Southwest. Much of the convection will once
again be focused along the various mountain ranges from the
Peninsular Ranges of southern California through the Sangre de
Cristos of Colorado. As usual in the desert Southwest, somewhat
limited moisture will be the limiting factor, even though areas
could be 2 sigma above normal for atmospheric moisture for this
time of year. Thus, the primary flood threat is for urban areas,
slot canyons, and arroyos where slow moving convection in chaotic
steering flow may result in long-duration localized heavy rain from
the most persistent convection. Any resultant flash flooding will
be isolated.
...Deep South Texas...
The Marginal Risk area was trimmed closer to the coast with this
update, now primarily for the urban area of Brownsville over to the
South Padre Island beaches. A stalled out front will begin to focus
an area of heavy rainfall associated with a slowly developing
tropical low over the western Gulf. Guidance has shifted east a
bit, resulting in lower forecasted rainfall totals for all of Deep
South Texas, with little if any rain expected west of Brownsville
through tonight. However...given the abundance of atmospheric
moisture available any stronger showers and storms will be capable
of very heavy rainfall rates which could cause isolated flash
flooding.
...Southeast...
The stalled out front that has been haunting the Southeast for well
over a week now remains in place from far southern South Carolina
through the northern Florida Peninsula. Besides the front itself
being strong...indicative of a tight gradient between the extremely
humid air mass over the Gulf and an outright autumnal air mass to
its north...instability remains very lacking since this front
hasn't moved in days, so diurnal heating is limited under the
extensive low cloud cover. Thus, almost all of the rain will be
light across this region. This is countered by the aforementioned
abundant amounts of atmospheric moisture available for any
convection that can form. Thus, a low-end Marginal remains in
place, with the greatest threat for flooding over the Florida
Peninsula well south of the front where diurnal heating can
actually make a difference.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...
As on Day 1, very few changes were needed to the inherited ERO risk
areas.
...Coastal Deep South Texas...
Weak easterly flow north of a developing tropical low over the
southwestern Gulf will advect increasingly deep tropical moisture
into coastal Deep South Texas on Monday. This is highly likely to
result in much more rainfall for South Padre Island and over to
Brownsville as compared with Day 1/Sunday. Given PWATs that could
exceed 2.5 inches, or 3 sigma above normal for this time of year,
any convection that moves west off the Gulf into the coast will be
capable of very heavy rain rates. Thus, the real question regarding
flood coverage will be how frequent storms producing those rates
move over urban areas. Think right along the coast widely scattered
instances of flash flooding can be reasonably expected. Any
training could quickly result in significant flash flooding. That
said, the system will only really just be getting organized, so
overall rains will likely be more intermittent in most areas. Urban
areas such as Brownsville will be at a higher likelihood for flash
flooding.
...Four Corners Region...
Convective coverage should shift northeastward a bit on Monday.
Thus, the flooding threat shifts into the Four Corners region and
away from the West Coast. Flooding concerns will be greatly
focused in area slot canyons and arroyos of southern Utah and
southwest Colorado Monday afternoon and evening with peak heating.
...Rest of the Gulf Coast...
Rinse and repeat weather pattern will continue, albeit shifted just
a bit south of Sunday's focus. Thus, most of the north Gulf Coast
will get a relative break from the potential for steady heavy rain,
as the heaviest rains shift south a bit away from the coast. The
exception will be into the Florida Peninsula, where a repeat day of
sea breeze showers and thunderstorms can be expected to the south,
with more widespread lighter rain expected further north.
From Mike Powell@618:250/1 to All on Sun Sep 8 12:58:00 2024
FOUS30 KWBC 081545
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1145 AM EDT Sun Sep 8 2024
Day 1
Valid 16Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST...
...16Z Update...
Very little changes were made to the overall D1 outlook with the
consensus remaining steady for low to medium end MRGL impacts
expected across portions of the West, Southeast, and Deep South
Texas. A small shift was made across South TX where the MRGL risk
was tightened even further to the coast to confine mainly
Brownsville and South Padre Island as the two main areas of focus
with the northern periphery of any banding associated with the
tropical wave to the south. The latest 12z HREF mean QPF output
depicts a sharp western gradient for precip through the period with
the best potential aligning with the overnight time frame as the
wave scoots more to the northwest, bringing some of the deeper
tropical moisture north into the tip of South TX. There's some
discrepancy on the timing of any banding with a 50/50 split amongst
CAMs and some global deterministic. There's enough merit to
warrant the continued MRGL to maintain continuity, especially with
a tropical disturbance positioned to the south. There's a chance
this materializes after the period, but as a precaution for the
low-end threat, decided against removal.
Over the Southeast, the main focus will reside along the coasts of
FL and perhaps over the coast of SC as convection will align near
the coastline during peak diurnal destabilization. The best
opportunity for heavy rain will likely occur with the sea breeze
across Northeast FL between Jacksonville to Melbourne with the HREF neighborhood probabilities and EAS both fairly aggressive in the
latest update for >3" of rainfall in the vicinity. The airmass in
place is sufficient for these higher hourly and intra-hour rates to
produce some localized flash flood concerns, especially within the
urbanized areas along the coast. This was enough maintain the MRGL
risk with no real changes to the risk positioning from previous
forecast.
Out West, the only change occurred over the northern extent of the
Peninsular range where 12z CAMs indicate a few cells developing
just north of where the risk area was forecast. HREF mean SBCAPE
values lie within the 500-1000 J/kg with a few CAMs a bit more
robust in the depiction. Considering a marginally more favorable
airmass overhead and the complex terrain additions over the area,
thought the expansion was favorable and was agreed upon in
coordination with the San Diego WFO. Otherwise, no other changes
were necessary from the previous issuance across the west.
Kleebauer
..Previous Discussions..
...Southwest...
A bit more widespread convection is expected with the monsoon this
afternoon across the Southwest. Much of the convection will once
again be focused along the various mountain ranges from the
Peninsular Ranges of southern California through the Sangre de
Cristos of Colorado. As usual in the desert Southwest, somewhat
limited moisture will be the limiting factor, even though areas
could be 2 sigma above normal for atmospheric moisture for this
time of year. Thus, the primary flood threat is for urban areas,
slot canyons, and arroyos where slow moving convection in chaotic
steering flow may result in long-duration localized heavy rain from
the most persistent convection. Any resultant flash flooding will
be isolated.
...Deep South Texas...
The Marginal Risk area was trimmed closer to the coast with this
update, now primarily for the urban area of Brownsville over to the
South Padre Island beaches. A stalled out front will begin to focus
an area of heavy rainfall associated with a slowly developing
tropical low over the western Gulf. Guidance has shifted east a
bit, resulting in lower forecasted rainfall totals for all of Deep
South Texas, with little if any rain expected west of Brownsville
through tonight. However...given the abundance of atmospheric
moisture available any stronger showers and storms will be capable
of very heavy rainfall rates which could cause isolated flash
flooding.
...Southeast...
The stalled out front that has been haunting the Southeast for well
over a week now remains in place from far southern South Carolina
through the northern Florida Peninsula. Besides the front itself
being strong...indicative of a tight gradient between the extremely
humid air mass over the Gulf and an outright autumnal air mass to
its north...instability remains very lacking since this front
hasn't moved in days, so diurnal heating is limited under the
extensive low cloud cover. Thus, almost all of the rain will be
light across this region. This is countered by the aforementioned
abundant amounts of atmospheric moisture available for any
convection that can form. Thus, a low-end Marginal remains in
place, with the greatest threat for flooding over the Florida
Peninsula well south of the front where diurnal heating can
actually make a difference.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...
As on Day 1, very few changes were needed to the inherited ERO risk
areas.
...Coastal Deep South Texas...
Weak easterly flow north of a developing tropical low over the
southwestern Gulf will advect increasingly deep tropical moisture
into coastal Deep South Texas on Monday. This is highly likely to
result in much more rainfall for South Padre Island and over to
Brownsville as compared with Day 1/Sunday. Given PWATs that could
exceed 2.5 inches, or 3 sigma above normal for this time of year,
any convection that moves west off the Gulf into the coast will be
capable of very heavy rain rates. Thus, the real question regarding
flood coverage will be how frequent storms producing those rates
move over urban areas. Think right along the coast widely scattered
instances of flash flooding can be reasonably expected. Any
training could quickly result in significant flash flooding. That
said, the system will only really just be getting organized, so
overall rains will likely be more intermittent in most areas. Urban
areas such as Brownsville will be at a higher likelihood for flash
flooding.
...Four Corners Region...
Convective coverage should shift northeastward a bit on Monday.
Thus, the flooding threat shifts into the Four Corners region and
away from the West Coast. Flooding concerns will be greatly
focused in area slot canyons and arroyos of southern Utah and
southwest Colorado Monday afternoon and evening with peak heating.
...Rest of the Gulf Coast...
Rinse and repeat weather pattern will continue, albeit shifted just
a bit south of Sunday's focus. Thus, most of the north Gulf Coast
will get a relative break from the potential for steady heavy rain,
as the heaviest rains shift south a bit away from the coast. The
exception will be into the Florida Peninsula, where a repeat day of
sea breeze showers and thunderstorms can be expected to the south,
with more widespread lighter rain expected further north.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...
...Coastal Deep South Texas...
The Slight Risk for the coastal sections of Cameron, Willacy, and
southeast Kenedy Counties remains largely unchanged, though trimmed
a hair on the west/inland side with this morning's forecast update.
The northern edge of the convection associated with Potential
Tropical Cyclone Six (hereafter, PTC6), will try to move into the
coast today into tonight. A strong front remains draped over South
Texas, along the coast, but extending west/inland over Deep South
Texas. This front is keeping dry air nearby, greatly limiting any
westward extent to the rainfall, but is also a source of forcing
for the very deep tropical moisture currently in place over most of
the Gulf. Thus, as PTC6 organizes today into tonight, waves of rain
may move into the coast over South Padre Island and Brownsville.
Widely scattered instances of flash flooding are probable,
especially where the storms are the most persistent today.
...Louisiana through Florida Gulf Coast...
Few changes were made to the Marginal Risk area...which will be
largely in place for afternoon sea breeze convection as deep
tropical moisture associated with PTC6 move east across the
Peninsula and along the Gulf Coast. There has been a nominal
southward shift in the guidance, so the Marginal was trimmed to
just the immediate Gulf Coast except over the Florida Peninsula
where sea breezes and outflow boundaries will be much more of a
factor in forcing storms capable of producing heavy rain and
resultant flash flooding.
...Four Corners...
No changes were made to the Marginal Risk area, where isolated
flash flooding is possible especially in slot canyons and arroyos.
Guidance has backed off a bit on the areal coverage of convection
in this region, so this area is a low-end Marginal.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR ALL OF THE GULF
COAST WEST OF PENSACOLA..
...Gulf Coast...
Potential Tropical Cyclone 6 (PTC6) will continue organizing over
the western Gulf based on the latest NHC forecast as it tracks
north and east over the northwestern Gulf. Heavy rains associated
therewith will impact all of the Gulf Coast, especially into
Louisiana and Texas. The latest guidance has been shifting the
axis of heaviest rainfall east, in response to very dry air north
of a strong front that will be guiding PTC6 northeastward. Thus,
the westward extent of the associated rainfall will be greatly
limited. Thus, the inherited Slight Risk was trimmed from the west,
especially over Texas. With that said, any heavy rain associated
with PTC6 will have well above average amounts of atmospheric
moisture to work with, making it more common for prodigious
rainfall rates to occur. Much of southern Louisiana has been very
wet in recent weeks, so saturated soils will only quicken the onset
of flooding once the heavy rain gets going.
Into Florida, the same stalled out front will shear some of the
moisture from PTC6's circulation, adding fuel for diurnally driven
showers and storms once again on Tuesday afternoon. Isolated flash
flooding will be possible once again.
...Utah/Colorado...
No changes were made to the inherited Marginal Risk area, where
numerous showers and storms may develop Tuesday afternoon. The most concentrated and heaviest showers and storms will be in the
Marginal Risk area from eastern Utah into western Colorado.
Isolated flash flooding will be possible in flood prone areas such
as slot canyons.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE
MISSISSIPPI DELTA...
Potential Tropical Cyclone Six (PTC6) is forecast to continue
strengthening up until landfall in southwest Louisiana on
Wednesday. Abundant associated moisture will advect north ahead of
and alongside the low center, resulting in multiple inches of rain
over almost all of Louisiana and adjacent southern Mississippi. For
Louisiana, much of the southern half of the state has seen abundant
rainfall over the past couple weeks, resulting in highly saturated
soils. A hurricane making landfall here will cause most of the rain
to convert to runoff. Thus, in coordination with LCH/Lake Charles,
LA; LIX/Slidell, LA; JAN/Jackson, MS, and SHV/Shreveport, LA
forecast offices, a Moderate Risk upgrade was introduced with this
update. The greatest risk of significant and potentially life-
threatening flash flooding will be in the hardest hit areas of the
I-10 corridor from New Orleans west through Baton Rouge to
Lafayette. A higher-end Moderate is considered in effect for this
portion of the I-10 corridor.
Considerable uncertainty persists on the north and west side of the
ERO risk areas. To the west, a notable and persistent drying trend
continues, partially due to eastward shifts in the track and
partially due to disagreement as to whether PTC6 will begin
extratropical transition around landfall. With the center expected
to track along the spine of Louisiana, locations to the west will
be on the dry side of the storm, which will quickly cut down on
rainfall totals. Given these trends, the Slight was trimmed on the
western side. To the north, the concern will be more about rapidly deteriorating antecedent conditions, as northern Louisiana,
Mississippi, and Arkansas have all been much drier than points
further south towards the Gulf Coast. This in turn will make the
watersheds much more able to absorb heavy rainfall before
significant flooding commences. Thus, the Slight and Marginal Risk
areas are quite conservative, as the first few inches of rain
should be largely beneficial in these areas.
The aforementioned eastward shift in track should mean wetter
conditions into Alabama and the Florida Panhandle, and the Slight
was expanded east for that reason, though Alabama has been
similarly dry away from the Gulf Coast as its neighbors to the
west.
With PTC6 really still trying to form, expect continued adjustments
in the track and expected rainfall associated therewith.
Uncertainty with locations and amounts have also precluded some
areas well away from the current forecast track of the center from
being upgraded, but may be with future updates, especially east of
the track.
...Intermountain West...
A Marginal Risk was introduced for this region as a strong digging
shortwave trough interacts with topography and above normal
moisture for this region. Forecast amounts may be high enough to
potentially need a targeted Slight with future updates in this
area.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...
...Gulf Coast...
Tropical Storm Francine will continue making headway to the north
and northeast towards Louisiana, with NHC forecasting the system to
become a hurricane. The combination of dry/cool air to the west
and northwest of the storm will lead to a tight rainfall gradient
on its west side. Further enhancing the western gradient will be
the increasing vertical wind shear with time out of the west-
southwest, which will attempt to shift convection downshear and
downshear left of its center (to its northeast). With QPF
continuing to trend downward, the Slight Risk for the TX coast was
removed this cycle. Across the immediate Brownsville and Port
Isabel area, recent heavy rains could make them more sensitive to
additional rainfall this morning. Even though the risk area was
reduced to Marginal, Slight Risk impacts cannot be ruled out.
Prospects increase gradually along the central Gulf Coast for
heavy rainfall. While initial bands from the periphery of tropical
cyclones can quickly erode existing instability ashore, portions of
Acadiana and Southeast Louisiana along with far southern MS have
had an exceeding wet week from a cyclone that ultimately merged in
with Francine, so keeping the inherited Slight Risk makes sense.
Also, guidance tends to bias slow with tropical cyclone
recurvature, and this Slight Risk area can account for some
possible track acceleration between now and its coastal approach.
As with any warm core cyclone, the low-level inflow will exceed
the mean flow, aiding precipitation efficiency. Plentiful effective
bulk shear will organize convection into curved bands within its
cyclonic circulation. Hourly rain totals to 3" will be possible
with any bands that manage to train.
...Florida...
An unusually far south synoptic front for Mid-September focuses
very high moisture east of Francine where, at times, effective bulk
shear could flirt with 25 kts. Where semi- organized and organized
convection manages to form, hourly rain totals to 3" and local
amounts to 6" are considered possible where storms merge, train, or
get tied to the boundary. The 00z NAM and 00z Canadian were
especially bullish with local maxima, both remained widely
scattered to isolated in coverage of the heaviest rainfall. This
kept the risk at the Marginal level.
Another area with sufficient instability and moisture is behind
the front near the northeast FL coast. Adding to possible issues
there is low level and upper level flow trying to zero out. Short
bands of efficient convection, which could lead to hourly totals to
2" and local amounts to 4", are possible. Kept this area in the
Marginal Risk to help support their flood watch.
...Inter-Mountain West and Southern Rockies...
A scattered convective footprint is anticipated across portions of
the West with the main threat residing over the Southern Rockies
into the Wasatch of UT. The threat is on the lower end of
probabilities considering the middling signal within the latest
HREF for >1" chances over the region. However, with precipitable
water values near 0.75" and sufficient instability, localized flash
flooding can't be ruled out in any dry washes/arroyos, burn scars,
slot canyons, or steep topography where hourly rain totals manage
to reach 1.5". The Marginal Risk area maintained showed good
continuity.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...
...Southeast...
Francine should begin extratropical transition and become highly
sheared around, if not before, the time of landfall Wednesday afternoon/evening, with a developing dry slot possible. This should
lead to a convective comma head and an inflow band/baroclinic
trough/developing front becoming increasing removed to the east of
its center which would both be foci for heavy/excessive rainfall.
The cities of Lafayette, Baton Rouge, and New Orleans (along with
their suburbs) would be most susceptible for flash flooding as
hourly rain totals to 4" (if not more) should be possible,
particularly within the trailing band/forming front to its
southeast. Acadiana and Southeast Louisiana are more susceptible
due to heavy rainfall from this past week caused by the cyclone
which merged in with Francine, so the soils will be more primed
relative to what is typical to the area. Continuity was generally
maintained in the risk areas.
...Intermountain West...
A deep layer cyclone moving into MT brings moderate to strong
upslope flow into western portions of the state, along with MU CAPE
of ~750 J/kg. Precipitable water values are sufficient for heavy
rain-related issues, so increased the risk area to Slight for
portions of western MT. Hourly rain totals to 1.5" with local
amounts to 3" are expected, which would be most impactful in burn
scars. Some of the QPF would be in the form of snow at the highest
elevations of the northern Continental Divide.
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR THE MID-SOUTH...
Mid-South/Southeast...
Francine is expected to be fully extratropical, with a well-
developed comma head and trailing front. Guidance suggests that MU
CAPE will be modest, 250-500 J/kg. Effective bulk shear should
continue to be sufficient for convective organization where enough
instability develops/exists. Precipitable water values remain
near 2", so hourly rain totals up to 1-2" should be possible, which
would be most problematic in urban areas and initially beneficial
elsewhere. This region has been quite dry lately. With the volume
of rain expected, and the concept of this being on the cusp of a
flash flood and longer duration flood event, kept the Slight Risk
area which showed minimal change from continuity. Across the
Southeast, there is a risk of training convection along Francine's
cold front, but its progression is expected to limit issues. Still,
hourly rain totals to 2" and local amounts to 4" should be
possible.
Western MT, Idaho Stovepipe & Vicinity...
One or two surface lows are expected to lift northward this
period through MT, drawing in sufficient upslope flow, moisture,
and instability from the east to be concerned about the potential
for organized convection. The guidance signal is not cohesive as of
yet, so left the risk across the area as Marginal. Should the
guidance get more agreeable or confidence increase, a Slight Risk
could become necessary in later updates. Hourly rain totals to 2"
and local amounts to 4" appear possible.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
CENTRAL GULF COAST...
...16Z Update...
...Central Gulf Coast and Florida...
Rainfall along the far south Texas Gulf coast should come to an
end by early afternoon, with the expectation that the risk for any
additional heavier rainfall should remain along the immediate
central and upper Texas Gulf Coast as Francine tracks to the north-
northeast. The greater threat will be with bands of convection
already focused along the central Gulf coast far to the northeast
of Francine, with additional bands expected later this evening and
overnight as the center of the system approaches from the
southwest. Updated 12Z hi-res guidance shows remarkably good
agreement on the heaviest rainfall footprint along the central Gulf
Coast, stretching from near Cameron, LA east through southeastern
LA and to the immediate coast of MS. Areal average totals through
early tomorrow morning are on the order of 2-3" with locally higher
amounts of 3-6" possible with training bands.
Additional storms are expected further east in vicinity of a
quasi-stationary frontal boundary draped through central Florida.
Enhanced moisture in the area from Francine will help contribute to
some locally heavy downpours with rain rates of 2-3" per hour,
which could lead to some isolated flooding/ponding issues in urban
areas.
...Intermountain West...
General footprint for the current Marginal Risk remains the same
with a small northwestward extension into southwestern Wyoming to
cover additional areas of scattered convection expected this
afternoon. Latest hi-res guidance indicates the potential for some
locally heavy storm total rainfall of 0.5-1", possibly as high as
1.5", as well as some low-end HREF probabilities (20-30%) of
exceeding 2-year ARIs. These totals may be enough to lead to some
isolated concerns for flash flooding in the usual more terrain
sensitive areas such as slot canyons, dry washes, and any burn
scars.
