• HVYRAIN: High Risk E US

    From Mike Powell@618:250/1 to All on Thu Aug 8 09:10:00 2024
    FOUS30 KWBC 080845
    QPFERD

    Excessive Rainfall Discussion
    NWS Weather Prediction Center College Park MD
    445 AM EDT Thu Aug 8 2024

    Day 1
    Valid 12Z Thu Aug 08 2024 - 12Z Fri Aug 09 2024

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

    ...WIDESPREAD AND NUMEROUS INSTANCES OF FLASH FLOODING WITH
    SIGNIFICANT AND CONSIDERABLE FLOODING LIKELY...

    ...Southeast and Mid Atlantic...

    There was a trend with the latest guidance for an eastward shift
    given the Debby's forward motion and expected heavy rain
    footprint. Debby will continue to move inland with a westward jog
    over towards the Carolina Piedmont during the morning and afternoon
    hours. To the northwest, an upper trough will scoot eastward
    through the Great Lakes and Ohio Valley with a surface cold front
    progged to move quickly to the east with frontal approach into the
    Mid Atlantic by the end of the period. The remnant circulation of
    Debby will hold steady through the morning, but the cyclone will be
    undergoing a transitional state to a more extra-tropical cyclone
    with more synoptic mid-latitude characteristics customary once
    inland and tracking northward. The combination of Debby's moisture
    flux, primary core, and the trough/front approach will allow for a
    robust swath of heavy rainfall to occur in-of the Carolinas,
    eventually moving north into the central Mid-Atlantic with sights
    on the Piedmont from southwestern Pennsylvania down through
    Maryland/West Virginia and western Virginia. The High Risk area
    covers portions of the southeast North Carolina, north-central
    North Carolina and much of central/northern Virginia where areal
    average of 6 to 10 inches are forecast.

    A Moderate Risk extends along the eastern South Carolina Piedmont
    near I-77 up through much of Southwest Virginia into the
    Shenandoah and adjacent Blue Ridge, stopping in Southwest
    Pennsylvania just north of the Mason Dixon line. Heavier rains
    will expand further away from the primary axis of heavier
    precipitation, but the coverage will be lower due to the confined
    low to mid- level moisture flux associated with Debby along the
    leading edge of the trough and advancing cold front. Some heavier
    tropical downpours will be forecast along the far western side of
    the setup due to convergent upslope flow against the Appalachians.
    With reduced forcing closer to the coastline the threat for heavier
    rain will be less than areas to the west, but non-zero.

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

    The monsoonal pattern should yield a targeted focus for heavy rain
    across portions of the Central and Southern Rockies. The primary
    threat will be across the Sangre de Cristos thanks to strengthening
    upslope component as a cold front banks against the terrain with
    strong upward motion along a prominent theta-E ridge situated
    across the Southeast CO Front Range into the adjacent terrain.
    Heavy thunderstorms capable of 1-1.5"/hr will cause issues within
    the complex terrain and the remnant burn scars that persist within
    the confines of the Sangre de Cristos. A Slight Risk remains in
    effect for portions of central Colorado and northern New Mexico
    given the increased risk for isolated to scattered flooding
    concerns. A Marginal Risk covers much of the Four Corners region,
    western Kansas and Nebraska, southwest South Dakota, eastern
    Wyoming and extreme southeast Montana.

    Campbell/Kleebauer


    Day 2
    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 the Northeast...

    The core of the heaviest rainfall is anticipated to occur along
    the convergent area of the cold front as Debby's remnants get
    absorbed into the mean trough to the northwest and shunted
    eastward; from the Piedmont region of the Maryland northward to
    Upstate New York, Vermont and New Hampshire. Debby's remnants will
    be moving rapidly to the north-northeast with the associated
    moisture streaming well ahead of any persisting circulation. The
    storm will have gone through full extra-tropical transition once
    into central PA and will advance all the way into Canada by the end
    of the forecast period. Widespread 2-4" of rainfall are forecast
    over central and western Pennsylvania through western New York
    state and the western side of Northern New England (North Country
    over into VT) before exiting into the Canadian Maritimes and
    eastern Quebec. Rainfall rates of 1-2"/hr are forecast within the
    zone of highest impact relayed above with the eastern flank of the
    storm becoming drier as it gains latitude.

    The Moderate Risk was maintained with very minor adjustments to
    account for the latest WPC forecast. There continues to be some
    uncertainty in the exact track of the remnant cyclone as it reaches
    the New York state border, but if the trends continue, the eastern
    side of the risk areas will likely be brought further west to
    tighten the corridor of heaviest rainfall as is expected with the
    setup. With rain diminishing across the Appalachians and western
    portions of the Carolinas and Virginia the Slight and Marginal
    Risks were significantly trimmed eastward. Some heavier bands are
    expected to setup over coastal areas of North Carolina that will
    keep an elevated threat for flooding concerns during this period.

    ...Four Corners, Central Rockies and High 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 south/central Colorado and
    northern New Mexico, where a Slight 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 Inter Mountain West and the
    Mogollon Rim in Arizona. Convection will also spread eastward into
    western Kansas and the panhandles of Oklahoma and Texas. A broad
    Marginal Risk covers much of the Four Corners region, east into
    western Kansas and northward into south-central Montana.

    Campbell/Kleebauer


    Day 3
    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 FOR MAINE...

    ...Southwest, Central Rockies and Plains...

    Diurnal heating and monsoonal moisture will keep showers and
    thunderstorms developing during the afternoon and evening hours for
    much of the Four Corners region. The area of more organized
    convection triggered by shortwave energy from the D2 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 to 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.

    ...Maine...

    The absorbed remnants of Debby will continue to track through the
    Canadian Maritimes meanwhile there will be lingering showers and
    thunderstorms. Recent heavy rain will have lowered FFGs
    significantly thus making the area sensitive to additional
    rainfall. Maintained a Marginal Risk for this period; however if
    the timing speeds up there may be an opportunity to downgrade with
    further updates.

    Campbell
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