FOUS30 KWBC 102030
QPFERD
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
329 PM EST Fri Feb 10 2023
Day 1
Valid 16Z Fri Feb 10 2023 - 12Z Sat Feb 11 2023
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EASTERN FLORIDA PANHANDLE INTO FAR SOUTH CENTRAL GEORGIA...
...16Z Update...
In coordination with the TAE/Tallahassee, FL forecast office, the
Marginal Risk was upgraded to a Slight for the ongoing line of
convection across a small area of the FL Panhandle into far
southern GA south of Valdosta. Radar storm total imagery and
surface observations in the area of heaviest rain suggest storm
total rainfall over 5 inches in the Slight Risk area, and the line
is nearly stationary. See the associated MPD #61 for additional
information. General consensus in the high resolution guidance is
for the line to begin moving east and weaken in intensity over the
next few hours. Assuming radar trends are supportive, will likely
issue a special update later this afternoon to account for this.
Elsewhere, expanded the MRGL just a bit to the west along the Gulf
Coast to include the Mobile, AL area. Soils are much wetter than
further east based on NASA sport imagery, and the forecast of
several hours of moderate intensity rain in this area this
afternoon and evening from the convection currently east of
Louisiana were factors included for the expansion.
Wegman
...Previous Discussion...
A stalling frontal zone is expected to result in a lingering axis
of significant tropospheric moisture (precipitable water values of
1.3-1.7 inches, above the 90th percentile climatologically per the
GEFS) from the FL Panhandle and southern AL northeastward into
south-central GA, SC, and extreme southeastern NC on Friday and
Friday night. Despite the presence of these anomalously high
PWATs, large-scale lift is still expected to remain fairly limited
(as the aforementioned upper-low slowly digs south into the far
western Gulf) with very little instability expected initially. So
a fairly narrow Marginal Risk area was mostly maintained, with
rainfall totals generally expected to range from 1-2 inches across
the area. Localized higher amounts are possible mainly from near
and to the west of the Florida Big Bend to south-central Georgia
due to the potential for training cells across the stalling
frontal zone...although expanded the southern boundary of the
Marginal a bit in the northern Florida peninsula in respons to a
QPF bullseye shown by the ECMWF and CMC. The 00Z HREF still
maintained a non-zero probabilities for greater than 3" in 6 hours
along this corridor...while an offsetting concern is that portions
of Florida is in D2 drougt. Given the relatively dry antecedent
conditions across the Southeast and already limited potential for
rainfall to exceed the high FFG, any instances of flash flooding
are expected to remain isolated and primarily confined to areas of
poor drainage during this time period. However, any rainfall could
lead to more sensitive soils ahead of a final surge of
precipitation on Saturday.
Bann
Day 2
Valid 12Z Sat Feb 11 2023 - 12Z Sun Feb 12 2023
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE EASTERN
FLORIDA PANHANDLE THROUGH SOUTHERN SOUTH CAROLINA...
...2030Z Update...
This afternoon's update includes a small expansion of the previous
Slight Risk area south and west to include areas of the FL
Panhandle and far southern GA that are getting soaked with heavy
rain today. While much less rain is expected Saturday as compared
with today, see no reason even the forecasted lighter rain won't
exacerbate already ongoing flooding in the area. Meanwhile much
for the rest of the Slight Risk area remains largely unchanged.
Antecedent flooding conditions led much of the previous Slight
Risk to be maintained in southern South Carolina.
The Marginal Risk area across much of NC was removed with this
update, in addition to the area having about average soil
moisture, most of the rain waits until the parent low is gathering
strength off the coast after 00Z, leading to a more likely
evolution of a widespread area of light to moderate rain with
embedded heavy elements. The quick movement of the low will not
allow this area of rain to stay in any one location too long
before the precipitation becomes more banded with the bands
oriented largely perpendicular to the mean southwesterly flow.
This will make the rain become increasingly off-and-on late
Saturday night, requiring heavier rainfall rates that are unlikely
to materialize.
Wegman
...Previous Discussion...
Maintained the Slight Risk for portions of the Southeast U.S. that
was introduced by yesterday's day shift from the Florida Big Bend
to the South Carolina Low-country. This upgrade is supported by
converging guidance regarding the upper-level low responsible for
much of the atmospheric lift, as well as the location of the
forecast heavy rainfall expected to occur over an area forecast to
become more saturated by the start of the outlook period due to
rain the previous day.