Putnam
...Previous Discussion...
...Gulf Coast...
Tropical Storm Francine will continue making headway to the north
and northeast towards Louisiana, with NHC forecasting the system to
become a hurricane. The combination of dry/cool air to the west
and northwest of the storm will lead to a tight rainfall gradient
on its west side. Further enhancing the western gradient will be
the increasing vertical wind shear with time out of the west-
southwest, which will attempt to shift convection downshear and
downshear left of its center (to its northeast). With QPF
continuing to trend downward, the Slight Risk for the TX coast was
removed this cycle. Across the immediate Brownsville and Port
Isabel area, recent heavy rains could make them more sensitive to
additional rainfall this morning. Even though the risk area was
reduced to Marginal, Slight Risk impacts cannot be ruled out.
Prospects increase gradually along the central Gulf Coast for
heavy rainfall. While initial bands from the periphery of tropical
cyclones can quickly erode existing instability ashore, portions of
Acadiana and Southeast Louisiana along with far southern MS have
had an exceeding wet week from a cyclone that ultimately merged in
with Francine, so keeping the inherited Slight Risk makes sense.
Also, guidance tends to bias slow with tropical cyclone
recurvature, and this Slight Risk area can account for some
possible track acceleration between now and its coastal approach.
As with any warm core cyclone, the low-level inflow will exceed
the mean flow, aiding precipitation efficiency. Plentiful effective
bulk shear will organize convection into curved bands within its
cyclonic circulation. Hourly rain totals to 3" will be possible
with any bands that manage to train.
...Florida...
An unusually far south synoptic front for Mid-September focuses
very high moisture east of Francine where, at times, effective bulk
shear could flirt with 25 kts. Where semi- organized and organized
convection manages to form, hourly rain totals to 3" and local
amounts to 6" are considered possible where storms merge, train, or
get tied to the boundary. The 00z NAM and 00z Canadian were
especially bullish with local maxima, both remained widely
scattered to isolated in coverage of the heaviest rainfall. This
kept the risk at the Marginal level.
Another area with sufficient instability and moisture is behind
the front near the northeast FL coast. Adding to possible issues
there is low level and upper level flow trying to zero out. Short
bands of efficient convection, which could lead to hourly totals to
2" and local amounts to 4", are possible. Kept this area in the
Marginal Risk to help support their flood watch.
...Inter-Mountain West and Southern Rockies...
A scattered convective footprint is anticipated across portions of
the West with the main threat residing over the Southern Rockies
into the Wasatch of UT. The threat is on the lower end of
probabilities considering the middling signal within the latest
HREF for >1" chances over the region. However, with precipitable
water values near 0.75" and sufficient instability, localized flash
flooding can't be ruled out in any dry washes/arroyos, burn scars,
slot canyons, or steep topography where hourly rain totals manage
to reach 1.5". The Marginal Risk area maintained showed good
continuity.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...
...2030Z Update...
...Southeast...
Minimal adjustments were made to the current Moderate Risk area
focused mainly on a narrowing corridor of heavier rain totals along
and to the east of the forecast path of Francine, which has been
extended a bit further northeastward into Mississippi. This region
covers an area with moderate to high (50-90%) probabilities of
rainfall totals exceeding 5" and rain rates in excess of 2-3" per
hour. The majority of this rainfall is expected to fall within 6-12
hours, exacerbating the potential for scattered to numerous
instances of flash and urban flooding. Storm total rainfall in the
broader Slight Risk area is expected to be on the order of 2-4".
The western edge of the Slight Risk has been trimmed eastward as
increasing shear will limit convection and subsequent rainfall
along the western side of the system track.
...Intermountain West...
Still expect post-frontal, moist upslope flow (PWs 1.0-2.0
standard deviations above the mean) will lead to periods of locally
heavy rainfall across portions of western Montana into central
Idaho. Newly available hi-res guidance shows storm total rainfall
of 1-3", backed by HREF guidance indicating high (70-90%)
probabilities of totals over 1".
Putnam
...Previous Discussion...
...Southeast...
Francine should begin extratropical transition and become highly
sheared around, if not before, the time of landfall Wednesday afternoon/evening, with a developing dry slot possible. This should
lead to a convective comma head and an inflow band/baroclinic
trough/developing front becoming increasing removed to the east of
its center which would both be foci for heavy/excessive rainfall.
The cities of Lafayette, Baton Rouge, and New Orleans (along with
their suburbs) would be most susceptible for flash flooding as
hourly rain totals to 4" (if not more) should be possible,
particularly within the trailing band/forming front to its
southeast. Acadiana and Southeast Louisiana are more susceptible
due to heavy rainfall from this past week caused by the cyclone
which merged in with Francine, so the soils will be more primed
relative to what is typical to the area. Continuity was generally
maintained in the risk areas.
...Intermountain West...
A deep layer cyclone moving into MT brings moderate to strong
upslope flow into western portions of the state, along with MU CAPE
of ~750 J/kg. Precipitable water values are sufficient for heavy
rain-related issues, so increased the risk area to Slight for
portions of western MT. Hourly rain totals to 1.5" with local
amounts to 3" are expected, which would be most impactful in burn
scars. Some of the QPF would be in the form of snow at the highest
elevations of the northern Continental Divide.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...
...16Z update...
The latest NHC track is very similar to the previous issuance
therefore no significant adjustments were needed for the Moderate
and Slight Risk areas that follow the core path of Francine as it
approaches the coast, makes landfall and then moves inland. The
latest guidance did show a decrease in QPF across portions of
eastern Texas and Oklahoma therefore the Marginal Risk area was
trimmed eastward by 1-2 tiers of counties to reflect the reduction
in threat for excessive rainfall.
The latest hi-res and global guidance continues to show convective
activity firing up along the higher terrain areas of western
Montana and central Idaho, with the potential for hourly rainfall
rates to pulse up to 1.5 inches/hour. The inherited Slight Risk
area encompasses where the majority of the guidance suggest an
elevated threat for excessive rainfall and isolated to scattered
instances of flash flooding concerns
Campbell
...Southeast...
Francine should begin extratropical transition and become highly
sheared around, if not before, the time of landfall late on
Wednesday, with a developing dry slot possible. This should cause
sheared convection initially expected to prefer the north and
northeast quadrants -- near and left of track -- and eventually a
convective comma head and an inflow band/baroclinic trough/
developing front becoming increasing removed to the east of its
center which would be foci for heavy/excessive rainfall. The
cities of Baton Rouge and New Orleans (along with their suburbs)
would be most susceptible for flash flooding as hourly rain totals
to 4" (if not more) should be possible, particularly within the
trailing band/forming front to its southeast. Southern Louisiana is
more susceptible due to heavy rainfall from this past week caused
by the cyclone which merged in with Francine, so the soils will be
more primed relative to what is typical to the area. Continuity was
generally maintained in the risk areas, though gradients were
sharpened across LA and the slower track for Francine inland caused
some retreat of the northern fringes of the risk areas.
...Intermountain West...
A deep layer cyclone moving into MT brings moderate to strong
upslope flow into western portions of the state, along with MU CAPE
of ~750 J/kg. Precipitable water values are sufficient for heavy
rain-related issues, so the Slight Risk area remains for portions
of western MT. Hourly rainfall rates up to 1.5 inches/hour will be
possible as individual cells pass through along with a daily
accumulations up to 3 inches in isolated locations; which would be
most impactful in the vicinity of burn scars. Some of the QPF
would be in the form of snow at the highest elevations of the
northern Continental Divide.
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-SOUTH & SOUTHEAST...
...Mid-South/Southeast...
Francine is expected to be fully extratropical, with a well-
developed comma head and trailing front. Guidance suggests that MU
CAPE will be modest, 250-500 J/kg. Effective bulk shear should
continue to be sufficient for convective organization where enough
instability develops/exists. Precipitable water values remain
near 2", so hourly rain totals up to 1-2" should be possible, which
would be most problematic in urban areas and initially beneficial
elsewhere. This region has been quite dry lately. With the volume
of rain expected, and the concept of this being on the cusp of a
flash flood and longer duration flood event, kept the Slight Risk
area which showed minimal change from continuity. Across the
Southeast, there is a risk of training convection along Francine's
cold front which shows a greater signal for excessive rainfall
overnight. Broadened the risk area to encompass the possible
training rain band, as enough moisture, instability, and possible
stalling to be concerned about hourly rain totals to 3".
...Western MT, Idaho Stovepipe & Vicinity...
One or two surface lows are expected to lift northward this period
through MT, drawing in sufficient upslope flow, moisture, and
instability from the east to be concerned about the potential for
organized convection. The Slight Risk raised by continuity has been
maintained across portions of western MT. Hourly rainfall rates may
intensify up to 2 inches/hour during this period, with a few
isolated locations possibly accumulating up to 4 inches by the end
of the day.
Roth/Campbell
Day 3
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024
...A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS ACROSS PORTIONS OF
THE MID-SOUTH AND SOUTHEAST...
A broad Marginal Risk of excessive rainfall exists for portions of
the Mid-South and Southeast with the weakening extratropical/Post-
Tropical Cyclone Francine. The guidance isn't showing a great deal
of overlap in potential heavy rain areas, so left the risk area
as-in, but in theory, portions of northern FL and the Southeast,
possibly including the southernmost Appalachians, have potential
for hourly rain totals to 2.5" and local amounts of 5" as enough
instability and low- level inflow/effective bulk shear exists for
possible issues. Should the guidance align on this idea QPF-wise
further, a Slight Risk could be added in later cycles.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
LOUISIANA AND SOUTHERN MISSISSIPPI...
...Southeast...
Francine has come inland over southern Louisiana but continues to
show a healthy circulation approaching the greater New Orleans
areas. Though some dry air has disrupted part of the core, intense
rainfall rates of 2-3.5"/hr are shown per MRMS, with storm totals
over 7 inches in south central LA. Rainfall tapers off notably to
the southwest of the center. Will maintain the Moderate Risk area
near the path of the center overnight given the likelihood of high
rainfall rates to continue as additional moisture is drawn in off
the Gulf across into southeastern LA and southern MS. The New
Orleans metro area will bear the brunt of the rainfall this evening
before translating northward, with the inflow band off to the east
responsible for the eastward bump in the outline. Francine will
continue inland overnight where the threat for excessive rainfall
extends northward trough much of Mississippi. Removed much of the
rest of Florida that was in a Marginal risk as lingering activity
should be diminishing over the next couple of hours.
...Intermountain West...
A deep layer cyclone moving across the northern Continental Divide
has brought afternoon convection and some flash flooding to parts
of Idaho into western Montana. Precipitable water values are
sufficient for heavy rain-related issues (0.75"), and the Slight
Risk area remains for portions of western MT and just into west-
central ID overnight. Hourly rainfall rates of 0.50-1.00" have been
observed and/or shown via MRMS which have been enough to reach
generally low FFG values in areas of complex terrain. With earlier
rainfall soaked in, additional rainfall may still exceed FFG values
overnight, but did trim off the western side of the risk area as
the focus should be mostly over western MT tonight closer to the
frontal boundary.
...A SLIGHT RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE
MID-SOUTH & SOUTHEAST...
...Mid-South/Southeast...
21Z update... The latest guidance favored a general
south/southeast trend with the QPF footprint associated with
Francine from the previous cycle. As such, this resulted in WPC
increasing amounts across eastern Tennessee, northern and central
Mississippi, and northern and central Alabama. This uptick
increased the threat for excessive rainfall and flooding concerns
for both rural and some major metropolitan areas. The Slight Risk
area was trimmed a little eastward across eastern Arkansas but also
expanded to the south and east to encompass more of central
Mississippi, central Tennessee, and west-central and eastern
Alabama. The latest guidance also suggested an increase in feeder
band convection across the Florida Panhandle thus the Marginal Risk
area was extended 1-2 counties to the south.
Campbell
Francine is expected to be fully extratropical, with a well-
developed comma head and trailing front. Guidance suggests that MU
CAPE will be modest, 250-500 J/kg. Effective bulk shear should
continue to be sufficient for convective organization where enough
instability develops/exists. Precipitable water values remain
near 2", so hourly rain totals up to 1-2" should be possible, which
would be most problematic in urban areas and initially beneficial
elsewhere. This region has been quite dry lately. With the volume
of rain expected, and the concept of this being on the cusp of a
flash flood and longer duration flood event, kept the Slight Risk
area which showed minimal change from continuity. Across the
Southeast, there is a risk of training convection along Francine's
cold front which shows a greater signal for excessive rainfall
overnight. Broadened the risk area to encompass the possible
training rain band, as enough moisture, instability, and possible
stalling to be concerned about hourly rain totals to 3".
...Western MT, Idaho Stovepipe & Vicinity...
21Z update... Multiple impulses within the flow will trigger
showers and thunderstorms that will track across the Northern
Rockies region. PW values around 0.50-0.75 inches will be pooled
over the region throughout this period and will bolster rainfall
efficiency wit each passing impulse. Per the CAM guidance any of
these storms will be capable of producing periods of moderate to
heavy rainfall as they pass over the favored upslope areas and then
exit into the adjacent High Plains. There was a trend for
increasing QPF coverage and amounts across portions of the Hi-
Line. The Slight Risk area was expanded eastward to now include
more of north-central and northeast Montana and southwestward to
extend into portions western Montana and into Idaho.
Campbell
One or two surface lows are expected to lift northward this period
through MT, drawing in sufficient upslope flow, moisture, and
instability from the east to be concerned about the potential for
organized convection. The Slight Risk raised by continuity has been
maintained across portions of western MT. Hourly rainfall rates may
intensify up to 2 inches/hour during this period, with a few
isolated locations possibly accumulating up to 4 inches by the end
of the day.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND NORTHERN ALABAMA...
...Mid-South into the Southeast...
As of 08z Tropical Storm Francine continues to move northward out
of Louisiana and into Mississippi. The consolidated core of heavy
rainfall to the north of Francine will lift north across MS today,
and eventually into portions of far eastern AR and western TN by
this afternoon. This is the corridor that will likely see the most
widespread axis of 3-6" of rainfall. However rainfall rates along
this axis will generally be on the decline through the day. At 12z
we still may seeing some 1-2" per hour rainfall just north of the
center, but by 18z we are probably below 1" per hour for the most
part, and by 00z we may be struggling to even get 0.5" per hour.
Thus even though this axis has the highest HREF EAS probabilities
of 1,2 and 3", the flash flood risk is likely mitigated by the
lower rainfall rates. In fact despite the high QPF in the HREF, the
probability of exceeding 3 or 6 hr FFG in the HREF is negligible
over this area. Now 3-6" is still a lot of rain, and will probably
see some urban and perhaps small stream flooding, however currently
not expecting to see more widespread higher end impacts. Thus the
Slight risk should cover the threat.
The higher risk of flash flooding may actually end up well east of
Francine over portions of central and northern AL. Areal averaged
rainfall may be lower here than areas along the track of Francine,
but rainfall rates and upper bound rainfall potential is likely
higher. A narrow axis of enhanced convergence appears likely, with
south southwesterly flow east of Francine, and easterly flow from
the building high over the western Atlantic. This axis may stay
rather persistent though the period, with around 1000 j/kg of
instability advecting northward behind Francine as well. This
environment does appear conducive for possible training convective
bands and several high res model solutions suggest we do see
periodic training convective bands across this area by this
afternoon and evening. HREF neighborhood probabilities of exceeding
5" are 50-70% and exceeding 8" are as high as 30-40%. Not
necessarily expecting widespread coverage of these amounts, however
localized swaths of 6-10" of rain are a possibility today/tonight.
This could lead to locally considerable flash and urban flood
impacts. For this reason we have upgraded to a small MDT risk where
the conditional threat of high end impacts appears greatest.
Slow moving/backbuilding convection also appears possible today
over portions of the FL Panhandle. Model agreement on the
potential is not quite as strong as over AL, but certainly some
potential for higher end impacts here as outer banding east and
southeast of Francine could train/backbuild at times near the
coast. Given a bit more uncertainty and higher FFG here, we will
not upgrade at this time, but do consider this area as a higher end
Slight risk.
...Montana...
We did trim the Marginal and Slight risk areas some with this
update. Overall not convinced we will see much in the way of true
flash flooding today over MT. Instability is lacking over much of
the state which will keep rainfall rates generally below flash
flood thresholds. However with strong and persistent forcing the
latest WPC deterministic rainfall forecast is 1-3" near and just
south of Great Falls. If this occurs it would be an event that
falls in the 5-25 year ARI range. So despite weak rates rainfall of
this magnitude could certainly cause some areal flood concerns.
Over northeast MT there should be more instability and the
potential for rainfall totals over 1"/hr. Current indications
suggest this stronger convection would be progressive in nature
likely limiting the flood risk...however given the high PWs and
instability forecast could certainly see pretty impressive sub
hourly rates which could cause some flood concerns in any more
sensitive areas. Thus even though the Slight risk appears lower
end...we opted to maintain it over areas that may either see a
higher ARI longer duration rain or the potential for at least short
duration high rates.
Chenard
Day 2
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE TENNESSEE VALLEY INTO THE SOUTHEAST...
By Friday the remnant surface circulation of Francine will likely
be somewhere near the AR/TN border. By this time the surface
feature will not be a main driver of heavy rainfall. However the
pattern will remain favorable for periodic training convective
bands from portions of TN into AL/GA and the FL Panhandle. While
the surface circulation of Francine will be dissipating, we will
still have plenty of mid level energy left over. Meanwhile we will
also have a trough of low pressure off the Southeast coast and a
high pressure to the north over the Western Atlantic. The
combination of these features will likely result in a corridor of
persistent low level convergence over TN/AL/GA. We should have
plenty of moisture and instability in place as well, and thus a
notable flash flood risk could continue. We will carry a higher end
Slight risk across portions of TN/AL/GA with this update.
Depending on what happens on day 1 (impacting soil and streamflow
conditions) can not rule out needing a MDT risk over a portion of
this area. However there is a bit more uncertainty on convective
details by Friday compared to Thursday, and so would like to
continue to evaluate future high res model runs, as well as see how
convection evolves today, before making any decision on an
upgrade. Either way flash flooding appears possible, and a locally
considerable threat could evolve.
Uncertainty increases with northeastward extent. There is some
flash flood risk over the western Carolinas, but unclear how far
northeast the risk will get. Quite possible the better convergence
and convective threat stays further west and southwest on Friday
as that appears to be the trend at the moment. Thus will only
carry a Marginal risk here. There is also some risk over the
coastal Carolinas where a low pressure may try to form offshore.
Again at the moment the most likely scenario sees the heaviest
rainfall stay offshore...however we could see some heavier
convective cells along coastal areas, warranting a Marginal risk.
From Mike Powell@618:250/1 to All on Fri Sep 13 08:21:00 2024
FOUS30 KWBC 130755
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
355 AM EDT Fri Sep 13 2024
Day 1
Valid 12Z Fri Sep 13 2024 - 12Z Sat Sep 14 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CENTRAL AND NORTHERN ALABAMA...
...Deep South...
Remnant circulation from Francine will be pivoting back south and
then southeast in retrograde thanks to a strong block from a stout
mid- level ridge positioned over the Great Lakes through
Ontario/Quebec. This will allow for the a migration of elevated
moisture centered under the disturbance to pivot back across MS/AL
where drier air has punched through under prevailing southwest flow
aloft, mainly above 500mb. This is well- documented within the 00z
sounding analysis out of KJAN where moisture is pooled between the
surface to 500mb with a large wedge of drier air focused above. The
sounding from KBMX was beginning to show signs of the dry tongue
and can be noted within the radar presentation as deep convective
development is struggling to initiate this evening. Tomorrow
afternoon and beyond is when the pattern shifts to a more favorable
convective regime across far Eastern MS into much of AL, especially
the northern 2/3's of the state thanks to the stacked synoptic
circulation sliding back overhead. Guidance is keen on the deeper
moisture return as indicated within the 700-500mb RH field leading
to an uptick in regional PWATs back above 1 deviation above normal
with ample mid- level energy accompanying.
The latest 00z HREF remains very aggressive within its depiction
for a broad axis of heavy rainfall in-of Northern and Central AL
during the late- afternoon and evening time frame in conjunction
with the increased upper forcing approaching from the west along
the tightened theta-E gradient located across much of the state.
Surface ridge to the northeast will also begin wedging south into
Northern GA with an axis of convergence developing upstream between
the approaching circulation from now Post-Tropical Francine. This
will place a good portion of AL into the crosshairs for a better
convective training episode as the deep layer flow aligns perfectly
from the southeast. HREF neighborhood probability for >5" is robust
between the area of KHUN down to KBHM indicating a 50-70% prob with
an EAS signal of 50-80% for at least 2", a signal correlated highly
for a higher end risk. Hourly rate probs are also aggressive in the
means with a corridor of elevated probs for between 2-3"/hr within
the HREF, along with some intra-hour rate potential exceeding 4"
given the efficient rainfall processes and expected instability
presence within the eastern fringe of the general circulation. All
these prospects weighted on the decision to maintain the inherited
MDT risk with some minor adjustments to correlate with the blended
mean QPF off the latest HREF, as well as the EAS prob field
coverage of higher probs for at least 2" of rainfall. The changes
made were not extensive in the grand scheme, so the overall
forecast was close to general continuity.
