• HVYRAIN: High Risk SE US

    From Mike Powell@618:250/1 to All on Tue Aug 6 09:07:00 2024
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    QPFERD

    Excessive Rainfall Discussion
    NWS Weather Prediction Center College Park MD
    427 AM EDT Tue Aug 6 2024

    Day 1
    Valid 12Z Tue Aug 06 2024 - 12Z Wed Aug 07 2024

    ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
    GEORGIA, SOUTH CAROLINA AND NORTH CAROLINA...

    ...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
    SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

    ...Southeast...

    Significant impacts are likely to continue across portions of
    southeast Georgia into eastern South Carolina and southeast North
    Carolina. Some uncertainty remains with regards to the structure
    of Debby by this time. Portions of the High risk area could end up
    within a dry slot at some point Tuesday...but even if this happens
    the extreme antecedent rainfall and expectation of areas of heavy
    convective banding near and north of the center more than supports
    a continuation of the High risk. Also some uncertainty with regards
    to how far north and inland the convective bands get into North
    Carolina, so some adjustments to the Moderate Risk area may be
    needed. There will likely be numerous instances of significant to
    catastrophic flooding for both rural and urban areas along with
    rising streams, particularly for the eastern South Carolina.

    A few feeder bands into Debby will pass across Florida during this
    period however both intensity and coverage along the west coast of
    Florida will be greatly diminished. The Slight Risk that was
    previously in place for portions of central and southern Florida
    was trimmed back to stop near northeast Florida. A Marginal Risk
    for that area reflects the level of threat as the impacts from
    Debby wains.

    ...Mid-Atlantic/southern portions of the Northeast...

    The Moderate Risk area was expanded to now include northern New
    Jersey, far southeast New york and portions of Long Island. During
    this period a stationary front draped across the region, and an
    approaching shortwave and upper jet streak from the west, and some
    connection to tropical moisture out ahead of Debby. Will likely
    have an MCS ongoing at 12z Tuesday moving east southeast across
    portions of Michigan and the Ohio Valley. This activity may end up
    surviving across the Mid- Atlantic, and we may see another
    convective cluster develop downstream of this MCS along/near the
    stationary front. The location and magnitude of the flash flood
    risk depends on a couple factors...does the MCS persist and merge
    in with frontal convection resulting in a training axis, and where
    exactly is the front by Tuesday afternoon. Overall this setup does
    have the look of a potential locally high impact flash flood event.
    There is enough forcing and instability, and a stationary front
    that could act as a focus for training/backbuilding cells. The
    exact location of where the greatest amounts will focus remain
    uncertain however the most favorable area seems to span from far
    northeast Maryland into central New Jersey, but this is dependent
    on the eventual location of the front and also the propagation of
    the upstream MCS.

    ...Southwest...

    The monsoonal moisture and diurnal heating will keep showers and
    thunderstorms developing during the afternoon and evening hours across
    the Southwest during this period. Convection to develop along the
    terrain, and then move westward into some of the lower elevations
    resulting in an isolated to scattered flash flood threat. The well
    defined shortwave currently over northern Mexico will approach
    southern Arizona by later Tuesday, which should bolster rainfall
    amounts and to some degree coverage of convection across southern
    Arizona. A Slight Risk remains in effect for portions of southern
    Arizona while a Marginal Risk remains in effect for much of the
    Four Corners region.

    ...Northern Rockies into Northern High Plains...

    Fast-moving thunderstorms are expected to pass over the Rockies and
    adjacent High Plains during this period. There is potential for
    training of cells over areas that have low 1-hour FFGs. Isolated
    instances of flash flooding may occur during this period therefore
    maintained the Marginal Risk area.

    Campbell/Chenard


    Day 2
    Valid 12Z Wed Aug 07 2024 - 12Z Thu Aug 08 2024

    ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
    SOUTH AND NORTH CAROLINA...

    ...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
    SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

    ...Southeast and Mid-Atlantic...

    A High Risk for excessive rainfall remains in effect for eastern
    South Carolina and southeast North Carolina as Debby continue to
    dump widespread heavy rain along its very slow track; likely
    resulting in continued significant rainfall and flooding over the
    High risk area. There is at least some flash flood risk that will
    extend up into the eastern Mid-Atlantic Wednesday as well...with
    moisture ahead of Debby likely continuing to interact some with the
    front to the north and right entrance region of the upper jet. A
    Slight Risk area was raised for portions of northern Delaware, far
    northeast Maryland, southeast Pennsylvania and southern New
    Jersey.

    ...Southwest...

    The increased moisture flux from the Gulf of California will make
    its way further north/east across the Four Corners region, helping
    maintain enhanced rainfall during the afternoon and evening. A
    Marginal Risk area covers much of the Four Corners region and
    extends into southeast Wyoming and western Nebraska.

    Campbell


    Day 3
    Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024

    ...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
    SOUTH AND NORTH CAROLINA...

    ...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
    SIGNIFICANT AND POTENTIALLY CATASTROPHIC FLOODING LIKELY...

    ...Eastern Seaboard...

    Debby continues to be a soaker for the Carolinas as it tracks
    inland once again while spreading heavy rain further north into the
    Mid- Atlantic region. Several more inches of rain expected during
    this period, bringing the multi-day totals into the 10 to 20 inch
    range with locally higher maxes. There will likely be widespread
    significant, possibly catastrophic flooding already ongoing by
    this period and will persist. A High Risk is in effect from South
    Carolina to north-central North Carolina. A Moderate spans from
    southern South Carolina to central/southeast Virginia. Further
    north, moisture from Debby will interact with a frontal system and
    ongoing MCS activity to further enhance convection across the
    northern Mid-Atlantic and Northeast. Although some of the stronger
    rainfall will have already occurred, antecedent conditions along
    with additional thunderstorms capable of producing heavy rainfall
    with keep the threat for excessive rainfall and local flooding
    concerns elevated. Much of the Mid-Atlantic and southern portion of
    New England have a Marginal Risk in effect.

    ...Four Corners, Central Rockies and High Plains...

    The Monsoonal pattern will persist through this period across the
    Four Corners region while increasing across portions of the Rockies
    and adjacent High Plains. Shortwave energy passing through the
    Rockies will help provide additional forcing for convection to
    focus along the higher terrain of Colorado and New Mexico. Model
    guidance suggests a concentration of cells capable of producing 1
    to 2 inches with very isolated maxes greater than 3 inches. A
    Slight Risk was maintained for portions of central Colorado and
    northeast New Mexico given the increased risk for isolated to
    scattered flooding concerns.

    Campbell
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