There will still be a quasi-stationary boundary draped over
portions of the Southeast U.S. with the flow aloft being forced to
back in response to a closed mid- and upper-level low that makes
its way into the Tennessee Valley on Saturday. This induces a
broadening area of precipitation as low pressure forms along the
front. Latest guidance shows an additional 1-3 inches of rainfall
from the Florida Panhandle to southeastern North Carolina. Despite
the expected lack of instability (GFS MU CAPE near 0 J/kg outside
of the Florida Peninsula and far southern Georgia), tropospheric
moisture levels are expected to remain very elevated, with
precipitable water values ranging from 1.25-1.75 inches (near the
90th percentile climatologically) with large-scale lift provided
from the traversal of the upper-low (and expected surface
cyclogenesis). Strong frontogenesis associated with a draped
stationary front from northern Florida to eventually the coastal
Carolinas will be the focus for rainfall rates to potentially
reach or exceed 2"/hr, otherwise rainfall rates are expected to
remain under 2"/hr. It is this additional rainfall on top of what
has fallen recently that should result in lower Flash Flood
Guidance across much of the Slight Risk area and was highly
considered when drawing the area. Otherwise, much of the forecast
rainfall and rates generally hovering around or just below 1"/hr
would be handled well by the largely sandy soils. WPC 48-hour
rainfall amounts of 3-5" between Friday and Saturday is forecast
for the region, which may lead to more scattered (as opposed to
isolated) flash floods and ponding across roadways/urban areas by
the beginning of the weekend. Surrounding the Slight Risk is a
larger Marginal Risk that encompasses regions where 1-2" of
rainfall are forecast and should lead to mainly localized flooding
concerns, which was confined a bit on the northern and western
fringes compared to the previous forecast.
Bann
Day 3
Valid 12Z Sun Feb 12 2023 - 12Z Mon Feb 13 2023
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHEASTERN VIRGINIA AND EASTERN NORTH CAROLINA...
...2030Z Update...
In coordination with the MHX/Morehead City, NC and AKQ/Wakefield,
VA forecast offices, maintained some of the inherited Marginal
Risk across SE VA and eastern NC this afternoon. Changes involved
trimming much of the southern end of the Marginal Risk area across
central NC, as the precipitation will rapidly end there early in
the Day 3 period Sunday morning with the advection of the dry slot
into the area. Meanwhile, also trimmed the northern end of the
inherited Marginal Risk area over the Northern Neck Peninsula of
Virginia due to very dry (under 10%) soil moisture based on NASA
Sport imagery.
Rainfall rates will be quite low during this period, maybe briefly
hitting 1/2 inch per hour rates, as the low winds up and moves off
the coast of NC. Ahead of the low, a line of stronger storms is
likely to impact eastern North Carolina Sunday morning, with
rainfall rates rapidly diminishing with the advection of the dry
slot after 18Z. This line of stronger storms will be capable of
much higher rainfall rates (perhaps briefly exceeding 2 inches per
hour), but it will be falling into coastal swamp and sandy soils,
which are unlikely to respond with much flash flooding. Any
urbanized areas of eastern NC are more likely to see isolated
flash flooding. Meanwhile across Virginia, lighter rain will fall
over much of the Day 3 period into Sunday night in the wraparound
region of the low. While there's high confidence all of Southeast
Virginia will see all rain, the longer time period (at least 18
hours) of light to occasionally moderate rain may result in
isolated instances of flash flooding, particularly in the
urbanized areas of Hampton Roads/Norfolk/Virginia Beach and inland
along I-64 through Richmond.
Wegman
...Previous Discussion...
Rain will continue to move northward from the Southeast U.S. into
parts of the southern Mid-Atlantic region in association with a
mid- and upper-level low. Present indications are that the higher
rainfall rates should be confined close to the coast where the
upper level support from a coupled jet structure and some
lingering mid-level frontogenesis will offset modest (at best)
instability across the region. As such...the main problems should
tend to be in areas of poor drainage. Broader coverage of more
stratiform rainfall is possible farther west...but neither the
rainfall rates nor the rainfall amounts appear to be problematic
at this based on input from the local offices...so did not extend
the Marginal Risk any farther west or north. The expectation is
that the rainfall will be shunted off the coast during the latter
part of Day 3 with a corresponding decrease in any excessive
rainfall potential.
Bann
Day 1 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8WQfCQZWE3KUc2krHsNczS7yOmhpN9iZ9tlybLddwos4= l2RGkdq9oK9qi61B6VeOwkKhtTr_s-K13kTNpayz4JFRXZk$=20
Day 2 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8WQfCQZWE3KUc2krHsNczS7yOmhpN9iZ9tlybLddwos4= l2RGkdq9oK9qi61B6VeOwkKhtTr_s-K13kTNpayzRlw9vWc$=20
Day 3 threat area:
https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8WQfCQZWE3KUc2krHsNczS7yOmhpN9iZ9tlybLddwos4= l2RGkdq9oK9qi61B6VeOwkKhtTr_s-K13kTNpayztQqUjWc$=20
$$
=3D =3D =3D
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