...Coastal Carolinas...
A weak inverted trough located off the Southeast coast will allow
for a targeted axis of heavy rainfall across the Eastern Carolinas,
mainly north of KCHS with a bullseye being forecast along the coast
of Southeast NC. The prevailing easterlies off the Atlantic within
the integrated water vapor field (IWV) from the GFS signal the axis
very well with a nose of elevated PWATs being advected right into
the coastal areas of NC during the first half of the period. 00z
CAMs also signal the threat for a period of heavy rainfall with
some varying degrees of the latitudinal push of where the heaviest
precip would occur. The greatest probabilities for >3" are centered
from Wilmington up through Morehead City into Cape Hatteras.
Considering the area FFG's, the main flooding risks will be
relegated to more urbanized areas where run off potential is
highest. The signal was good enough for a continued MRGL risk with
some expansion further inland, mainly west of Raleigh where some
convergence from the damming high to the north and the prevailing
easterlies may intersect allowing for a small axis of enhanced
precip away from the coastal plain. Not anticipating a large areal
coverage of flash flooding in any case for the region, but a few
pockets are plausible leading to a low to middle ground MRGL for
the forecast.
...Florida Panhandle...
Deep tropical flow pulling north from the Gulf will allow for an
axis of heavy convection to push north out of the Gulf into the Big
Bend of FL. Areal FFG indices for 1/3/6 hr rates are pretty
substantial, so the threat is low-end within the MRGL risk
category. Still, the signal for amounts >5" is relatively high
right along the coast leading to the MRGL continuation from
previous forecast.
Kleebauer
Day 2
Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEASTERN U.S...
...Southeast...
The remnant mid and upper circulation from Francine will get stuck
under the blocking pattern positioned across the northern and
eastern U.S with little propagation of any kind during the period.
This will allow for a continuation of broad, cyclonic flow across
the Southeast with a steady convergence pattern positioned over
parts of the Deep South. A steady southeast flow pattern will
preside over much of AL and Western GA with a sharp convergence
signal focused over the central portion of the outlined area thanks
to the persistent anti-cyclonic "wedging" occurring around the
southern flank of the ridge, banking against the cyclonic regime
centered around Francine's remnants. Ensemble QPF distribution is a
bit further south compared to the previous forecast allowing for an
adjustment in that direction for the SLGT risk prospects. Totals
are generally favored to be within 1-3" overall, but some pockets
of higher totals are depicted within the 00z deterministic suite
leading to some locally impactful flash flood prospects, especially
when coupled with the previous period(s) of rainfall lowering the
area FFG's across AL/GA.
The SLGT risk is on the higher-end of the spectrum with a higher
risk potential relying on the contingency of how the D1 period
plays out across the region. There's a non-zero chance of an
upgrade, but the slowly degrading axis of instability may thwart
the potential all together. Regardless, the SLGT risk is sufficient
just given the variables at hand.
...Arizona...
A surge of deep tropical moisture from now Tropical Storm Ileana
will make progress into the southern portion of AZ by the middle to
end of the forecast period. With sufficient diurnal heating
Saturday afternoon, environment across Southern and Southeast AZ
will destabilize allowing for isolated to scattered thunderstorms
to initiate in-of the terrain situated within Pima/Santa Cruz
counties to possibly as far east as Cochise county in proxy to the
Huachuca mountains. MUCAPE forecast is relatively favorable for
convective development with the ensemble SBCAPE forecast between
750-1250 J/kg off the latest HREF mean. The combination of
increasing moisture anomalies and environmental buoyancy adds
favor to an isolated flash flood risk, especially within the
terrain and adjacent locales. A MRGL risk was maintained from
previous forecast, but trimmed on the northern edge given the
expected timing of the moisture push to be delayed for a greater
convective threat within Pinal/Maricopa counties. This area will
have more favorability later on D3.
From Mike Powell@618:250/1 to All on Sat Sep 14 08:48:00 2024
FOUS30 KWBC 140809
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Sat Sep 14 2024
Day 1
Valid 12Z Sat Sep 14 2024 - 12Z Sun Sep 15 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEASTERN U.S...
...Southeast...
Current radar/satellite composite depicts a broad cyclonic gyre
situated across the Southeastern U.S with a north-south band of
convection bisecting the a good portion AL into Western TN around
the northeast periphery of the remnant cyclone. The surface
reflection from what was Francine will become more diffuse over the
next 6 hrs with a degrading surface signature, leaving much of the
flow tied to the leftover mid and upper lows associated with the
storm. That said, the cyclonic spin from the tandem 850-500mb
low(s) will allow for a continued favorable moisture stream around
the circulation with small mid-level perturbations stuck over the
same areas allowing for locally enhanced ascent within the confines
of AL/GA/TN. PWATs generally residing within the +1 standard
deviation anomaly across the Deep South will be sufficient enough
within the pattern to yield another round of scattered to
widespread convection across similar areas that were impacted
today. The difference is the magnitude of the convective pattern
will be a bit less vigorous thanks to the weakening mid and upper
low centers as denoted within all the recent guidance height
fields. Still, the combination of the moisture anomaly and semi-
favorable ascent under the remnant mid and upper lows will be
plenty to offer another round of convection across the Deep South.
To the northeast, a stout surface ridge will nose down further into
the interior Southeast with a formidable easterly regime focused
in-of Northern GA down into the Northwest side of AL. The
combination of the cyclonic flow from the mid-level remnants and
wedging associated with the surface ridge will create a tightened
axis of convergence within the state of AL down into Western GA.
The consensus this evening within the 00z CAMs suite was very well
represented within the HREF mean QPF and associated probability
fields, highlighting the semi-narrow corridor of higher rainfall
potential located just to the south and west of Huntsville,
bisecting much of North-Central AL down into the Southwest side of
GA. This area has been the beneficiary of some significant rainfall
the past 24-48 hrs leading to a degraded FFG footprint where flash
flood prospects will be greater, relative to average. Signals for
1-2"/hr rainfall rates would be sufficient given the antecedent
conditions across the area, and with the setup likely to garner
some locally enhanced rainfall rates >2"/hr as indicated within the
prob fields, there's a higher likelihood for continued flash flood
concerns within the confines of those areas in AL and GA mentioned
above. A higher end SLGT risk is the forecast for those areas
extending from Northwestern AL near the AL/MS line, southeast
through Columbus, GA, an area that includes the Birmingham metro.
SLGT risk extends further northwest and southeast, respectively
with a broad MRGL extension around the periphery as heavy rain
concerns exist from as far north as Western KY to as far Southeast
as Northeastern FL.
...Arizona...
A surge of deep tropical moisture from now Tropical Storm Ileana
will make progress into the southern portion of AZ by the middle to
end of the forecast period. With sufficient diurnal heating
anticipated this afternoon, the environment across Southern and
Southeast AZ will destabilize allowing for isolated to scattered
thunderstorms to initiate in-of the terrain situated within
Pima/Santa Cruz counties to possibly as far east as Cochise county
in proxy to the Huachuca mountains. MUCAPE forecast is relatively
favorable for convective development with the ensemble SBCAPE
forecast between 500-1250 J/kg off the 00z HREF mean. The
combination of increasing moisture anomalies and environmental
buoyancy adds favor to an isolated flash flood risk, especially
within the terrain and adjacent locales. A MRGL risk was maintained
from previous forecast with only minor adjustments based within the
HREF blended mean QPF footprint, and aligning with the edges of the
10% or great probabilities within the neighborhood probability
output for >2" rainfall potential. This area will have more
favorability later on D2.
...South Florida...
Surface trough located in-of South FL will maintain a prevailing
westerly flow across the Southern part of the Peninsula, placing
the mean steering pattern pointed towards the population centers
along the Southeast FL coast in time for the sea breeze convective
initiation this afternoon. Area PWATs are very elevated this
evening with the 00z sounding out of KMFL depicting a solid 2.40"
PWAT reading placing the area within the +1-2 standard deviation
anomaly for the period. This environment will be conducive for
convective generation just inland along the penetrating sea breeze
in the afternoon with locally strong convective cores producing
rainfall rates between 2-3"/hr on initiation, moving towards the
coast thanks to the prevailing steering flow. HREF probabilities
for >3" are solidly between 50-80% along the population centers
extending from PBI down through MFL with the highest probs situated
between PBI and FLL. Lower end 5+" probabilities are also
highlighting the area of interest, so the threat is within the
middle grounds for flash flood concerns leading to an addition of a
MRGL risk situated along the Southeast FL coast encompassing the
population centers.
Kleebauer
Day 2
Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST U.S...
...Southeast...
Despite the remnant mid and upper circulation from Francine
degrading in its presentation, another period of enhanced
convective activity will spawn underneath the primary upper forcing
lingering over the Southeastern U.S. There's a bit more emphasis on
an axis of convection developing across portions of the Lower
Mississippi Valley in-of AR and MS, extending southeast into parts
of the Deep South, mainly Southern MS and AL. Areal PWATs will
remain elevated within +1 standard deviations, enough of an anomaly
to spur up some potential heavier convective cores that could
result in some flash flood prospects, especially over any urban
zones where run off potential is greatest. Ensemble bias corrected
QPF and the 75th percentile of the NBM indicate a fairly stout axis
of heavier precip likely to occur under the influence of the upper
low and remnant energy pivoting around the general circulation. The
primary concerns will continue to be those locally heavy rates that
will reach between 1-2"/hr with some intra-hour potential for
heavier rates that typically cause more issues in this part of the
country. A SLGT risk was added to the previous forecast with an
adjustment southwest to the overall SLGT/MRGL risk areas to align
with the current trends and ensemble forecast consensus on the
heaviest precip location. The MRGL expands into the FL Panhandle to
cover the area that has been hit pretty hard in the past 36 hours
as isolated storms are still forecast for those areas that have
seen copious amounts of rainfall in the above time span.
...Southwest...
Remnant moisture from now Tropical Storm Ileana will continue to
advect northward into the rest of the Desert Southwest with a
trajectory towards the Four Corners, wrapping back eastward into NM
during the afternoon hours Sunday. The ensemble QPF footprint upped
the total precip forecast compared to the previous periods, a trend
that was noted when assessing the QPF differential from 12z to 00z.
This places the convective pattern towards those areas we've been
highlighting all summer for flash flood concerns thanks to a
plethora of burn scars located within the terrain across parts of
NM. The previous forecast was relatively unchanged over AZ into the
Four Corners, but the MRGL risk was expanded to include much of NM
with a focus into the terrain over the Sangre de Cristos and the
Sacramento Mountains. General QPF average between 0.25-0.75" is
expected, but maxima of >1" will be plausible anywhere within the
confines of the forecast risk area. A targeted SLGT risk is still
possible, but the scattered nature of the convection forecast
doesn't instill enough confidence at this time. A reassessment with
the addition of CAMs guidance could sway that the other way, so
stay tuned for future updates.
From Mike Powell@618:250/1 to All on Sun Sep 15 08:51:00 2024
FOUS30 KWBC 150911
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
511 AM EDT Sun Sep 15 2024
Day 1
Valid 12Z Sun Sep 15 2024 - 12Z Mon Sep 16 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CENTRAL GULF COAST INTO THE MISSISSIPPI...
...Central Gulf Coast into MS...
The setup that has resulted in a convective bands and locally
heavy rainfall the past couple days will continue today, just
shifted a bit to the southwest. Lingering mid level energy and low
level convergence will result in convection today from coastal AL
northwestward across MS and into eastern AR. The persistence of the
convergence will allow for some training/backbuilding potential
locally increasing rainfall totals. The overlap of strongest
convergence and higher instability looks to focus closer to the
Gulf Coast today, across portions of coastal MS/AL and the western
FL Panhandle. The flash flood risk is likely highest across these
areas, with 5"+ HREF neighborhood probabilities as high as 50-80%.
HREF guidance indicates one round of potential backbuilding cells
this afternoon, with another round possible late tonight into
early Monday. Given this signal the Slight risk was expanded into
this region, and do consider this area to be a higher end Slight
risk, with locally significant flash flooding a possibility given
what should be very efficient rainfall producers.
The flash flood risk is a bit lower further north across MS and AR
given weaker instability and what should generally be more
transient convective cells. Nonetheless some training/backbuilding
is still possible, and soil saturation is still above average due
to rainfall from Francine a few days ago. Thus still think a
Slight risk is warranted, with isolated to scattered flash flooding
possible.
....Coastal Carolinas...
An area of low pressure off the southeast coast may bring heavy
rainfall into portions of coastal SC/NC between 06z-12z Monday. The
GFS remains a fast outlier in bringing heavy rainfall inland from
this system, and thus is not a preferred solution. While there is
still some spread amongst the other model solutions, some form of
consensus is developing. This consensus would suggest that some
bands of locally heavy rainfall may get into coastal portions of
the Carolinas early Monday. Still think the greater flash flood
risk ends up after 12z Monday, but can not rule out some heavier
totals right along the coast by 12z...and thus will introduce a
Marginal risk.
...South TX...
A small Marginal risk was added to far southern TX. Will likely be
an active convective day across northeast MX today, with easterly
low level flow advecting in moisture off the Gulf, and westerly
mid/upper flow bringing in tropical moisture off the Pacific. Low
level convergence should be enhanced in this pattern, and also
have some mid/upper level energy to work with as well. In general
expect the bulk of convective activity to be over northeast MX
where convergence is strongest, however there is at least some
threat far south TX can see some activity as well. Guidance
indicates that any convection that organizes over south TX should
drop back south into MX, however there is an opportunity for some
slow moving cells later this morning or afternoon before this
propagation takes hold.
...Southwest...
A Marginal risk covers portions of AZ and NM today where a
localized flash flood risk may exist. This risk area was reduced in
size to just cover areas where convective coverage should be a bit
greater and instability more pronounced. Rainfall over the
Marginal risk area could locally exceed 1" which could produce an
isolated flash flood over any more susceptible basins.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE CAROLINAS INTO FAR SOUTHEAST VIRGINIA...
...Carolinas into Virginia...
An area of low pressure will likely move into portions of NC on
Monday bringing a heavy rainfall threat to the area. Confidence is
increasing that this system will have a notable core of heavy
rainfall near the center and heavy convective banding to the east
of the center. The threat of flash flooding across portions of
eastern NC appears to be increasing, with locally significant
impacts possible. With that said there remains some model spread
regarding the location of landfall and the structure/intensity of
the system.
Like was mentioned in the day 1 discussion, think the 00z GFS
remains a less likely fast and west outlier with this system on
day 2. After discounting that solution the overall model consensus
improves, although not without some notable spread. The 00z NAM
and Gem Reg are two of the stronger and further south solutions,
with the 00z CMC and UKMET further east over coastal NC. With the
high res models you have the 00z 3km NAM and ARW stronger/quicker
and further south near the SC/NC border, and the HRRR and ARW2 a
bit slower/weaker and north. Overall it appears that the earlier
the low begins to deepen the stronger, quicker and further south it
moves ashore. Where as the models with a slower developing low
keep it a bit weaker and east over NC. At the moment do not really
have a strong preference, with things likely becoming clearer today
as the low begins to become more well defined offshore.
The above described uncertainties will keep us from upgrading to a
MDT risk at this time. However do consider this a higher end
Slight risk across portions of southeast NC. Wile the details still
need to be ironed out, what we can say is that the flash flood
risk from this system is increasing and locally significant flash
flooding is possible over portions of eastern NC (and possibly
northeast SC depending on the track). Just about every model
solution brings a low pressure ashore with locally heavy rainfall
near and east of the center...however enough spread still exists
with the track and intensity of the system to keep the risk level
at Slight for now. Once the low develops and confidence on the
evolution of the system continues to increase, a MDT risk upgrade
may be needed.
...Central Gulf Coast...
Some flash flood risk will likely linger across portions of the
central Gulf Coast into southeast AR on Monday. However the pattern
becomes gradually less favorable with time, and thus currently
expecting that only a localized flash flood risk will linger into
Monday.
...Southwest to Four Corners... A strong deep layered low moving
into the CA will result in an uptick in large scale forcing over
portions of the Southwest on Monday. Broad south to southwesterly
flow ahead of this feature will also advect moisture into the
region, and should have plenty of instability and moisture in
place for locally heavy rainfall within convection. Given the
increased forcing convective coverage should be notable over the
region, suggesting at least an isolated flash flood threat. Cell
motions should be fairly quick off to the northeast limiting the
duration of heaviest rainfall and the overall flash flood risk.
However with the persistent forcing in place areas could see
multiple convective cells/clusters move through over the period
adding to rainfall totals.
...NV/ID...
An axis of higher areal averaged rainfall is likely closer to the
stronger forcing underneath the deep layered low over portions of
northern NV, southeast OR and into ID. On Monday morning there is
some signal for potential training rainfall bands over portions of
NV, although instability looks rather weak, suggesting rainfall
rates may struggle to get high enough for flash flooding.
Nonetheless will need to be monitored and some areas of 1-2" of
rainfall are possible, which for some of this area could be around
a 5yr ARI event. Thus even without higher rates some areal flooding
is a possibility. By Monday afternoon we should see some increase
in instability over the broader region likely allowing for some
heavier convective cells. However currently expecting these cells
to be rather quick moving. Thus overall think a Marginal risk
should suffice, with localized flooding concerns a possibility.
...ND/MN...
A Marginal risk was added across portions of ND and MN. Model
guidance shows a lifting warm front, mid level vort energy moving
through, and divergent flow aloft between two jet streaks...likely
enough forcing to get convective development Monday into Monday
night. Convective details differ from model to model, but the
overall ingredients are similar and instability should be
plentiful. Overall the threat is there for an axis of training
and/or backbuilding convection within the Marginal risk area, and
some flash flooding could evolve.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER SOUTHEASTERN
NORTH CAROLINA AND THE NORTHWESTERN CORNER OF SOUTH CAROLINA...
...Carolinas...
PTC8 continues to churn off the Carolina coast with an ill-defined
center of circulation and convective clustering to the north of
the analyzed low off the coast of SC. The beginning wave of
tropical rainfall has started across Northeast SC into Southeast
NC with rates anticipated to pick up as the deeper tropical
moisture flux located offshore begins to propagate into the coast.
There has been relatively modest uncertainty to exactly where the
heaviest rainfall will occur due to a less defined surface low
center that provides more consensus on the specific track and
subsequent heavy rain axis. One of the notable observations through
the past 6-8 hrs has been the QPF shield displaced further north
away from the analyzed surface center provided by NHC. A lot of
this is likely due to the moderate shear impinging the southern
flank of the circulation allowing for an offset of the precip field
to the north and northeast of the primary circulation. This has
caused issues within the NWP suite of forecasts, especially higher
resolution guidance that tends to need a more defined surface
initiation to outline short range prospects within the precip
field and track of the surface reflection. There is enough data to
project the current QPF swath to be displaced further north than
some of the CAMs and global deterministic output with a surface
reflection likely to come ashore a bit further north than even the
00z consensus. For that, the trend is for 4-8" totals with locally
up to 10" possible across the Northeastern corner of SC through
Southeast NC, an agreed axis within the neighborhood probability
fields from the latest HREF. The signal for >5" is very robust with probabilities between 70-90% located across Southeastern NC with
the strongest signal located near Wilmington, an area of greatest
concern considering the urbanization factors.
The storm will induce some pretty good rainfall across Northern SC
and much of NC through the periods as the circulation moves inland
and progresses westward into the Carolina Piedmont. Bands of
tropical rainfall with efficient warm rain processes will pivot
inland with the prospects of flash flooding increasing over areas
that see training. As a result, a broad SLGT risk is centered
across much of North Carolina and the northern periphery of South
Carolina. A targeted MDT risk is in place over the northeast corner
of SC and much of Southeastern NC as the consensus for higher
rainfall amounts is most prolific within that zone.
...Northern Plains and Upper Midwest...
Aloft, a shortwave will eject out of the Central Plains and push
quickly to the northeast, entering the Northern Plains of the
Dakotas into Northwestern MN by later this evening providing ample
mid-level ascent conducive for convective initiation. At the
surface, a pre-frontal trough will develop in-of the Southern
Canadian Prairies, shifting southward into Northern ND and MN by
early this evening, setting the stage for a strong surface
convergence axis located across Eastern ND and Northwest MN. This
is thanks to the tandem mid-level ascent pattern and convergence
signature located within a formidable theta-E tongue bisecting the
area. The signal has grown considerably in the past succession of
runs with the latest CAMs all generally robust with its
presentation of locally enhanced rainfall located over the above
regions. With the mean flow generally parallel to the pre-frontal,
surface based trough, there is a growing concern for training cells
that develop upstream over ND and progress east-northeast along the
slow-moving trough axis. The 00z HREF blended mean QPF is quite
robust in its depiction with an areal average of 1.75-2.5" of total
precip located from just west of Grand Forks up through the
International border in Northern MN. A bullseye of 3-5" is co-
located within the axis of the surface trough positioning extending
over much of Northwestern MN, an area that typically does not favor
these types of convective outputs. HREF EAS probabilities for 1"
(80-90+%) and 2" (30-50%) are incredibly high for an event that is
not tied to a strong organized synoptic complex, only adding
emphasis to the potential with such high agreement within this
evenings CAMs. Neighborhood probs are just as impressive with a
strong >3" signal (70-90+%) and modest >5" (15-30%) output with the
latter tied closer to the Canadian border where the model agreement
is strongest for the heavier totals. A SLGT risk was added given
the favorable forecast from this evening and a strengthening
consensus amongst CAMs members for a locally enhanced flash flood
threat across the northern tier of the ND/MN area.
...Western U.S...
Upper air analysis indicates a broad closed upper circulation
positioned over Northern California with an expectation to
strengthen as we progress through the period. Increasing moisture
on the eastern flank of the mean trough will allow for a better
convective environment as the region enters a strong upper forcing
regime with a correlated instability maximum located over the Great
Basin. The approach of the upper low will initiate a blossoming of
convection across the Eastern and Northern Sierra, extending into
NV and eventually ID as we progress through today and this evening.
MUCAPE between 500-1000 J/kg will be most common within the
interior, plenty to induce a swath of enhanced convection over an
area not typically acclimated to see a decent amount of rainfall.
The current forecast calls for a multi-region impact of
convectively driven rainfall that could easily exceed 1" in any
area located from the Great Basin, Four Corners, and portions of
the Southwestern U.S. A fairly robust and widespread probability
for >1" (70+%) is well documented within the 00z HREF neighborhood
probability output over the above locations, a strong enough signal
to warrant a continuation of the previous MRGL risk forecast.
...Deep South...
Lingering low-level moisture with the proximity of a quasi-
stationary front across the Deep South will allow for another
period of locally enhanced rainfall that could cause some flash
flood concerns, generally over the Lower Mississippi Valley into
the Central Gulf coast. The signal is more diffuse than previous
periods, but the probability fields still indicate a formidable
potential for >3", especially along the Gulf coast where the
surface based instability is most prominent. This is a low-end MRGL
risk with a better chance for flash flooding to occur over those
areas that have been hit hardest the past several days from
Francine and its remnants.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID ATLANTIC, FLORIDA PANHANDLE, SOUTHERN ROCKIES, AND WESTERN
MONTANA...
...Mid Atlantic...
The remnant cyclone will be perusing through the Mid Atlantic come
Tuesday with its moisture flux steadily pushing northward through
the Central Mid Atlantic, carrying a potential for locally enhanced
rainfall and flash flood prospects. As of this time, the consensus
is still muddled on exactly where the remnant circulation will be
progressing, as well the forecast instability positioning on the
eastern flank of the circulation. PWAT anomalies will be creeping
closer to 1-2 standard deviations above normal, a historical
characteristic for heavy rain and flash flood concerns within the
Mid Atlantic. Ensemble instability fields are generally more
defined in-of the Blue Ridge in VA and across Southeastern VA.
These would likely be the areas with the most interest when it
comes to any higher risk. The shifts within the ensemble mean QPF
add too much variance to conclude the prospects for a higher risk,
at this time. With coordination from the local WFOs across the Mid
Atlantic, a MRGL was maintained from the previous forecast with
some adjustments based on the latest ensemble QPF footprint.
...Florida Panhandle...
Lingering low-level moisture flux tied a quasi-stationary front and
subsequent weak surface reflection along the Gulf Coast will allow
for one more period of isolated to widely scattered thunderstorms
within the Western FL Panhandle, bisecting an area that is
currently under more sensitive FFG indices compared to
climatological averages. The most prone location resides near
Apalachicola where recent 5-day rainfall totals are upwards of
8-15" leading to a fairly water-logged position along the immediate
Gulf coast. Despite the signal not being as impressive as previous
periods, the instability gradient right along the coast could
provide the necessary element to maintain a risk for locally heavy
rainfall, especially any cells tied to the boundary running
parallel to the coast. This is a lower-end MRGL risk categorically,
but with forecasts within the deterministic still pointing towards
cells producing >2" of rainfall, this was enough of a signal to
warrant the MRGL risk inherited from the previous forecast.
...Southern Rockies...
Ejecting shortwave trough out of the Four Corners will provide
ample mid and upper support for a generation of scattered
convection located across the Southern Rockies down into the
Sacramento Mountains in NM. The setup is geared towards locally
enhanced rainfall prospects tied to the terrain with the best
opportunity for impacts likely occurring over burn scars located
within the Sangre de Cristos and the Sacramento Mountains near
Ruidoso. Those two locations are very sensitive, and the setup
would certainly be conducive to providing local impacts within the
two separate MRGL risk areas. Areal QPF average is between
0.25-0.5", but deterministic output still signals potential for
upwards of 1.5-2" of rainfall, plenty to cause issues within the
complex terrain and over any sensitive grounds. A MRGL risk was
added over the above locations to account for the threat.
...Western Montana...
Approaching closed upper low over the Great Basin will pivot
northeastward while tilting negative as it progresses into the
Interior Mountain West, eventually situating over MT by the end of
the forecast cycle. Large scale ascent pattern will be maximized
downstream of the mean trough with ample mid-level moisture
advection being carried poleward into Big Sky Country, pushing PWAT
anomalies into the +2 to +3 standard deviation range. This is a
prominent signal when coupled with a strong upper forcing regime
and is co-located with an increase in boundary layer instability
when assessing the area MUCAPE field(s). Current QPF distribution
signals a scattered array of 1-2" totals with the most precip
footprint of the maxima tied closer to the terrain thanks to the
orographic elements typically favored in these types of setups. The convectively driven pattern is one that maintains prominence on
local flash flood prospects and still has some room for a targeted
higher risk pending the convective outlook once into better range
of the CAMs. For now, maintained general continuity with some
extension on the southern periphery of the risk area over MT.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER SOUTHEAST VIRGINIA...
...Mid Atlantic...
The disturbance that pushed ashore yesterday will continue to churn
over the Southern Mid Atlantic with a funneling of moisture off the
Atlantic aimed across far Northeastern NC and the adjacent VA
Tidewater through the morning and afternoon hours. Modest SBCAPE
located across the above areas will justify enough instability to
trigger heavier convective cores capable of producing heavy
rainfall in an area that has a historically lower FFG due to the
urbanization factors over places like Norfolk, Virginia Beach, up
to Williamsburg. The pattern has produced a relatively robust precip
footprint as it is with models struggling to handle the overall
magnitude of the rainfall on the eastern flank of the low. 00z HREF
probability fields are still pretty exuberant for >3" (50-70%)
across the Tidewater with an extension of 30-50% further inland
towards Richmond. PWAT anomalies are sufficiently around +2
standard deviations with the 00z sounding out of MHX producing an
observed PWAT of 2.27", a similar airmass that will be advecting
north with time. This pinpoints the Southeastern VA area as the
most notable area for potential flash flood capabilities for the
period when you couple the moisture anomalies and anticipated areal
instability presence for tomorrow. This allowed for a general
maintenance of the SLGT risk from the previous forecast with only
an minor adjustment necessary on the western and northern flank of
the risk area.
Across the rest of Western NC and Western VA, the general
circulation of the remnant low will drift into the Carolina
Piedmont and become fairly stationary over the course of the
forecast. The weakening of what's left over from the disturbance
will mitigate the precip field enough to limit the flash flood
concerns further inland, although still within the low end of the
MRGL risk. In fact, some of the 24 hr totals within the means are
pretty high across Western NC and VA, but a lot of the rainfall
comes from a persistent, steady light to moderate rain with
embedded heavier echoes during the afternoon and evening hours with
marginal diurnal destabilization. Forecast 1"/hr rainfall
probabilities are relatively tame with basically no calls for 2"/hr
within the prob fields. The signal was sufficient to maintain the
MRGL for the forecast with most of the rain accumulating over a
long period of time and not so much a classic flash flood prospect.
Those areas along the Blue Ridge into the Shenandoah will have the
best opportunity for flooding in this setup, but with higher FFGs
across the region, the prospects are lower than normal for a
heightened flash flood concern.
...Florida Panhandle...
Lingering low-level moisture flux tied a quasi-stationary front and
subsequent weak surface reflection along the Gulf Coast will allow
for one more period of isolated to widely scattered thunderstorms
within the Western FL Panhandle, bisecting an area that is
currently under more sensitive FFG indices compared to
climatological averages. The most prone location resides near
Apalachicola where recent 5-day rainfall totals are upwards of
8-15" leading to a fairly water-logged position along the immediate
Gulf coast. Despite the signal not being as impressive as previous
periods, the instability gradient right along the coast could
provide the necessary element to maintain a risk for locally heavy
rainfall, especially any cells tied to the boundary running
parallel to the coast. 00z HREF neighborhood prob output signals a
stripe of 25-50% in any given location across the I-10 corridor
down towards Apalachicola for >3" potential with a 10-15% prob for
5" over the same area. The convective coverage is more scattered
in nature, so the previous MRGL has merit given the probabilities
and anticipated rates hovering between 1-2"/hr.
...Southern Rockies and High Plains...
A strong, broad upper low over the Great Basin will continue to
slowly migrate to the northeast with increasing large scale forcing
downstream of the mean trough. Model consensus has grown over the
past succession of runs for a broader ascent pattern to develop
across the Southern Rockies with some aid of modest surface based
instability and terrain to generate a period of convection
beginning later this afternoon and early evening. Further east into
the NM and TX High Plains, a region of relatively favorable SBCAPE
between 500-1500 J/kg will allow for convection to develop
downstream of the mountains with cells becoming rooted at the
surface and helped along by shortwave energy ejecting out of the
adjacent terrain. 00z CAMs are more robust with the convective
pattern than even 24 hours ago, matching well with the recent ML
output from the ECMWF AIFS and Graphcast with some precip focus
over those areas. The HREF probs for >1" were much higher than
previous forecast as a result with some lower end probs for >2" and
3" located across parts of the Southern Rockies and High Plains.
The focal points for flash flooding potential will continue to be
the complex terrain and burn scars situated within the Sangre de
Cristos along with the Sacramento Mountains, further south. The
High Plains area is low-end within the MRGL risk threshold, but the
favorable pattern lends credence to the chance at an isolated cell
or two over-performing and causing some flood concerns. This also
matches the current UFVS First Guess Field forecast for the period
allowing for the two previous MRGL's in the region to be bridged
together and expanded east to account for the threat.
...Northern Rockies and High Plains...
Approaching closed upper low over the Great Basin will pivot
northeastward while tilting negative as it progresses into the
Interior Mountain West, eventually situating over MT by the end of
the forecast cycle. Large scale ascent pattern will be maximized
downstream of the mean trough with ample mid-level moisture
advection being carried poleward into Big Sky Country, pushing PWAT
anomalies into the +2 to +3 standard deviation range. This is a
prominent signal when coupled with a strong upper forcing regime
and is co-located with an increase in boundary layer instability
when assessing the area MUCAPE field(s). The latest 00z
deterministic and relevant ensembles, including the HREF provide a
strong consensus on the potential for locally heavy rainfall
extending from Western MT, where the signal is the strongest, over
into the High Plains of Eastern MT and points south near the Bad
Lands of SD. Ensemble QPF output is still within that 1-2" range
for an areal average, but pockets of higher QPF are dispersed all
across the above area thanks to the approach of the potent mid-
latitude cyclone. Considering the overall setup and growing
convective support of the CAMs downstream of the previous MRGL
risk, the risk area was expanded to include much of the rest of MT
down into Northeast WY and Western SD, areas that have more
favorability for flooding potential due to lower FFGs and higher
run off capabilities given the soil type and topographic makeup in
those locations.
A SLGT risk was mulled over for Western MT, but with coordination
from the local WFO in Great Falls, decided against the upgrade with
retaining focus for the higher impacts anticipated the next period
(Wednesday).
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MONTANA...
Negatively tilted mean trough and closed low over the Northern
Rockies will become vertically stacked over Big Sky Country leading
to a prolific axis of deformation developing in-of the Northern
Rockies encompassing Northwest MT. Ensemble mean QPF is quite
impressive given the synoptic regime with very little surface based
instability to work with. The whole pattern is driven by powerful
upper forcing within a well-defined axis of deformation on the
western side of a broad, occluding surface cyclone. Precip totals
off the latest deterministic are gaudy with a wide swath of 2-4"
with locally as high as 5+" located within the terrain to the east
of Great Falls. There has been some adjustments within the ensemble
means for the heaviest axis of QPF with a small displacement to the
east. When assessing the 500mb height anomalies between ensemble
runs and the current deterministic, there is a subtle, but
noticeable difference in the closed upper reflection being a bit
more into East-Central MT leading to a heavier QPF distribution
being a bit further east than previous iterations. This also
follows with the ML outputs recently with the trend having the axis
of deformation a bit further east of Great Falls, a general marker
for where the heaviest precip will focus. Ensemble bias corrected
QPF is also a touch east with totals relatively similar in the 2-4"
range with a max of just over 5". This will be something to
monitor going forth, but the signal is still very much present for
a locally impactful rainfall thanks to a strong mid-latitude
cyclone entering the interior Northern U.S. A SLGT risk was
maintained with a small extension eastward to reflect the recent
trends in guidance.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF MONTANA...
...Montana...
Current WV satellite imagery is a sight this evening with a broad
upper circulation engulfing much of the Inter Mountain West through
Big Sky Country in MT. Height falls have been impressive with this
disturbance as it pivots northeastward with an intensifying
dual surface low reflection located across Central MT with
pressures now down below 1000mb. The expectation is for the
cyclone to continue to intensify through the evening with an
occlusion phase occurring later this morning allowing for a strong,
well- defined deformation axis to center itself over the Northern
Rockies, east of Great Falls. PWAT anomalies approaching the upper
end of climatological peak for not just this time of year, but all
years according the latest NAEFS and ECENS moisture return
intervals. PWAT anomaly forecast is between +3 to +5 deviations, a
solid 97th to 99th percentile outcome given climatology which is
impressive to say the least. The strong large scale forcing
focused on the western side of the occluded cyclone will induce a
focused TROWAL across the aforementioned area above, in good
agreement amongst all guidance, including the CAMs and global
deterministic. Regardless of which model output you assess, the
prospects for 3-5" of rainfall with locally higher are being
forecast in-of the terrain east of Great Falls and the I-15
corridor in MT.
Latest HREF probabilities are pretty stout within both the
neighborhood and EAS fields across that general area with the
neighborhood probs running between 50-80% for >5" and even some
15-30% probs for the >8" totals being expressed. EAS probabilities
are >80% for at least 2", a testament to both the expected rainfall
and the consistency among the CAMs. This is no different from the
totals expressed within the global deterministic, and even the
ensemble blends are signaling over 4" in places. The bottom line
is; this will be a pretty formidable storm for the Northern Rockies
and adjacent Great Plains of MT with scattered instances of
flooding increasingly likely as we move through the daytime,
Wednesday. The storm will drift north and begin losing steam the
back end of the forecast leading to the organized TROWAL to fade
with lighter rates before tapering off later in the evening. The
previous SLGT risk was maintained given the strong continuity from
guidance. The only limitation to this being a higher risk is the
lack of sufficient surface instability with most of the positive
buoyancy relegated above the boundary layer. Still, rates of
1-1.5"/hr are still possible in the stronger cores, enough to
solidify the SLGT risk forecast.
...Mid Atlantic...
The period can be characterized by uncertainty within the guidance
as a disheveled surface and mid-level reflection left over from
what was Potential Tropical Cyclone 8 (PTC8) continues to meander
over the Mid Atlantic with a broad PWAT anomaly positioned across
Northern NC through the Delmarva. The interesting aspect of the
forecast is multiple deterministic outputs have some prospects of
heavy rainfall across parts of the Mid Atlantic, but there is a
large disagreement on where exactly that could take place. The
issue is the lack of an organized surface reflection that can help
bring everything together, but still has a presence that can allow
for smaller zones of opportunity to crop up. Assessing the ensemble
QPF has helped curb some of the discontinuity within the
deterministic leading to two areas of focus within the next 24 hrs.
The first is across the coast of NJ where an axis of convective
potential seems reasonable as the weak surface low analyzed over
Eastern NC begins to drift northeast with some resemblance of
organization as it enters over the Gulf stream. A modest jet streak
is forecast to develop over the Western Atlantic with the surface
low likely to lie within the left exit region of the jet allowing
for some modest strengthening as it moves northeast off the coast
of DE/NJ. The synoptic evolution is moving towards the "classic"
axis of deformation prospects as the low begins to intensify and
the western side of the cyclone exhibits a band of heavier precip
close to the DE/NJ coastal plain. This was outlined within the
latest HREF ensemble blended mean QPF with the matched mean QPF
displaying a relatively significant heavy QPF signature within the
Southern and Central NJ coast impacting areas from just north of
Cape May up through Atlantic City to just to the east of Toms
River. These areas aren't as flood prone compared to some of the
focused urban corridors inland, but the combination of some heavy
surf with rainfall rates between 1-2"/hr could cause some localized
flooding within the coast. Considering some of the uncertainty, the
only risk to add was a MRGL as the probabilities for >3" and >5" are
still favorable (40-60% and 15-30% respectively), but with only
modest agreement amongst guidance, it would be hard to issue
anything higher.
The second area of interest lies within much of Northern and
Eastern VA where the closed 500mb low will continue to spin south
of the region prompting some weak ascent focused over a region of
relatively modest instability and deep layer moisture. This is
another scenario where the guidance has precip, but the focus is
anywhere from Southern PA down to Hampton, VA. HREF probabilities
signal a broad axis of 40-60% probs for >3" somewhere from the WV
Panhandle down to Virginia Beach with some low end >5" probs
lingering within the same corridor. Ensemble QPF footprint depicts
an areal average of 1-1.5" located across the above zone with a
maxima centered over Northern VA. The prospects for heavy rainfall
also include areas along the Potomac, including the DC metro which
has notoriously lower FFGs thanks to urban sprawl. There is too
much uncertainty again for anything more than a MRGL risk, however
the environment is favorable for somewhere to see a heavy cell or
two that could cause issues. For that, have opted to add a second
MRGL risk within the Mid Atlantic with points between the two risk
areas void from the threats due to a greater model consensus
pointing at a relative QPF min within that area of the Delmarva.
...North-Central Sierra and Western NV...
Another broad closed upper low will come ashore over Northern CA
from the Pacific leading to increased moisture and favorable upper
forcing across the terrain of CA/NV beginning later this afternoon.
Despite cooler surface temps, steep low and mid-level lapse rates
with a decent outline of positive buoyancy within the forecast
soundings out of KREV signal the potential for some heavier
convective cell initiations across the North-Central Sierra over
into Western NV. There has been an uptick in the ensemble QPF
across this area the past succession of runs leading to a bit more
potential than what was thought prior. Historically, including with
the past system, we've seen these strong upper lows tend to
overachieve at times with the magnitude and spatial coverage of the
convection downstream. In this case, it's favoring the prospects
for locally heavy rainfall in the complex terrain and over an area
of lower FFGs like Reno which carry the urbanization factor. The
latest UFVS First Guess Field has also introduced a recommended
MRGL risk for the first time in its runs as of late with the uptick
in the signal. A MRGL risk was added to account for the threat with
the threshold still on the lower half of the risk spectrum.
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
A non-zero flash flood threat does exist across portions of the
Upper Midwest, however the antecedent dry conditions will allow for
much of this rain to be beneficial over the area of interest
(Northern IA, Southeastern MN, Western WI, and the Arrowhead of
MN). Urban corridors will be the most favorable spots for flash
flood prospects. Rates generally capped around 1-2"/hr, falling shy
of the current FFG forecast in the area. If the signal becomes more
impressive in the next 24 hours, a targeted MRGL is possible.
Kleebauer
Day 3
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE TRANSVERSE RANGES OF CALIFORNIA, UPPER MIDWEST, AND THE URBAN
CORRIDOR OF SOUTHEAST FLORIDA...
...California...
A fairly stout closed upper low over the Pacific will be the
driving force behind increased forcing and region moisture
advection into the southern half of California this period. Recent
wildfires have brought about a plethora of burn scars within the
terrain of the Transverse Ranges that have become increasingly
susceptible to flash flooding prospects due to hydrophobic soils
allowing for consistent run off capabilities. The pattern evolution
supports a strong mid and upper forcing regime to be co-located
within the Central and Southern CA coasts, as well as the immediate
terrain to the east of the coastal plain. Latest 00z HREF mean QPF
distribution pinpoints a regional maxima within the confines of
the Transverse Range, intersecting an area with multiple burn scar
locations that will likely exhibit flash flood effects thanks to
the current antecedent soil environment. There's a consensus within
both the mean QPF and probability fields for >1" of rainfall in-of
the aforementioned area with even some prospects for locally >2"
as noted within the neighborhood probabilities for D1. With
coordination from earlier updates by the local WFOs involved
prompting the MRGL risk, and with the signal remaining prolific
enough from the latest guidance, the MRGL risk was maintained to
project continuity.
...Upper Midwest...
An increasingly favorable pattern will invoke a period of
scattered to widespread thunderstorm activity capable of producing
locally enhanced rainfall cores leading to 1-2"/hr rain rates in
the strongest convective cores. Steepening lapse rates, increasing
bulk shear, and modest surface to mid-level buoyancy will create an
environment conducive for not only thunderstorm initiation, but
prolonged updrafts capable of maintaining significant storm
structure within a core of elevated PWATs advected ahead of a mean
trough to the west. Latest HREF EAS signals for >1" are pretty high
(60-80%) across much of Northwest WI, an area where most CAMs are
pinpointing as the primary area of focus for this afternoon's and
evening's convective prospects. Neighborhood probs are a bit less
emphatic than what normally constitutes a higher impact potential,
but still fairly significant nonetheless for potential totals
ranging between 2-4" over portions of Eastern MN into Northwest WI.
The antecedent conditions present within the zone of highest
probabilities is borderline unfavorable for flooding with NASA
SPoRT indicating soil moisture percentiles closer to 40-50% over
the area. Considering the favorable environment and SPC outlining
the area with a Slight Risk for severe weather, believe the setup
is good enough for a targeted MRGL within the core of the best
probabilities based off the latest hi-res ensemble.
...Southeast Florida...
A meandering frontal boundary in-of South FL will become a focal
point for elevated convective coverage, especially just inland of
the urban metro of Miami up to West Palm Beach when sea breeze
initiation becomes tied within the frontal positioning later this
afternoon. The steering pattern is favorable for convective
development to drift back over the urban metro, settling within a
modest PWAT anomaly situated over the area. The core of convection
will likely drop rates between 1-3"/hr with intra-hour rates >3"
more than plausible, a factor that typically causes flash flood
concerns within the urban corridor. HREF blended mean QPF has a
bullseye situated within the Fort Lauderdale/Pompano Beach area
with a small secondary max near Miami. This signal is fairly
classic for a lower category risk, especially when you add the
steering pattern leading to favorable motions back over the cities
along the Southeast FL coast. The previous forecast was maintained
with only a very minor adjustment along the western periphery of
the risk area.
Kleebauer
Day 2
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE URBAN CORRIDOR OF SOUTHEAST FLORIDA, NORTHEAST NEW MEXICO, AND
THE DESERT SOUTHWEST...
...Southeast Florida...
A secondary period of impact from active sea breeze convection and
propagation into the urban corridor is forecast to transpire on
Friday afternoon. The signal is not as robust as the previous
period, but any heavy rainfall prior would precede this potential
event with lower FFG signals leading to a bit more sensitivity to
flash flood prospects. The ensemble mean QPF is a touch less
aggressive as stated, so it will remain a period to monitor to see
if the threat trends higher once we move closer to the range of the
CAMs. For now, maintained general continuity as the signal
persists for a flash flood threat in-of the urban corridor
extending from Miami up through West Palm Beach.
...New Mexico...
The prolific closed upper circulation out across the Southwest will
begin pressing eastward with an increased upper forcing pattern
thanks to a broad diffluent axis downstream of the mean
trough/upper low. Most of the latest global deterministic eject a
rapidly moving shortwave out of Chihuahua, traversing over the
Eastern half of NM by the afternoon and evening hours Friday. A
tongue of modest instability based off the mean MUCAPE forecast
from all the deterministic is forecast to bisect that eastern side
of the state with the best focus aligned from the Sacramentos up
through the western side of the Caprock and into the High Plains of Northeastern NM. Flash flooding threats will likely be tied to
urban areas and some of the remnant burn scars located within the
Sacramento Mountains, however these setups are notoriously tricky,
especially with the propensity for convection to become anchored
along the instability gradient favored in the terrain and adjacent
Caprock. Some of the deterministic output is pretty reasonable for
isolated flash flood prospects, especially when you add the
increasingly favorable upper pattern. A small MRGL risk was added
to encompass the areas above with the threat lying on the lower-end
of the risk threshold.
...Desert Southwest...
Our deep closed upper reflection will migrate slowly to the east
with the circulation likely crossing Southern CA into the interior
Desert Southwest as we move towards the end of the period. As of
now, the current QPF output is not exactly what would constitute a
favorable flash flood scenario. However, considering the expected
pattern evolution and the advection of more moist, buoyant air
northward ahead of the upper low, you have to acknowledge the
prospects of a more scattered pulse convective pattern over an
area that is much more conducive for flash flood potential. Recent
ECMWF AIFS ML output was actually more aggressive than the recent
deterministic in the spatial coverage of precip, and even for the
magnitude of the expected setup. This tends to signal a red flag
for the deterministic not being aggressive enough with such a
strong mid and upper pattern. A low-end MRGL was placed over
portions of Southern CA/NV into Western AZ to outline the favored
area for scattered convective development with modest storm cores
capable of locally heavy rainfall.
From Mike Powell@618:250/1 to All on Fri Sep 20 09:54:00 2024
FOUS30 KWBC 200821
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
421 AM EDT Fri Sep 20 2024
Day 1
Valid 12Z Fri Sep 20 2024 - 12Z Sat Sep 21 2024
......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHERN UNITED STATES......
...Southern California/Western Arizona Deserts...
An upper low moving down the coast early today will move inland over
the Desert Southwest later today. Increasing coverage of showers
and thunderstorms are expected as moisture will accompany the low
into the usually dry areas of southeast California, southern Nevada,
and northwestern Arizona..with greatest coverage tied to the
steepest low- to mid-level lapse rates at the center of the upper
low where upper level temperatures will be coldest. That is also
where steering flow becomes weak...leading to slow moving storms.
With any more persistent showers and storms, expect isolated flash
flooding with any burn scars, dry washes, and other flood-prone
areas.PWATs between 0.5 and 0.75 inches in these areas will peak at
about 1.5 sigma above normal for this time of year.
...New Mexico...
Broad upper level diffluence ahead of the positively tilted low
over the Desert Southwest will cause a somewhat narrow 50-100 mile
wide plume of training showers and possibly embedded thunderstorms
to develop over eastern New Mexico and into portions of the far
western Texas and Oklahoma Panhandles. Given the sensitivities of
some of these areas to prolonged rainfall, the Marginal risk
remains in place, especially for any burn scars or flood prone
areas.
...South Florida...
Today looks to be the final day of heavy rainfall potential is
expected in South Florida...especially in the urban corridor from
West Palm Beach south through Homestead. Highly efficient rainfall
producing thunderstorms should be slow-moving on the leading edge of
an advancing cold front. The front will clear South Florida by
early Saturday turning the predominant flow more northeasterly. This
will cut down on the precipitable water values some, while also
greatly limiting, though not eliminating the coverage of storms by
Saturday. Thus, the Marginal flash flooding threat should end once
the convection simmers down late this afternoon or this evening.
Bann
Day 2
Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024
......THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE CENTRAL ROCKIES AND CENTRAL PLAINS......
A broad upper trough with evolution of a closed low over the West
will translate eastward on Saturday with an evolving setup capable
of showers and thunderstorms across the Rockies through the
Central Plains, especially once the trough axis picks up forward
speed. A combination of favorable ascent and increased moisture
advection thanks to a developing LLJ pattern east of the Front
Range will lead to thunderstorms capable of producing several
heavy cores with locally enhanced rainfall. The 20/00Z HREF
neighborhood probabilities...available only during the first half
of the day 2 outlook period on the overnight shift...focuses brief
periods of 30 percent or greater probabilities of exceeding 1 inch
per hour developing out over the plains of eastern New Mexico that
ease into West Texas by early evening...with the threat continuing
into Saturday night in response to the development of a low level
jet. Farther north...the signals for locally heavy rainfall also
appear across portions of the Plains states as thunderstorms form
along and ahead of a cold front tied to an amplifying upper trough
building across the northern US. As mentioned in a previous discussion...antecedent conditions and the fact that much of the
area is rural will help mitigate some of the flash flooding concern
due to amount of infiltration possible.
Bann
Day 3
Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE SOUTHERN
GREAT LAKES SOUTHWESTWARD ACROSS PORTIONS OF THE CENTRAL PLAINS...
An upper trough initially over the central and southern Rockies on
Sunday morning will move eastward on Sunday into early Monday
morning...with showers and thunderstorms developing ahead of the
front in an airmass characterized by precipitable water values
generally in the 1.6 to 1.8 inch range. Given the moisture in place
and enough mid- and upper-level shear to support some convective organization...the possibility exists for excessive rainfall due to
some intense downpours. Tended to favor the cluster of models
guidance led by the ECMWF/GFS/CMC with respect to the potential for
heaviest amounts somewhere from southeast Kansas into
Missouri...but did not go as far east as suggested by the
aggressive UKMET. Suspect there is a non-zero threat of excessive
rainfall farther to the southwest where storms may also be able to
organize but for now favored the better moisture and the region of
better upper difluence.
From Mike Powell@618:250/1 to All on Sat Sep 21 09:52:00 2024
FOUS30 KWBC 210835
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
435 AM EDT Sat Sep 21 2024
Day 1
Valid 12Z Sat Sep 21 2024 - 12Z Sun Sep 22 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS...
...New Mexico into Texas/Oklahoma Panhandles...
The large-scale flow across the southern Great Basin will continue
to draw moisture northward at the low levels while becoming increasingly diffluent aloft in response to the approach of an upper low moving
across the southern Great Basin today. Precipitable water values
are forecast to exceed 1.5 inches across parts of West Texas by
late afternoon while the right entrance region of an upper level
jet rounds the base of an upper trough and tracks over the region.
This combination should help support multi-cell storms capable of
producing isolated rainfall rates of an inch or more and storms
total rainfall amounts in excess of 2 inches over portions of
northeastern New Mexico into the Texas panhandle during the
afternoon or evening hours with an associated risk of flash
flooding. There was one round of showers and thunderstorms
that developed on Friday evening over southeast Kansas and
weakened as it moved into southwest Missouri...followed by a second
flare up of convection in the overnight hours on a track a bit to
the south of the first round. There was some concern is more about
the cumulative effect of multiple rounds lowering the flash flood
guidance here ahead of anything developing later in the day...but
warming cloud tops and limited coverage of 1 inch amounts was
enough to keep the area in the Marginal risk category for the time
being.
Bann
Day 2
Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY TO THE SOUTHERN HIGH PLAINS...
The focus for areas of heavy rainfall will continue to be along a
plume of deeper moisture...precipitable water values generally at
or above 1,75 inches...from the western portion of Texas
northeastward to where it begins to encounter a cold front moving
in from the northern United States. An upper level shortwave trough
over the central and southern Rockies on Sunday morning will eject
eastward during the day...aiding the formation of storms over parts
of western Texas capable of heavy downpours around the time of
maximum heating. Farther to the east...model guidance tends to
generate at least some convection capable of producing isolated 2
to 3 inch rainfall amounts over portions of Missouri as moisture
flux convergence occurs ahead of a cold front pushing southward.
Mesoscale guidance...available through the first 12 hours of the
Day 2 period for the overnight EROs...does show low-end potential
for 2 inch per hour rates embedded within a broader area where
probabilities of 1 inch per hour rates are on the order of 10 to 25
pct. There was already a Slight Risk introduced here and only
needed a bit of realignment to fit the latest guidance.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER A PORTION OF
THE MISSISSIPPI VALLEY EASTWARD INTO THE OHIO VALLEY...
As the large scale upper trough continues to move eastward on
Monday...the plume of available moisture gets elongated and the
moisture flux along the front becomes less well defined. There will
be some broader upper support along the front which will have taken
on a more east-west orientation...so there is at least some
potential for problems from run off with 1 to 2 inches or rainfall
over portions of Illinois into Indiana. But with convection
expected to become increasingly elevated and with flash flood
guidance tending to be 2.5 inches per hour or greater...will
maintain a Marginal Risk area for now and evaluate the need for an
upgrade if rates/amounts pick up in later model cycles.
From Mike Powell@618:250/1 to All on Sun Sep 22 08:37:00 2024
FOUS30 KWBC 220834
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
434 AM EDT Sun Sep 22 2024
Day 1
Valid 12Z Sun Sep 22 2024 - 12Z Mon Sep 23 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY TO THE SOUTHERN HIGH PLAINS...
The focus for areas of heavy rainfall will continue to be within a
plume of deeper moisture ahead of a large-scale upper trough making
its way out over the plains towards the Mid-Mississippi Valley. The
airmass already in place had precipitable water values generally
at or above 1,75 inches...from the western portion of Texas
northeastward to where it encounters a cold front moving in from
the northern United States. Latest model runs still support two
areas of more concentrated rainfall warranting a Slight Risk area--
namely over western Texas and from southern Missouri into southern
Illinois. The southern area should be more driven by diurnal
instability strong enough to support spotty 2 or 3 inch rainfall
amounts and rainfall rates topping an inch per hour. The area
farther east will have the added advantage of a surface front to
help focus moisture convergence but the upper support is not
expected to be as strong as it was on Saturday. For that
reason...confidence in occurrence is somewhat diminished. Made a
small southward adjustment to the Slight Risk area based on latest
guidance and a trimmed a bit of the northern periphery of the
Marginal given the placement of the cold front.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID MISSISSIPPI VALLEY...
Surface low pressure begins to develop on Monday as shortwave
energy moving through the southern stream trough...leading to
renewed showers and thunderstorms in the Mid-Mississippi Valley
early in the day that shifts into the Ohio Valley by late Monday
night/early Tuesday morning. The best overlap of ingredients and
forcing looks appears to be from the afternoon into the early
evening across portions of southeast Missouri into parts of
southern Illinois. After that...low level flow should take on an
increasing component parallel to the front. Between the weakening
magnitude of moisture flux convergence at the front and the loss of
daytime heating/destabilization...rainfall rates should be waning.
There are a few ensemble members which generate over an inch of
rain over the Ohio Valley as upper flow backs in response to energy
in the northern stream...but the expectation is that the flash
flooding risk is minimal with those rainfall rates and dry
antecedent conditions.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE Eastern OHIO VALLEY AND ACROSS CENTRAL OF WESTERN TEXAS...
Ohio Valley...
As energy in the southern stream interacts with digging energy in
the northern stream late on Day2...the flow begins to back and
become diffluent leading to increasing coverage of rainfall by the
time Day 3 begins. The flow should continue to become increasingly
diffluent aloft with precipitable water values now about 1.75
inches in place. Spaghetti plots of QPF from the ensembles show
minimal support for amounts greater than 2 inches...and most of
that support is from just a few GEFS members. With antecedent
conditions being so dry...it is presumed that only isolated
instances will occur that run off problems occur.
Texas
Light southeasterly on-shore flow across far southern Texas will
start to draw moisture northward again on Tuesday afternoon. As it
does so..a weakening cold front will help provide some focus for
showers and a few thunderstorms in central or western Texas. The
NAM and ECMWF were the only two operational runs from the 22/00Z
model suite to generate 2+ inch amounts of QPF while the GFS had
support from the GEFS favoring a weaker/drier solution. Given the uncertainty...opted to maintain a Marginal risk for this cycle.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-MISSISSIPPI VALLEY EASTWARD TO THE SOUTHERN APPALACHIANS...
Precipitation over the southern Plains will continue to spread
north ahead of a positively-tilted upper shortwave trough drifting
east through the central Plains. Deepening moisture (PWs ~1.75-2
inches) supported by southwesterly inflow, and large-scale ascent
generated by the upper shortwave, are expected to produce a broad
area of precipitation spreading northeast early in the period from
the Ozarks through the lower Missouri and mid Mississippi valleys.
There is the potential for a stripe of locally heavy amounts, with
the 00Z HREF showing high neighborhood probabilities for
accumulations of 3 inches or more from central Missouri eastward
through the St. Louis metro. However, limited instability should
help to keep rainfall rates in check through the morning hours. A
better chance for heavy rainfall rates may develop later in the day
as trailing energy and daytime heating contribute to storms
redeveloping near the Ozarks and tracking northeast through the mid
Mississippi Valley and into the lower Ohio Valley later this
afternoon into the evening.
Moisture spreading further east is expected to fuel showers and
storms developing later today across the central and southern
Appalachians. Some of the hi-res guidance continue to show some
potential for isolated heavy amounts across the region, with the
greatest threat centered over western North Carolina, where the
HREF is showing high neighborhood probabilities for accumulations
of 2 inches or more.
..THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO VALLEY AND THE CENTRAL TO SOUTHERN APPALACHIANS...
The shortwave trough moving across the mid Mississippi Valley on
Day 1 will begin to interact with a northern stream trough
amplifying over the northern Plains early in the period. Models
show the upper pattern quickly amplifying over the Mississippi
Valley on Tuesday. At the surface, low pressure is expected to
track northeast from the mid Mississippi Valley to the Great Lakes,
with a trailing cold front extending back through the Ohio Valley.
An axis of deeper moisture (PWs ~1.50-1.75 inches) coinciding with
deep southeasterly flow ahead of the upper trough will fuel
showers and storms along the low track and ahead of the trailing
cold front. Models do not present a signal for widespread heavy
amounts but show the potential for locally heavy amounts,
including for portions of the central to southern Appalachians,
where relative soil moisture is slightly higher and flash flood
guidance values lower.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST, TENNESSEE VALLEY, AND THE SOUTHERN APPALACHIANS...
Although disagreements on the track continue, the consensus of the
models show a powerful tropical cyclone tracking north through the
eastern Gulf of Mexico during this period. Apart from the faster
Canadian solutions, which bring heavy amounts into the Florida
Panhandle, most of the deterministic models and their ensemble
means show moderate to heavy rains brushing the coast, but the
heaviest amounts remaining out over the open waters through 12Z
Thu. Therefore, just a Marginal was maintained from South Florida
and the Keys northward along the Florida Gulf Coast through the
Panhandle. However, should more guidance begin to trend toward a
faster solution, future upgrades may be necessary.
Despite their disagreements with the cyclone track, the models are
presenting a growing signal for heavy amounts developing well to
the north. The cold front pushing east into the Ohio, Tennessee,
and lower Mississippi Valley prior to the period is forecast to
slow across the region, providing a focus for moisture ushered in
by strong southeasterly to easterly flow ahead of the system. With
some longitudinal disagreement, the GFS and ECMWF show PWs
increasing to 1.75-2 inches along an axis of strong low level
convergence extending north from the northern Gulf Coast into the
Tennessee Valley. Meanwhile, an upper low that will begin to
develop over the mid Mississippi Valley on Wednesday is expected to
drop south, enhancing lift across the region. While there is some
west-east disagreement on its placement, most of the 00Z
deterministic runs showed a stripe of 2-4 inch accumulations
extending northeastward from northern Alabama into the southern
Appalachians.
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE OHIO/Tennessee VALLEYS AND ADJACENT PORTIONS OF THE MID-ATLANTIC ...
One area of showers and thunderstorms that produced spotty moderate
to heavy rainfall overnight along a quasi-stationary front
overnight should be weakening as it reaches portions of the Mid-
Atlantic region by morning. However...shortwave energy in the
northern stream will be developing a closed low that deepens with
time as it tracks from the Upper Midwest into the Mid-Mississippi
Valley later tonight. This backs the flow aloft and supports some
convection with locally heavy rainfall rates in a diffluent flow
regime over the eastern Ohio Valley as well as thunderstorms ahead
of the low track and ahead of a trailing cold front. Model guidance
continues to show the potential for locally heavy amounts while not
presenting a signal for widespread heavy rainfall
totals...including portions of the central and southern
Appalachians where relative soil moisture is slightly higher and
where flash flood guidance was lower.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AND EASTERN TENNESSEE VALLEY...
Only minor adjustments were needed to the previously issued Slight
risk area from portions of Alabama and Georgia into southeast
Tennessee...while a targeted Slight Risk area was introduced along
the coastline of the Florida panhandle. It appears that a
predecessor rainfall event will take shape somewhere in the
Southeast US as moisture streams northward from the Gulf of Mexico
ahead of Potential Tropical Cyclone Nine and interacts with a cold
front dropping southward from the Tennessee Valley. There is still
some disagreement among the various models with respect to where
the axis may set up but the area covered by the Slight risk has the
best overlap of different solutions so only minor adjustments were
needed to what is effectively a high-end Slight Risk area. Along
the coast...model QPF has increased along portions of the Florida
coastline to the point where a Slight seems warranted.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AT POTENTIAL TROPICAL CYCLONE NINE
MOVES INLAND...
There should be an increasing threat of widespread and potentially
significant rainfall as Potential Tropical Cyclone Nine approaches
the coast and eventually makes landfall sometime around 27/00Z
based on the latest guidance from the National Hurricane Center.
Moderate to heavy rainfall may develop across portions of Florida
panhandle into the Southeast US well before PTC Nine make landfall
as strong transport of Gulf moisture interacts with deepening mid-
and upper-layer closed low over the central/southern Mississippi
Valley. Primary changes were to extend the Moderate risk area
northward along the NHC path of PTC Nine where the guidance had
shown an increase in rainfall amounts...with the Moderate risk now
into the southern Appalachians where terrain effects look to result
in locally enhanced rainfall amounts.
With an unusually deep low best seen in the mid- and upper-levels
located to the west...at least some of the moisture being drawn
inland by Nine will begin to get drawn westward over portions of
the Tennessee Valley and into the Mississippi Valley. As a
result...part of the Slight risk area from the Southeast US gets
pulled westward into the Tennessee Valley around the mid- and
upper-level low. Given the model spread and the poor run to run consistency...refrained from taking the Slight risk area too far
westward at this point.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE VIRGINIA/NORTH CAROLINA PIEDMONT NEAR THE STATE LINE...
...16Z Update...
...VA/NC border...
In coordination with RNK/Blacksburg, VA; RAH/Raleigh, NC; and
AKQ/Wakefield, VA forecast offices, a Slight Risk area was
introduced with this update along the VA/NC border in the Piedmont
area. Tonight, much of the guidance, including the 12Z HRRR, HREF,
and both ARWs are suggesting an area of training convection will
develop. The heavy rain will be forced by a stationary surface
trough over the area combined with a local maximum of atmospheric
moisture with PWATs over 1.75 inches. A strong and deep upper level
trough over the Mississippi Valley will greatly increase the upper
level divergence to its east. This too will greatly add lifting
support to the storms as they form with the convective maximum
after sunset this evening. HREF probabilities are up to 30% of
exceeding FFGs in the western part of the Slight risk area, as well
as over 80% of exceeding 3 inches of rain in the neighborhood
probabilities and almost 50% of exceeding 5 inches through
tonight.
The factors working against heavy rain and flash flooding are some
antecedent dry conditions, with at least average soil moisture
across this area, and marginal instability generally around 500 to
1,000 J/kg. The instability will limit to some extent the
widespread coverage of the heavy rain, but the excellent forcing
may be able to make up for that. Portions of the Slight Risk area
saw heavy rainfall last week, which at least has kept those area
from having totally dry soils...but average soil moisture can
sometimes work against flash flooding as some clays can be
hydrophobic when they're really dry, resulting in extra runoff.
Such is not expected to be the case here.
...Midwest...
Elsewhere across the Upper Ohio and Tennessee Valleys expect
continued off and on showers and storms, but general struggles with organization. Very dry soils north of the Ohio River should also
generally limit the flooding threat...whereas along the west facing
slopes of the Appalachians, localized upslope may enhance it a bit.
Regardless any flash flooding in these areas should be confined to
urban and flood-prone locations.
Wegman
...Previous Discussion...
One area of showers and thunderstorms that produced spotty moderate
to heavy rainfall overnight along a quasi-stationary front
overnight should be weakening as it reaches portions of the Mid-
Atlantic region by morning. However...shortwave energy in the
northern stream will be developing a closed low that deepens with
time as it tracks from the Upper Midwest into the Mid-Mississippi
Valley later tonight. This backs the flow aloft and supports some
convection with locally heavy rainfall rates in a diffluent flow
regime over the eastern Ohio Valley as well as thunderstorms ahead
of the low track and ahead of a trailing cold front. Model guidance
continues to show the potential for locally heavy amounts while not
presenting a signal for widespread heavy rainfall
totals...including portions of the central and southern
Appalachians where relative soil moisture is slightly higher and
where flash flood guidance was lower.
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AND EASTERN TENNESSEE VALLEY...
Only minor adjustments were needed to the previously issued Slight
risk area from portions of Alabama and Georgia into southeast
Tennessee...while a targeted Slight Risk area was introduced along
the coastline of the Florida panhandle. It appears that a
predecessor rainfall event will take shape somewhere in the
Southeast US as moisture streams northward from the Gulf of Mexico
ahead of Potential Tropical Cyclone Nine and interacts with a cold
front dropping southward from the Tennessee Valley. There is still
some disagreement among the various models with respect to where
the axis may set up but the area covered by the Slight risk has the
best overlap of different solutions so only minor adjustments were
needed to what is effectively a high-end Slight Risk area. Along
the coast...model QPF has increased along portions of the Florida
coastline to the point where a Slight seems warranted.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHEAST UNITED STATES AT POTENTIAL TROPICAL CYCLONE NINE
MOVES INLAND...
There should be an increasing threat of widespread and potentially
significant rainfall as Potential Tropical Cyclone Nine approaches
the coast and eventually makes landfall sometime around 27/00Z
based on the latest guidance from the National Hurricane Center.
Moderate to heavy rainfall may develop across portions of Florida
panhandle into the Southeast US well before PTC Nine make landfall
as strong transport of Gulf moisture interacts with deepening mid-
and upper-layer closed low over the central/southern Mississippi
Valley. Primary changes were to extend the Moderate risk area
northward along the NHC path of PTC Nine where the guidance had
shown an increase in rainfall amounts...with the Moderate risk now
into the southern Appalachians where terrain effects look to result
in locally enhanced rainfall amounts.
With an unusually deep low best seen in the mid- and upper-levels
located to the west...at least some of the moisture being drawn
inland by Nine will begin to get drawn westward over portions of
the Tennessee Valley and into the Mississippi Valley. As a
result...part of the Slight risk area from the Southeast US gets
pulled westward into the Tennessee Valley around the mid- and
upper-level low. Given the model spread and the poor run to run consistency...refrained from taking the Slight risk area too far
westward at this point.
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST UNITED STATES...
Even though Helene should still be well off-shore during the
period...abundant moisture should be streaming northward ahead of
the storm. The models have been increasingly aggressive with
rainfall amounts coming off the Gulf into the Florida panhandle
with locally excessive amounts of rainfall along the coastline. Of
greater concern is the moisture that continues to stream northward
into portions of northern Georgia and upstate South Carolina where
a surface cold front acts to focus a predecessor rainfall event.
Some pieces of guidance...including the HAFS A and B parent grids
...drop 5 or 6 inches or rainfall within a broader area of 2 to 4
inches of rainfall. The overall agreement on placement of the axis
is really very good with only subtle east/west variations...which
necessitated an expansion of the moderate risk area and now
includes some of the complex terrain of the southern Appalachians.
Slight and Marginal risk areas surrounding covered the risk of
convection forming along broad synoptic scale flow getting drawn
into the environment ahead of Helene.
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
SOUTHERN APPALACHIANS...
Given the increasing confidence about the overlap of the axis of
a predecessor rainfall event on Day 1 period and the amount of
rainfall expected as the center of Helene approaches the southern
Appalachians during the latter part of the Day 2 period...there has
been a growing number of models generating close to 10 or 11
inches of rainfall for the 2-days combined in the complex terrain.
That raises concerns about impactful flooding becoming widespread
with considerable and potentially life-threatening flash and urban
flooding as well as areas of significant river flooding over
portions of Florida, the Southeast U.S. and the southern
Appalachians. Widespread minor to moderate river flooding is likely
and major river flooding is possible. Landslides are possible in
areas of steep terrain in the southern Appalachians given these
rainfall amounts. This prompted the upgrade to a high risk over
portions of northwestern Georgia into far western South Carolina
that was mainly confined to the terrain. What effectively amounts
to a high-end moderate risk surrounds the high that extends along
much of the track as Helene makes landfall along the Florida
panhandle coastline around 27/00Z (give or take a couple of hours)
based on the latest NHC guidance. In terms of model guidance...the
tracks tended to be clustered well resulting in NHC guidance being
close to most model runs. For the Day 2 period...that resulted in
the highest rainfall totals being close to each other where the
orographic forcing was consistent. A second axis of excessive
rainfall branched off to the northwest from Helene's track towards
a deep-layered low developing over the southern portion of the
Mississippi Valley as moisture gets entrained into that system
leading to scattered convection that could produce locally heavy
amounts. Mainly confined the Slight risk to the Tennessee Valley
and only a Marginal risk extending back towards the core of the
upper system.
Bann
Day 3
Valid 12Z Fri Sep 27 2024 - 12Z Sat Sep 28 2024
...INTRODUCED A MODERATE RISK OF EXCESSIVE RAINFALL OVER A PORTION
OF THE UPSLOPE REGION INTO THE SOUTHERN APPALACHIANS...
Based on latest NHC track guidance...Helene should be in northern
Georgia and tracking on a more northwestward course during the day.
This suggests the flow of deep moisture should be on-going to the
terrain of western Carolinas into southwest Virginia. Latest model
guidance shows several inches of rainfall in addition to what falls
in the predecessor 48 hours...felt a focused Moderate risk placed
where there was best model consensus was warranted. It was also a
multi-day rainfall total farther south along the NHC track in the
eastern Tennessee Valley that was the main reason to keep a Slight
Risk where model QPF be a little too light to support one. That
portion of the Slight Risk area extends westward to the deep-
layered low where steepening low level lapse rate and the
additional moisture streaming off of Helene could lead to some
showers and thunderstorms capable of producing locally heavy
downpours.
From Mike Powell@618:250/1 to All on Sat Sep 28 08:59:00 2024
FOUS30 KWBC 280758
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Sat Sep 28 2024
Day 1
Valid 12Z Sat Sep 28 2024 - 12Z Sun Sep 29 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
OHIO AND TENNESSEE VALLEYS AS WELL AS FOR CENTRAL FLORIDA...
...Ohio/Tennessee Valleys...
The combined upper low and remnant circulation of what is left of
Helene is expected to slow/stall over the Ohio and Tennessee
Valleys. With very little if any instability to work with the bulk
of rain today will likely be stratiform with hourly rates around
0.5 inch/hour or lower. At best there may be some isolated flash
flooding, therefore maintained the broad Marginal Risk that is in
place over the region.
...Florida...
A trailing cold front across the Florida peninsula will be present
during this period along with abundant instability and moisture
within proximity to this feature. The development of strong
thunderstorms can be expected, especially on the western side of
the peninsula. Recent heavy rain has increased soil sensitivity for
much of the area therefore maintained a Marginal Risk for excessive
rainfall for the western and central peninsula.
Campbell
Day 2 Valid 12Z Sun Sep 29 2024 - 12Z Mon Sep 30 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC/APPALACHIANS AS WELL AS THE NORTHERN FLORIDA
PENINSULA...
...Mid-Atlantic/Central Appalachians...
During this period the remnant low of Helene will get picked up by
the upper level jet and track eastward into the Mid-Atlantic and Central/Northern Appalachians. There will be an increase in
available instability which may lead to localized enhanced rainfall
rates for portions of the Shenandoah Valley and west to the
Allegheny Front in Maryland, West Virginia, and Virginia; which may
result in isolated to widely scattered instances of flash
flooding. The level of threat continues to be handled well with the
Marginal Risk that is currently in effect. There may be a need for
a Slight Risk with future updates depending on model trends and
observations.
...Florida...
Convection will continue to fire up in proximity to a stalled
frontal boundary draped across northern/central Florida. Storm
motion may be fast-moving with some tracking over areas hard-hit
by Helene, but because progressive nature of the convection a
Marginal Risk remains appropriate for this period.
Campbell
Day 3
Valid 12Z Mon Sep 30 2024 - 12Z Tue Oct 01 2024
...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-ATLANTIC/CENTRAL APPALACHIANS...
The lingering upper low over the East and accompanying moisture
from Helene will continue to produce some showers and thunderstorms
over portions of the Appalachians and Mid-Atlantic into Monday.
Heavy intensity and accumulations are not expected to be widespread
or heavy, however the recent wet period keeps an elevated threat
for excessive rainfall and isolated flooding concerns. A Marginal
remains in effect from south-central Virginia to central
Pennsylvania.
From Mike Powell@618:250/1 to All on Sun Sep 29 09:54:00 2024
FOUS30 KWBC 290815
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Sun Sep 29 2024
Day 1
Valid 12Z Sun Sep 29 2024 - 12Z Mon Sep 30 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE INTERIOR MID-ATLANTIC...
...Mid-Atlantic/Central Appalachians...
Remnant moisture from Helene will remain caught up in the upper
trough pattern across the Ohio Valley while the mean trough begins
to slowly advance eastward through the period. During this period
precipitable water anomalies will be on the order of 1 to 2
standard deviations above normal and will encounter increased
forcing from the upper jet and modest lift through the column over
much of the Mid-Atlantic. This setup will support the development
of scattered to numerous thunderstorms that will likely have slower
storm motions. Recent rains have lowered local FFGs and increased
sensitivity across the region. Areal average QPF for the region
will be in the 1 to 3 inch range with isolated maximums in excess
of 3 inches will be possible. A Slight Risk for excessive rainfall
is in effect for extreme western Maryland, eastern West Virginia, northern/central/south-central Virginia and north-central North
Carolina. A Marginal Risk area spans from northeast
Georgia/northwest South Carolina to southern Pennsylvania and
southeast Ohio.
...Florida...
An unstable airmass coupled with lingering deep layer moisture
will promote isolated to widely scattered thunderstorm activity
capable of localized flooding, especially within urbanized zones.
Any of these storms will have the capability to produce 2 to 4
inches, especially along the coastline. A Marginal Risk for
excessive rainfall is in effect for central portions of the
peninsula.
Campbell
Day 2
Valid 12Z Mon Sep 30 2024 - 12Z Tue Oct 01 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE INTERIOR MID-ATLANTIC...
High pressure to the north, and a developing surface reflection
off the Mid-Atlantic coast thanks to the upstream trough will
create a persistent northeasterly flow setup with greater upslope
emphasis within the terrain across Western Virginia into the
far eastern West Virginia. There will be an enhancement of forcing
through the column of atmosphere near the Appalachian/Blue Ridge
spine that may result in hourly rainfall rates of 1 to 1.5
inches/hour during this period. Soils near saturation from recent
rains will keep the threat for flash flooding elevated for much of
this region thus maintained a Slight Risk for excessive rainfall
for extreme south-central Pennsylvania, western Maryland, eastern
West Virginia and northern/central Virginia. A Marginal Risk area
spans from northeast Tennessee/northwest North Carolina to southern Pennsylvania.
Campbell
Day 3
Valid 12Z Tue Oct 01 2024 - 12Z Wed Oct 02 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
From Mike Powell@618:250/1 to All on Mon Sep 30 09:17:00 2024
FOUS30 KWBC 300809
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
409 AM EDT Mon Sep 30 2024
Day 1
Valid 12Z Mon Sep 30 2024 - 12Z Tue Oct 01 2024
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
EASTERN WEST VIRGINIA, VIRGINIA AND NORTHERN NORTH CAROLINA...
The latest guidance continues to show the south/southeast trend of
where the heaviest rain is expected to occur Monday across the
Virginias. During this time the slow moving upper low will be transferring
its energy to a coastal low feature located offshore Virginia and
North Carolina. This will likely lead to a prolonged local
enhancement in hourly rainfall rates near the Blue Ridge of 1 to
1.5 inches/hour during this period. Soils near saturation from
recent rains will keep the threat for flash flooding elevated for
much of this region. A Slight Risk for excessive rainfall remains
in effect for extreme eastern West Virginia, northern/central
Virginia, and north-central North Carolina. The Marginal Risk was
extended to cover much of coastal North Carolina in association
with the developing coastal low potentially locally increasing
rainfall totals due to slow- moving convection especially on the
western side of Pamlico Sound.
Campbell
Day 2
Valid 12Z Tue Oct 01 2024 - 12Z Wed Oct 02 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Campbell
Day 3
Valid 12Z Wed Oct 02 2024 - 12Z Thu Oct 03 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
From Mike Powell@618:250/1 to All on Tue Oct 1 09:29:00 2024
FOUS30 KWBC 011241
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
841 AM EDT Tue Oct 1 2024
Day 1
Valid 12Z Tue Oct 01 2024 - 12Z Wed Oct 02 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHEAST
Virginia INTO NORTHEAST NORTH CAROLINA AS WELL AS FOR CENTRAL FLORIDA...
...Virginia/North Carolina...
During this period an area of low pressure will slide off the
Virginia and North Carolina coast while periods of heavy rainfall
persists from Richmond south and east through Hampton Roads and
portions of far northeastern North Carolina and the northern Outer
Banks. A majority of the rain is expected through about mid-
afternoon. Rain amounts and coverage with decrease as the low moves
further offshore. The inherited Marginal Risk was maintained albeit
with minor adjustments to reflect the latest model guidance and
WPC forecast.
...Central Florida...
A disturbance moving along the stationary front over central
Florida may support more widespread shower and thunderstorm
activity from Tampa through Orlando to the Space Coast Tuesday
afternoon. Training of the convection and impacts with the urban
centers may result in isolated flash flooding. The inherited Marginal
Risk was maintained with a minor southward expansion.
Campbell/Wegman
Day 2
Valid 12Z Wed Oct 02 2024 - 12Z Thu Oct 03 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
There will be a lingering potential for convection near the
eastern coastline that could result in bursts of modest or very
local heavy rainfall intensities. The threat for excessive rainfall
and flooding concerns will remain below the 5 percent threshold for
this period.
Campbell
Day 3
Valid 12Z Thu Oct 03 2024 - 12Z Fri Oct 04 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
There is a non zero possibility for locally modest to heavy
rainfall to move northward from the Gulf of Mexico along the Gulf
Coast anywhere from the Mississippi River delta to west of the
Florida bend. There is a fair amount of model spread that further
serves to decrease confidence on where any of these storms may
occur. At this time a trends suggest that the bulk of the heaviest
rainfall will remain south of the coastline.
From Mike Powell@618:250/1 to All on Wed Oct 2 08:09:00 2024
FOUS30 KWBC 020757
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
357 AM EDT Wed Oct 2 2024
Day 1
Valid 12Z Wed Oct 02 2024 - 12Z Thu Oct 03 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
There will be a lingering potential for convection near the
eastern coastline that could result in bursts of modest or very
local heavy rainfall intensities. The threat for excessive rainfall
and flooding concerns will remain below the 5 percent threshold for
this period.
Campbell/Hamrick
Day 2
Valid 12Z Thu Oct 03 2024 - 12Z Fri Oct 04 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
For a few days now there has been a signal for moderate to possibly locally heavy rainfall to move northward from the Gulf of Mexico into far
southeast portions of Louisiana and up near the Mississippi River
delta. This is associated with moisture convergence along a quasi-
stationary frontal boundary over the northern Gulf of Mexico. There
have been a fair amount of spread on where and how much will occur
and that has not resolved much for this round of guidance. In
general consensus keeps the highest QPF over the Gulf while a
couple of solutions suggest 2-3+ inches may reach the coastal water
and the wetlands. In coordination with the local forecast office
in New Orleans the forecast QPF will likely not lead to any impacts
of significance for the wetland areas of southeast Louisiana
especially given their recent drier stretch. The Marginal Risk area
was removed for this period.
Campbell
Day 3
Valid 12Z Fri Oct 04 2024 - 12Z Sat Oct 05 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
As mentioned during the Day 2 period there has been a multi-day
pattern where the convection is just offshore the Gulf Coast but
within the convection there could be a few inches of isolated heavy
rainfall. The spread for this period favors a placement more over
the Gulf than the Gulf Coastline. There is a non zero potential for
localized moderate, possibly heavy rainfall intensities however
nothing that is expected to become problematic at this time.
From Mike Powell@618:250/1 to All on Thu Oct 3 09:27:00 2024
FOUS30 KWBC 030828
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
428 AM EDT Thu Oct 3 2024
Day 1
Valid 12Z Thu Oct 03 2024 - 12Z Fri Oct 04 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
The latest guidance continues to have heavier QPF over the
Southeast Louisiana Parishes with areal averages of 2 to 3 inches
for areas along and south of I-10 from near New Orleans through
Biloxi towards Mobile, with some of the CAMs as much as 3 to 5
inches near the coastline. Spread persists in the placement of the
local maxima with the highest potential likely over either St.
Bernard or Plaquemines Parishes, both areas that are very difficult
to flood due to the swampy land structures and sandier soils that
limit flash flood potential.
Campbell
Day 2
Valid 12Z Fri Oct 04 2024 - 12Z Sat Oct 05 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
During this period there will be broad cyclonic over the Western
Gulf along with deep tropical air that advects into the flow. This
will result in convection that breaks out across the Gulf and
creeps inland into South Texas. A majority of the guidance
maintainsthe highest QPF offshore however there could be 0.75 to
1.5 inches that occurs in South Texas that should provide
beneficial moisture to the area rather than pose as an threat for
local flooding concerns.
Campbell
Day 3
Valid 12Z Sat Oct 05 2024 - 12Z Sun Oct 06 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Campbell
d
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From Mike Powell@618:250/1 to All on Fri Oct 4 09:42:00 2024
FOUS30 KWBC 040733
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
333 AM EDT Fri Oct 4 2024
Day 1
Valid 12Z Fri Oct 04 2024 - 12Z Sat Oct 05 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Wegman
Day 2
Valid 12Z Sat Oct 05 2024 - 12Z Sun Oct 06 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Wegman
Day 3
Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR SOUTHWEST
FLORIDA AND THE LOWER KEYS...
A tropical wave or weak and slow-moving disturbance over the Gulf
will drift east into the Florida Peninsula Sunday. Multiple inches
of rain are expected through the day from Tampa south into the
Keys. The heaviest rain is expected Sunday afternoon and evening,
where added instability from diurnal heating will locally increase
rainfall rates. The slow-moving nature of any storms will be the
greatest contributor to flash flooding. FFGs have recovered since
Helene last week, so despite the forecast of 2-4 inches of rain
broadly over the Florida Gulf coast, only isolated flash flooding
is expected at this point.
From Mike Powell@618:250/1 to All on Sat Oct 5 07:43:00 2024
FOUS30 KWBC 050745
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
345 AM EDT Sat Oct 5 2024
Day 1
Valid 12Z Sat Oct 05 2024 - 12Z Sun Oct 06 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP
SOUTH TEXAS...
...Deep South Texas...
Easterly flow to the north of a developing but weak low over the
western Gulf that is the focus for widely scattered thunderstorm
activity over the Gulf is forcing storms to move inland from the
Gulf over Padre and South Padre Islands this morning. A cold front
will push south across Deep South Texas today, which will push the
tropical moisture south into Mexico, so the threat for flash
flooding is confined to the immediate coast down to Brownsville for
the day today. The flood threat should abate by tonight.
...South Florida...
The urban corridor of southeast Florida, including Miami may see
occasional heavy rain from westward-moving thunderstorms this
afternoon and evening. The storms should continue moving and weaken
as they move inland, so the threat while non-zero, should remain
under Marginal Risk levels. Better agreement in the CAMs may
necessitate a Marginal Risk issuance for this area with the midday
update.
Wegman
Day 2
Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
SOUTHERN FLORIDA PENINSULA...
A trough developing ahead of a slowly developing tropical low over
the Gulf will form over the southern Florida Peninsula on Sunday.
The air mass the trough will be developing in will be incredibly moisture-rich...with PWATs anywhere from 2.2 to 2.6 inches. This
will be close to the climatological maximum for this time of
year...between the 95th and 99th percentile and more than 3 sigma
above normal. The predominant mid-level wind flow will switch from northeasterly to southwesterly during the period, resulting in
chaotic storm motions. The heaviest rainfall totals will be along
both coasts, from Tampa south through Cape Coral on the Gulf Coast
as well as from the Treasure Coast south through Miami on the
Atlantic side. This bimodal distribution of QPF suggests
organization will be somewhat lacking, depending more on mesoscale
phenomena such as sea breezes or differential heating between the
urban areas and their surroundings.
This will likely be the first day of several where heavy rain will
be common and widespread across the Florida Peninsula. In the
increasingly likely event a tropical cyclone eventually develops in
the Gulf and moves towards the Florida Peninsula mid to late week,
this will be the first day of the predecessor rain event, or PRE.
Given high FFGs areally and this being the first day of the PRE, a
Marginal Risk was left intact for this area, but further increases
in forecast rainfall may require a Slight Risk for some of the
urban areas with future updates.
Wegman
Day 3
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
FLORIDA PENINSULA...
Conditions across the Florida Peninsula will continue to
deteriorate on Monday. The PRE is expected to continue to
intensify, making for a second full day of periodic heavy rainfall
across the Peninsula. Both overall rainfall amounts and coverage
north up the Peninsula will increase on Monday as compared with
Sunday. All but the fastest guidance suggest that should a tropical
cyclone form over the Gulf, it will remain well west of the Florida
Peninsula through Monday night. This means the PRE will remain in
full force across the Peninsula. Once again, there will be a
bimodal distribution of the precipitation...but much more
pronounced. The focus will remain along both coasts, albeit heavier
and spread north. Given the expected heavy rainfall from Day
2/Sunday, by Monday most soils should be at or near saturation.
This means most of the day's rains will convert to runoff in most
areas.
With heavier rainfall expected in the urban areas along both
coasts, a Slight Risk upgrade was introduced with this update, in
coordination with MFL/Miami, FL; TBW/Tampa, FL; and MLB/Melbourne,
FL forecast offices. With better definition as to how and where any
tropical cyclone may develop over the Gulf, this will improve the
forecast for the PRE on Monday. This will apply both to where the
heaviest rain may occur as well as how heavy the amounts will be.
Further, with the hindsight of where Sunday's rainfall will occur,
it's possible additional upgrades may be needed with future
forecasts...though a preponderance of the guidance suggests any
direct effects of a potential tropical cyclone will hold until
after this period. Depending on how the heaviest rain areas align
on Monday with Sunday, some areas may be approaching 2 day totals
of 10 inches of rain by Tuesday morning. These amounts of rain in
any urban areas could approach Moderate Risk level impacts.
From Mike Powell@618:250/1 to All on Sun Oct 6 09:59:00 2024
FOUS30 KWBC 060758
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
358 AM EDT Sun Oct 6 2024
Day 1
Valid 12Z Sun Oct 06 2024 - 12Z Mon Oct 07 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE MIAMI-
FORT LAUDERDALE METRO...
In coordination with the MFL/Miami, FL forecast office, a Moderate
Risk upgrade was introduced with this update.
A weak surface low is developing over the eastern Gulf well east of
T.S. Milton, but in the same plume of incredibly deep tropical
moisture. The low will drift southeastward toward south Florida.
This will increase the easterly flow over the Straits of Florida
between the Bahamas and Florida. This increased easterly flow in an
air mass with PWATs between 2.25 and 2.5 inches will both increase
the concentration and intensity of resultant thunderstorms over the
Gold Coast.
Portions of the Ft. Lauderdale and Miami metros saw up to 3 inches
of rain on Saturday. Rainfall amounts today and especially tonight
may accumulate to over 6 inches in that time. With saturated soils
and urban effects, the threat for ponding and flooding in the area
has greatly increased. Expect numerous areas of ponding and
flooding from today's rainfall, which necessitated the Moderate
Risk upgrade.
Elsewhere, the surrounding Slight Risk was expanded to include the
Gulf Coast from Tampa south and the Atlantic coast from Cape
Canaveral south. The aforementioned low will enhance rainfall rates
along much of the Gulf Coast from Tampa south, where heavy rainfall
Saturday caused localized flash flooding. This additional rainfall
on saturated soils will cause renewed flash flooding, mostly in
urban areas. Further north up the Atlantic coast, similar
convergence as further south is expected, but with lesser forcing,
resulting in lower rainfall amounts for the Treasure and Space
Coasts. Widely scattered instances of flash flooding are expected.
Finally, for the middle of the Florida Peninsula, the area will be
furthest from the strongest forcing, resulting in a local minimum
of rainfall. Since the area will not have tidal flooding concerns
and is somewhat less developed, only isolated flash flooding is
expected. The threat for heavy rainfall will continue up to the
Georgia coast however, where a local convergence zone may develop
as indicated in several of the CAMs. The Marginal was expanded
north to account for this possibility.
Wegman
Day 2
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHERN FLORIDA PENINSULA AND THE KEYS...
The weak surface low that will bring heavy rain to south Florida on
Sunday will continue eastward over the Peninsula and into the
Bahamas by Monday afternoon. Heavy rain will likely continue over
the southern Peninsula through at least Monday morning. There is
considerable disagreement on the track, strength and the speed of
the low, which will play a major role in how long the heavy rain
persists into Monday afternoon. With somewhat better agreement that
rainfall amounts will at least be lower than on Sunday, the Slight
risk for the area that was inherited was largely maintained, albeit
including some of the interior of the Peninsula as soils become
saturated area-wide, resulting in close to 100% conversion of heavy
rain to runoff. An internal higher-end Slight is in place for the
Gold Coast, for the potential that should amounts from Sunday
verify, then another targeted Moderate Risk upgrade will be
necessary. Meanwhile, a second day of very heavy rain and
thunderstorms is expected to impact all of the Keys, which at high
tide times may cause flooding due to poor drainage.
With some of the other CAMs moving the low across the southern
Peninsula and east to the Bahamas, portions of the Gulf Coast,
especially north of Ft. Myers/Cape Coral up through Tampa are in
the rare decreasing trend in the forecast rainfall amounts on
Monday. Thus, the Slight that extended up through Tampa was
downgraded to a Marginal north of Ft. Myers/Cape Coral with this
update.
Wegman
Day 3
Valid 12Z Tue Oct 08 2024 - 12Z Wed Oct 09 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PENINSULA...
The PRE (predecessor rain event) will enter its third and final day
over the Florida Peninsula on Tuesday. The trough that occasionally
spins up weak surface lows will remain in place across the
Peninsula. The circulation of what will then be Hurricane Milton
will make its approach to the Gulf Coast of Florida, likely near
Tampa very late Tuesday night/early Wednesday morning. At this
time, expect little of the rainfall directly associated with
Milton's core to yet impact the Florida Peninsula. Of course, with
future updates and forecast changes from NHC, this may change.
Given the inherent uncertainty with the track and speed of this
small hurricane, the Slight over the Peninsula was maintained,
albeit with some filling in in the middle of the Peninsula near the
Orlando area. There remains a bimodal distribution of the rain with
much more expected along the Gulf coast from Tampa down through
Naples, but with a secondary maximum for the Space and First
Coasts.
With the latest guidance, there is some offset of the axis of
heaviest rainfall ahead of Milton. The northward trend will reduce
amounts in portions of the hard-hit Gold Coast in favor of more
rain for the northern Peninsula towards the First Coast. By Tuesday
the greatest overlap will be over the Gulf Coast. It is here than
an internal higher-end Slight was introduced for the Tampa through
Naples area due to increasing amounts of heavy rain moving into the
coast, being moved along by the predominant southwesterly flow in
the trough ahead of Milton's core circulation. Heavy rainfall both
the past couple days as well as expected both today and Tuesday
should keep all of the soils on the Florida Peninsula at or near
saturation, resulting in nearly all of the rainfall converting to
runoff. Thus, progressively less rainfall will be needed to result
in flooding.
Despite Milton and directly associated rains largely holding to the
Day 4/Wednesday period, it appears that similar to Helene, most of
the rain over the Florida Peninsula will be associated with the
PRE, while the heaviest rain (as well as wind and storm surge) will
conclude the multi-day rain event with Milton moving through,
causing much more widespread flooding.
From Mike Powell@618:250/1 to All on Mon Oct 7 09:52:00 2024
FOUS30 KWBC 070823
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
423 AM EDT Mon Oct 7 2024
Day 1
Valid 12Z Mon Oct 07 2024 - 12Z Tue Oct 08 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE GOLD
COAST AND THE UPPER FLORIDA KEYS...
Very few changes were made to the inherited EROs for this period.
A leading low, front, and surface trough over the eastern Gulf and
Florida well ahead of Hurricane Milton continues to spread rain
over nearly all of the Florida Peninsula this morning. The low will
shift east over the Everglades and South Florida today. Ahead of
the low, additional showers and thunderstorms capable of very heavy
rainfall rates due to the extremely moist environment will
continue. Portions of the Gold Coast saw 2-4 inches of rain
yesterday, resulting in localized flash flooding, so the potential
for that much rain again today supports the continuance of the
Moderate Risk. The trailing front south and west of the low will
remain stationary near Southwest Florida and the Keys. This will
keep the threat for heavy rainfall ongoing through tonight. The
Slight Risk area remains in place for these regions as far
southwest Florida is the swampy Everglades and the Keys present
very small targets for heavy rain. If training cells were to move
over the Keys during high tide then flooding is probable.
HREF probabilities suggest the heaviest rain today all over south
Florida are more likely to remain offshore, so the heavy rain
threat is slightly lower today compared to yesterday/Sunday.
Convergence along the Atlantic coast remains prevalent today, so
the Slight Risk remains in place to the north through the Space and
Treasure Coasts. With the center of the low tracking over south
Florida and off to the east towards the Bahamas, this should give
the likely landfall areas of the Gulf Coast north of Ft. Myers
through Tampa a general reprieve from heavy rain for most of today.
Wegman
Day 2
Valid 12Z Tue Oct 08 2024 - 12Z Wed Oct 09 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PENINSULA...
There will be a general lull in the overall general rainfall across
Florida on Tuesday. The "calm before the storm". The low impacting
most of the state Monday will be well east of the Bahamas by
Tuesday, while the rainfall directly associated with Hurricane
Milton will likely remain off or just arriving at the Gulf Coast
before 12Z Wednesday. Nevertheless, widespread light to moderate
rainfall will impact the state Tuesday and Tuesday night. Thus, any
resultant flooding will be largely due to favorable antecedent
conditions due to generally widespread saturated soils across the
Peninsula. Further, continued extremely moist atmospheric
conditions consistent with the deep tropical air mass with PWATs
around 2.25 inches will still support locally heavy rainfall with
any convection that manages to form.
On the Atlantic side, a convergence zone may remain over the Space
and Treasure Coasts on Tuesday, which could locally organize any
convection and result in locally heavier rainfall over these mostly
urban areas. Meanwhile over the Gold Coast, a local minimum of
rainfall may develop, but the heavy rain there the past couple days
as well as some chance of shower and thunderstorm activity may
cause widely scattered localized flooding. No changes were made to
the ERO risk areas.
Wegman
Day 3
Valid 12Z Wed Oct 09 2024 - 12Z Thu Oct 10 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR CENTRAL AND
NORTH FLORIDA AS WELL AS FAR SOUTHEASTERN GEORGIA...
The main circulation of Hurricane Milton will cross the Florida
Peninsula on Wednesday. There remains considerable uncertainty as
to where the center will make landfall, which will of course play a
huge role as to where the heaviest rainfall amounts will be. See
the NHC forecast for the latest track. The storm is expected to be
undergoing increasing shear, beginning extratropical transition,
growing in size, and ingesting dry air. Thus, there is decent
agreement that at landfall, the storm will be weakening. This
should help to reduce the rainfall south of the circulation center
as dry air ingestion associated with extratropical transition
should quickly erode the rain-producing storms southwest of the
center. Very dry air behind a southward moving front will greatly
cut into rainfall amounts on the northern fringe of the
circulation. Thus, expect there to be a tight gradient of rainfall
both on the south and north sides of the circulation, but for
different reasons. The ERO risk areas have been expanded in both
directions due to track and storm size uncertainties, especially
towards the north, with the Moderate Risk expanded into far
southeastern Georgia, and the surrounding Slight and Marginal
expanded into the Low Country of coastal southern South Carolina.
This may be optimistic, but these expansions were made primarily
due to uncertainties, and will likely be refined in the coming
days.
For the moment, there are several reasons a High Risk was not
considered in no particular order: 1) The storm will be weakening
prior to landfall which may temper rainfall amounts a little bit,
2) Milton's small size should reduce the areal coverage of the
heaviest rain, 3) The primary rain footprint of Milton over north
Florida will largely miss where the hardest hit areas of the PRE in
south Florida are. Of course there is some overlap, but not nearly
as much as with Helene, 4) The core of the heaviest rain at the
moment is expected to track from north of Tampa northeast through
around Jacksonville. This area has been relatively dry in recent
days compared to areas further south, 5) Milton will be moving
quickly and accelerating. This will reduce the time the heaviest
rain has along and north of the track to result in widespread and
numerous flash floods, 6) The inclusion of CAMs and higher
certainty with the track may help to narrow down where a potential
High Risk may be needed with future forecasts. For now with Day 3
being outside the CAMs time range, the large Moderate will
suffice.
From Mike Powell@618:250/1 to All on Tue Oct 8 07:44:00 2024
FOUS30 KWBC 080816
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
416 AM EDT Tue Oct 8 2024
Day 1
Valid 12Z Tue Oct 08 2024 - 12Z Wed Oct 09 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
FLORIDA PENINSULA...
No significant changes were made to the ERO risk areas for this period.
"The calm before the storm" will be in place over much of Florida
today. This isn't to say it won't rain anywhere...but the risk of
heavy rain causing flooding on most of the Peninsula today will
most certainly be the lowest of the next few days. A stationary
front in the same tropical moisture plume as Milton is draped
across southern Florida. This is providing a focus for heavier
showers and storms south of the front from Miami south through the
Keys. Since the storms are tracking parallel to the front, training
is a significant concern this morning. As any mesolow tracking
along the front moves off, the heavy rain threat in the Keys should
diminish by this afternoon due to increasing influence of Milton.
For the rest of the Florida Peninsula, most of today should be dry,
especially the further north you go. This is because drier air
associated with a jet across the Panhandle is sinking southward
with a front and will ultimately catch Milton and force
extratropical transition as it approaches Florida. For today
however, it will keep things largely dry along and north of the
eventual track of the storm.
The rainfall threat (and the basis for the ongoing Slight) is
largely tied to any showers and storms that may form...most likely
from mesoscale effects like differential heating and sea breezes.
Due to an excess of atmospheric moisture over most of Florida with
PWATs of 2.25 to 2.75 inches today, any storms that form, even if
disorganized and not associated with any discernible forcing...will
have a superabundance of moisture to work with to convert to
rainfall. Much of south Florida has seen multiple inches of rain
over the past few days. While not prolific enough to cause
widespread flooding concerns, they have kept soils close to
saturated. This will play a role in the eventual flooding concerns
with Milton. 00Z HREF guidance highlights portions of the Treasure
Coast with the highest chances of 3 inches or more of rain today
not directly associated with Milton.
A higher end Slight remains in place for the Gulf Coast from Tampa
south through Naples. This is largely tied to the initial rain
bands from Milton which will begin to impact the coast in the
predawn hours Wednesday before the start of the Day 2 period.
Wegman
Day 2
Valid 12Z Wed Oct 09 2024 - 12Z Thu Oct 10 2024
...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL FLORIDA...
...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING ARE
EXPECTED WITH LIFE-THREATENING AND CATASTROPHIC FLASH FLOODING PROBABLE...
With the ingestion of some of the longer range CAMs into the model
suite, the forecasted rainfall directly associated with Hurricane
Milton's crossing of the Florida Peninsula has increased markedly
from previous forecasts. The latest storm total rainfall has over
11 inches of rain for Tampa, 8 inches near and north of Orlando,
and 5.5 inches for Jacksonville. The corridor of the High risk
following the entire length of I-4 has seen the most impactful
increases in forecasted rainfall. Obviously there has been a nearly
historic increase in Milton's strength in the last 24 hours, and
all of this added energy and Gulf moisture will mean more rainfall
to come along its track, despite its fast forward speed.
The heaviest rainfall amounts along I-4 and north will generally
will be immediately along and north of the most probable track of
Milton's center. As mentioned in the NHC discussions, Milton is
expected to be well into the process of extratropical transition by
the time the storm crosses the Peninsula. This process should
greatly limit the amount of rainfall expected south of the center,
as dry air entrainment into the southwest quadrant of the storm is
an essential aspect of the extratropical transition process. Thus,
nearly all of the impressive tropical moisture associated with
Milton will be focused along and north of the center, and therefore
represents the greatest threats for flash flooding. The ERO risk
categories drop off a bit slower on the south side since the
predecessor rainfall event (PRE) of the past couple days and today
was largely focused on south Florida, as the stationary front that
has been there has persisted. Inflow into Milton's circulation and
the eventual development of a cold front south of the center
should still lead to bands of rain criss-crossing south Florida
from west to east. Since the PRE saturated this area the most...the
lesser amounts of rain expected here may still cause flooding
impacts...so the ERO risk categories south of the track may be a
bit generous.
Meanwhile on the north side of the circulation, abundant dry air
associated with a separate cold front and jet streak over the
Panhandle and far north Florida should act as a very effective sink
for the portion of Milton's rainfall that drifts too far away from
the center. Since this area has been quite dry in recent days, the
threat for flooding will be much more constricted, and therefore
the ERO risk categories are much more compact.
The additional rainfall now in the forecast will be one factor for flooding...while another one will be storm surge and tidal
flooding, especially south of the center where the flow will be
onshore and to a slightly lesser but still impactful extent, to
the north of the center on the Atlantic side where there will also
be onshore flow. At the time of high tide and the storm surge, the
added water from the Gulf/Atlantic will effectively block effective
drainage from the rainfall falling over the interior. The 6-12
inches (with locally higher amounts of rainfall) having nowhere to
drain due to the high tide and storm surge flooding will also work
to exacerbate the flooding impacts from Milton since that rain
water will have nowhere to drain.
Given all of the above, and despite the small to medium size of the
storm and the acceleration of the storm in the forecast, a High
Risk was introduced in coordination with TBW/Tampa, FL;
MLB/Melbourne, FL, and JAX/Jacksonville, FL forecast offices. It's
probable that the greatest flooding impacts will be with storm
surge primarily, but also the portions of the nearshore rivers,
streams, and creeks (especially near landfall) where the inland
flood waters from heavy rainfall are unable to drain to the Gulf.
Wegman
Day 3
Valid 12Z Thu Oct 10 2024 - 12Z Fri Oct 11 2024
...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR ORLANDO AND
PORTIONS OF THE FIRST AND SPACE COASTS...
A rare double upgrade was introduced with this update, with
potential for further adjustments with future updates. The latest
guidance suggests that the wraparound rain to the north and
northwest of the center of Milton will persist well into the day on
Thursday. Thus, associated forecast rainfall has increased
markedly from the previous forecasts. For the Moderate Risk area,
expect an additional 2 to 4 inches of rain after the 12Z/8am
Thursday start of the day 3 period. With a High Risk on Day 2,
further increases in the forecast rainfall may require a High Risk
in this general area to continue into Thursday. Obviously,
adjustments in the speed and track of Milton as it races off the
coast will result in additional big changes to this ERO forecast.
Essentially, the Day 3 ERO is just a continuation of the Day 2
rainfall with Milton. The vast majority of the rainfall for the
period will fall during the day Thursday, and expect trimming and
downgrades through the day as the rain ends from west to east.
Expect widespread and catastrophic flooding from the Day 2 period
to continue through the Day 3 period as the rain ends.
From Mike Powell@618:250/1 to All on Thu Oct 10 08:03:00 2024
FOUS30 KWBC 100827
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
427 AM EDT Thu Oct 10 2024
Day 1
Valid 12Z Thu Oct 10 2024 - 12Z Fri Oct 11 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
EAST-CENTRAL TO NORTHEAST FLORIDA...
Heavy rainfall from Milton continues to quickly move east across
the Florida Peninsula. Additional rain of 1 to 4 inches is likely
over the next couple hours along portions of the central and
northern FL east coast. However based on recent radar and HRRR
trends it appears likely that most, if not all, of this heavier
rain will be offshore by 12z this morning. Thus while areas of
considerable flash flooding will continue over the next few hours,
the expectation is that additional flash flooding after 12z will be
minimal. For that reason we will carry only a Marginal risk on the
new day 1 ERO that goes into effect at 12z this morning. The
ongoing High risk remains valid for the next couple hours until
12z. Keep in mind that even after the heavy rain ends, significant
areal and river flooding will continue to be a concern in areas
that have received significant rainfall.
Chenard
Day 2
Valid 12Z Fri Oct 11 2024 - 12Z Sat Oct 12 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Chenard
Day 3
Valid 12Z Sat Oct 12 2024 - 12Z Sun Oct 13 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Chenard
d
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From Mike Powell@618:250/1 to All on Fri Oct 11 08:27:00 2024
FOUS30 KWBC 110721
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
321 AM EDT Fri Oct 11 2024
Day 1
Valid 12Z Fri Oct 11 2024 - 12Z Sat Oct 12 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Easterly low level flow will bring showery conditions to the
central and southeast Atlantic coast of FL today into tonight.
However with PWs below 2" and instability limited not currently
expecting rainfall rates or totals to reach levels of concern for
flash flooding. Thus we will continue to carry no risk area in the
ERO.
Chenard
Day 2
Valid 12Z Sat Oct 12 2024 - 12Z Sun Oct 13 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...
Moisture and instability return northward into southern FL Saturday
into Saturday night, with PWs increasing towards 2.25" and CAPE
over 1000 j/kg. Meanwhile a weak wave moving into the eastern Gulf
of Mexico may result in an uptick in low level flow and
convergence. Thus expect we should gradually see an uptick in
convective coverage this weekend over the Keys into south FL.
Easterly low level flow and westerly upper level winds support the
potential for slow moving convective cells near the coastal
convergence axis. Timing remains a bit uncertain, and the better
convective coverage may end up more over the Keys this period
(where flash flooding is harder to come by), but given the
increasing moisture and instability there is at least a conditional
threat of localized flash flooding Saturday into Saturday night
into the more urban areas of southeast FL. If deep convection does
indeed develop then urban flash flooding is possible from the
upper Keys into southeast FL.
Chenard
Day 3
Valid 12Z Sun Oct 13 2024 - 12Z Mon Oct 14 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...
The risk on day 3 is a continuation of the threat on day 2 over
Southeast FL. The overall thermodynamic ingredients remain the
same, however there is a chance that forcing will be stronger by
Sunday. Still some timing and positional differences amongst the
guidance, but the general consensus is for a weak wave to move
across FL Sunday, which should both result in a subtle uptick in
mid/upper forcing and locally enhance lower level convergence.
Thus currently expecting a greater chance of loosely organized
convective clusters Sunday into Sunday night...and with wind fields
still supporting slow moving cells...this activity could tend to
anchor along the coastal convergence axis for a while. Thus
isolated urban flash flooding is considered possible. Certainly a
chance that a Slight risk may be needed for one of these weekend
days...but given some uncertainty with the synoptic evolution and
convective coverage, think a good first step is just introducing
Marginal risks for both days 2 and 3 (Sat and Sun) and continue to
monitor trends.
From Mike Powell@618:250/1 to All on Sat Oct 12 08:57:00 2024
FOUS30 KWBC 120752
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
352 AM EDT Sat Oct 12 2024
Day 1
Valid 12Z Sat Oct 12 2024 - 12Z Sun Oct 13 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...
Moisture and instability return northward into southern Florida
today which should result in deeper convection and the potential
for higher rainfall rates with PWs increasing towards 2.25".
Meanwhile a weak wave moving into the eastern Gulf of Mexico should
result in an uptick in low level flow and convergence. Thus expect
we should gradually see an uptick in deeper convective coverage
over the Keys into south FL and the adjacent waters today into
tonight.
The overall synoptic pattern still looks to favor slow moving
convective cells along and near the coastal convergence axis given
easterly low level flow and westerly upper level winds. Timing and
the CAPE distribution over land remain a bit uncertain. Some model
guidance wants to keep the better instability and convergence
offshore or over the Keys, while others shift these ingredients
more into the Southeast Fl coastal urban corridor. This decreases
confidence with regards to convective evolution.
The ingredients in place in the vicinity certainly support the
potential for a localized higher end flash flood event somewhere
within the southeast FL urban corridor today and/or tonight.
However the signal in the 00z HREF guidance is mixed, and overall
just not seeing enough in the probability fields to suggest we are
at Slight risk level coverage or confidence at this time. Thus we
will maintain the Marginal risk and continue to monitor
observational and model trends.
Chenard
Day 2
Valid 12Z Sun Oct 13 2024 - 12Z Mon Oct 14 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...
The risk on day 2 is a continuation of the threat on day 1 over
Southeast Florida. The overall thermodynamic ingredients remain
similar, however there is still spread with the mesoscale to
synoptic setup. The 00z GFS and ECMWF have trended quicker with the
weak wave and suggest the better organized convective risk Sunday
will now be offshore and over the Bahamas. However the Gem Reg and
some HREF members appear slower and focus more convection over
south FL.
The uncertainty in these details appears too large to go with
anything more than a Marginal risk at this time. Overall do tend to
think the flood risk Sunday is trending down compared to what it
looked like last night (given the trends noted in the ECMWF and
GFS), but favorable moisture and instability parameters suggest at
least a localized flash flood risk could persist.
Chenard
Day 3
Valid 12Z Mon Oct 14 2024 - 12Z Tue Oct 15 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
From Mike Powell@618:250/1 to All on Sun Oct 13 08:55:00 2024
FOUS30 KWBC 130801
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
401 AM EDT Sun Oct 13 2024
Day 1
Valid 12Z Sun Oct 13 2024 - 12Z Mon Oct 14 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER FLORIDA KEYS INTO THE SOUTHEAST FLORIDA URBAN CORRIDOR...
No changes were made to the inherited Marginal risk area over the
southeast FL urban corridor. Isolated convection will be ongoing at
12z this morning along the coastal convergence axis...with another
round of activity likely this afternoon. It is the afternoon
convection that appears to have better instability to work with,
and thus a better chance of more robust development. Coverage
remains a question, but high res guidance suggests a weak surface
trough/wave moving across south FL should help locally enhance
convergence this afternoon.
With any deeper convection tapping into the stronger westerly flow
aloft, the tendency may be for cells to have more of an eastward
motion this afternoon, limiting rainfall duration as cells move
offshore. However if low level easterly flow is strong enough
and/or the low level convergence axis is persistent enough, then
we could see at least some cell training for a period of time.
Overall the ingredients are in place to suggest a localized flash
flood risk is a possibility. HREF guidance supports isolated 3"
totals, but there is minimal model support for amounts much higher
than that. The Marginal risk should cover this isolated urban
flash flood threat...as not seeing enough support for Slight risk
level impacts/coverage at this time.
Chenard
Day 2
Valid 12Z Mon Oct 14 2024 - 12Z Tue Oct 15 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Chenard
Day 3
Valid 12Z Tue Oct 15 2024 - 12Z Wed Oct 16 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
From Mike Powell@618:250/1 to All on Wed Oct 16 09:26:00 2024
FOUS30 KWBC 160802
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Wed Oct 16 2024
Day 1
Valid 12Z Wed Oct 16 2024 - 12Z Thu Oct 17 2024
The probability of rainfall exceeding flash flood guidance is less than
5 percent.
Kleebauer
Day 2
Valid 12Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024
...2030Z Update...
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Kleebauer
Day 3
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN NEW MEXICO INTO SOUTHERN COLORADO...
Longwave pattern will evolve into a fairly prolific upper
trough/closed-low over the Great Basin, slowly migrating eastward
with sights on the Four Corners. Large scale ascent will become
maximized downstream of the mean trough with a pronounced diffluent
pattern in-of the Southern Rockies, including the Sangre de Cristo
and San Juan mountain ranges where heavier precip is forecast. A
cold front will slowly propagate eastward from the primary
disturbance with a return flow setup pressing westward across the
Eastern NM plains on the western flank of a sprawling surface high
east of the Mississippi. A secondary cold front to the north will
plunge southward thanks to a strong ridge of high pressure
developing upstream in wake of the primary western low, along with
a lee side trough developing over the Front Range. The tandem of
boundaries and backed flow will converge over Northern NM with a
strong isentropic ascent pattern initiating later Friday evening
through Saturday AM and beyond.
Convective signals are most pronounced in the initial stages of
the pattern evolution mainly due to the return flow setup where
more unstable air will advect northwestward into the Eastern NM
plains before banking against the lee of the Southern Rockies. Both
ensemble and deterministic output for heavy precip are locked in
across the area encompassing much of Northern NM, but especially
within the confines of places like Raton and points south where
topographic enhancement within the convergent axis will generate
local precip maxima when the convergence pattern truly solidifies
late in the period. Totals of 1-2" of precip are forecast on the
southern edge of the San Juans down into portions of the Sangre de
Cristos, closing in on some of the prevalent burn scars in the
region. A secondary maxima of 1-2" is forecast over that main axis
of convergence over Northeastern NM in vicinity of the eastern
flank of the Sangre de Cristos, including places like Taos and
Angel Fire over into parts of the plains in the lee of the mountain
chain. The period before will be more of a primer for the main
event this period through the following D4 time frame. The
combination of better moisture return and large scale forcing via
diffluent mid- level flow and jet coupling provides ample support
for a continuation of the previous MRGL risk, as well as an
expansion of the risk area further southwest and east to match the
ensemble QPF footprint in areas of highest convective potential.
From Mike Powell@618:250/1 to All on Thu Oct 17 08:57:00 2024
FOUS30 KWBC 170804
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
404 AM EDT Thu Oct 17 2024
Day 1
Valid 12Z Thu Oct 17 2024 - 12Z Fri Oct 18 2024
Relative progressive nature of precipitation across Northern NM
will alleviate much of the flash flood concerns this afternoon,
however the very low FFG's in place over each existing burn scar
within the Sangre de Cristos will offer an opportunity for
localized flooding within each impacted zone. Current HREF probs
for >1" of precip is around 50-60% within the neighborhood derived
analysis. This is still very lacking within the EAS prob field
leading to a pattern of less widespread heavier precipitation
totals and more relegated to very localized maxima likely over the
tops of the mountain chain. The best depiction is outlined over the
San Juan Mountains where a majority of the precipitation will fall
as snow or a rain/snow mix mitigating the threat for flash flooding
during the period. This has allowed for a continuation of the nil
ERO across the CONUS, but still a non-zero presence within the burn
scars of Northern NM.
The probability of rainfall exceeding flash flood guidance is less than
5 percent.
Kleebauer
Day 2
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE FOUR CORNERS REGION THROUGH NORTHERN NEW MEXICO...
Longwave pattern will continue evolving into a fairly prolific
upper trough/closed-low over the Great Basin, slowly migrating
eastward with sights on the Four Corners. Large scale ascent will
become maximized downstream of the mean trough with a pronounced
diffluent pattern in-of the Southern Rockies, including the Sangre
de Cristo and San Juan mountain ranges where heavier precip is
forecast. A cold front will slowly propagate eastward from the
primary disturbance with a return flow setup pressing westward
across the Eastern NM plains on the western flank of a sprawling
surface high east of the Mississippi. A secondary cold front to the
north will plunge southward thanks to a strong ridge of high
pressure developing upstream in wake of the primary western low,
along with a lee side trough developing over the Front Range. The
tandem of boundaries and backed flow will converge over Northern NM
with a strong isentropic ascent pattern initiating later Friday
evening through Saturday AM and beyond.
There has been very little deviation in the guidance to amass much
of a change compared to the previous forecast, so a general
continuity was maintained from the inherited MRGL risk across the
Four Corners into Northern NM. Ensemble guidance is in relative
agreement on the placement of two maxima during the forecast
period; one located Northeast AZ into Southeast UT with the initial
diffluent signature ahead of the evolving upper low over the Great
Basin, and the secondary maxima across Northeastern NM where the
convergence pattern aided by increasing large scale ascent will
generate a period of heavier precip Friday evening into Saturday
morning and beyond. Each areal maxima is similar with regards to
precip magnitude with a general neighborhood 1-2" max with a small
chance at upwards of 2.5" across the western maxima within the best
upper ascent within the nose of the 100kt 250mb jet streak. The
secondary maxima is generally within the last 12-hr window
encompassing 00-12z Saturday with the heaviest QPF footprint
actually highest in D3 (More on that in the D3 period below). In
any case, the very dry antecedent conditions, slot canyons, and
flashier river basins within the Four Corners area will offer a
greater risk of flash flooding with a secondary threat over the
remnant burn scars in Northern NM. This allowed for a maintenance
of the previous MRGL risk.
Kleebauer
Day 3
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS
OF THE SOUTHERN HIGH PLAINS INTO SOUTHEASTERN COLORADO...
Closed upper circulation will slowly migrate eastward with a
persistent diffluent channel focused in-of the Eastern NM High
Plains up into the Colorado Front Range for Saturday. A uni-
directional flow located within the lowest 600mb will persistent
through much, if not all the forecast period allowing for moist
unstable air to be pulled poleward into the confines of the Caprock
and points north. ECENS PWAT anomalies are forecast to run upwards
of 3-3.5 deviations above normal with an M-Climate Percentile
(Forecasts relative to the last 20 years during time of year) that
is peeking into the seasonal max, a testament to the anomalous
nature of the moisture advection regime. With the poleward
advancement of the airmass, regional theta-E's spike with a theta-E
ridge positioning located right over the NM/TX border running clear
into Front Range of CO and nearby KS. This signal is consistent
with a persistent band of rainfall likely to be situated within the
theta-E ridge mentioned above and just out ahead of the slow-moving quasi-stationary front trailing the very slow moving to perhaps stationary
low pressure across Southern CO.
Precip totals of 2-4" with locally higher amounts are increasingly
likely within that corridor between Clovis up through Southeastern
CO with the max likely driven by repeating convection as small
mid-level perturbations rotate around the general upper circulation
and move overhead of the areas above. The only saving grace for a
more significant flash flood potential is the antecedent conditions
leading in are very dry with the 0-10cm relative soil moisture
percentiles running between 5-20% across the area anticipated to
see the most rainfall. Initial rains will be beneficial, but will
add through the course of the period with waves of heavier rain
likely exacerbating areas by later in the period into early Sunday.
The previous SLGT risk was generally maintained with a MRGL
surrounding. Both risks were expanded somewhat on the southern and
northern peripheries to account for a trend in an expansion of
precip latitudinally in either direction. The heaviest will likely
encompass Eastern NM into Southeast CO with these areas likely
trending towards the higher end of SLGT risk if the signal remains
and/or amplifies further.
From Mike Powell@618:250/1 to All on Fri Oct 18 08:41:00 2024
FOUS30 KWBC 180833
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
433 AM EDT Fri Oct 18 2024
Day 1
Valid 12Z Fri Oct 18 2024 - 12Z Sat Oct 19 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN INTERMOUNTAIN REGION AND ADJACENT SOUTHERN HIGH PLAINS...
Overall...the synoptic pattern continued to maintain good
continuity with previous runs...showing ab upper trough/closed-
low over the Great Basin that eases eastward with time and the
strongest large scale ascent maximized downstream of the mean
trough with a pronounced diffluent pattern. This has allowed the
the Day 2 outlook from Thursday to propagate into the Day 1 outlook
without significant change. Ensemble guidance is in
relative agreement on the placement of two maxima during the
forecast period; one located Northeast AZ into Southeast UT with
the initial diffluent signature ahead of the evolving upper low
over the Great Basin, and the secondary maxima across Northeastern
NM where the convergence pattern aided by increasing large scale
ascent will generate a period of heavier precip this evening and
into the early morning hours of Saturday morning. Did make a minor
adjustment in moving the Marginal risk area around most of the
Sangre de Cristo area to avoid too much overlap with the WPC winter
weather desk deterministic snowfall forecast. Slot canyons, and
flashier river basins within the Four Corners area will offer a
greater risk of flash flooding with a secondary threat over the
remnant burn scars in northern New Mexico, This allowed for a
maintenance of the previous MRGL risk.
Bann
Day 2
Valid 12Z Sat Oct 19 2024 - 12Z Sun Oct 20 2024
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN HIGH PLAINS INTO SOUTHEAST COLORADO...
...Southern Plains...
Forecast rainfall continues to slowly increase across the southern
Plains in response to a quasi-stationary stationary upper low which
begins to tap deeper moisture will begin to tap into increasing
Gulf moisture. By Day 2/Friday the moisture plume will be well
established. Given the steering flow changes will be slow, the
excellent forcing east of the upper level low and increasing
instability will all favor the development of storms on Saturday capable
of producing heavy downpours that lead to flooding/flash flooding
over an otherwise mostly parched/arid area. While individual storms
may be progressive given the low level wind speeds involved, the
placement of the strongest forcing moves little on
Saturday/Saturday night suggesting the threat of repeat convection
or training of cells...especially over northeast New Mexico.
Modified the previous Marginal risk area to avoid the highest
terrain of the Sangre de Cristo range given the elevation forcing
the predominant precipitation type as snow.
...Northwestern Washington State...
As the Day 3 period begins at 12Z on Saturday...a well defined
atmospheric river should be settling into Washington from the
north, resulting in local rainfall totals of 3-5 inches for the
period. Integrated Vapor Transport values at the peak could exceed
900 kg/m/s, which is a bit stronger than the typical A.R., but
nothing major. When added to Friday's rain totals, this will likely
be enough rain to result in isolated flash flooding. Even if the
highest elevations see snow, the large majority of the rainfall
into the mountains will be in the form of rain. Saw little reason
to make to many changes to the region of greatest excessive risk
based on terrain- forced upslope/downslope regions at this point.
Only the first 12 hours of the Day 2 period were covered by the
18/00Z HREF probabilities...but the probabilities of 2 inch amount
of liquid precipitation are roughly 50 percent in the terrain of
the Olympics and the far northern Cascades.
Bann
Day 3
Valid 12Z Sun Oct 20 2024 - 12Z Mon Oct 21 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN AND CENTRAL HIGH PLAINS...
There threat of excessive rainfall is expected to linger into
Sunday from parts of northeast New Mexico and adjacent portions of
the Texas panhandle northeastward into southwest Kansas in a region
of upper level difluence east of the closed low/upper trough
back closer to the four-corners region. The additional rainfall in
this period does not look to be blockbuster in nature...perhaps 1
to 1.5 inches...but it will be in addition to the amounts that fall
mainly in the Day 2 period. During the period... model guidance
suggests a weakening of the upper level feature near the four-
corners and surface high pressure building southward east of the
front range will help lead to decreasing coverage and rainfall
intensity with time.
From Mike Powell@618:250/1 to All on Thu Oct 24 09:01:00 2024
FOUS30 KWBC 240815
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
415 AM EDT Thu Oct 24 2024
Day 1
Valid 12Z Thu Oct 24 2024 - 12Z Fri Oct 25 2024
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MIDDLE MISSISSIPPI VALLEY...
Introduced a Marginal Risk for portions of the Middle MS Valley,
from northwest IL and southeast IA into northern MO (and adjacent
portions of far northeast KS). Strengthening southwesterly low-
level flow (850 mb) to 40-50 kts this evening will usher in PWATs
to near 1.25" (well above the 90th percentile, per SPC sounding
climatology), along with sufficient instability (ML CAPE 1000+
J/kg) and forcing (potent shortwave trough crossing the Central
Plains) for convective development. Strong vertical shear (~50
kts) with idealized curving hodographs will likely favor discrete
convection (with both the mean flow and bunkers right vectors
favoring storm motions of 20-30 kts), and despite relatively low
PWs (particularly compared to warm season convection) any
supercells will likely be capable of 1-2"/hr rainfall rates (per
40-km neighborhood HREF probabilities for 1"/1-hr exceedance of
40-50%). While 1-2" totals should be well tolerated by soils (and
largely beneficial, given dry antecedent conditions), localized
training could result in too much rainfall (2"+) too quickly (in a
matter of 2-3 hours). Given this threat, an isolated instance or
two of flash flooding is possible.
Churchill
Day 2
Valid 12Z Fri Oct 25 2024 - 12Z Sat Oct 26 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.
Churchill
Day 3
Valid 12Z Sat Oct 26 2024 - 12Z Sun Oct 27 2024
The probability of rainfall exceeding flash flood guidance is less
than 5 percent.