• HIGH RISK Excessive Rainf

    From Dumas Walker@21:1/175 to All on Wed Sep 25 20:26:00 2024
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    QPFERD

    Excessive Rainfall Discussion
    NWS Weather Prediction Center College Park MD
    853 PM EDT Wed Sep 25 2024

    Day 1
    Valid 01Z Thu Sep 26 2024 - 12Z Thu Sep 26 2024

    ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
    NORTHEAST GEORGIA THROUGH FAR WESTERN SOUTH CAROLINA AND WESTERN NORTH CAROLINA...

    ..LIFE-THREATENING FLASH FLOODING INCREASINGLY LIKELY OVER THE
    SOUTHERN APPALACHIANS..

    Only minor adjustments were made to the ongoing risk areas based
    primarily on recent radar and HRRR trends. A significant rainfall
    event will continue to evolve through the overnight hours into
    Thursday across portions of the FL Panhandle into the southern
    Appalachians. Trends support a slightly further east rainfall axis
    through 12z Thursday, and thus the risk areas were nudged in this
    direction. Recent trends also suggest the rain axis will only push
    a a bit further east before stalling, and then even coming back a
    bit west by late tonight as strong south southeasterly flow
    increases.

    The heaviest rainfall rates will likely occur along the
    eastern edge of the rain shield where better instability will be
    present, supporting localized 2-3" per hour rainfall. Even as
    instability erodes overnight enough weak instability combined with
    the upslope flow into the terrain will support hourly rainfall
    periodically exceeding 1 to 1.5". Additional rainfall through 12z
    within the MDT and High risk areas will generally be within the 2-5" range...however localized amounts of 5-7" are possible. Flash
    flood coverage and severity is expected to increase across the MDT
    and High risk areas (GA into the southern Appalachians) through
    the overnight hours as conditions continue to saturate.

    As Helene moves northward across the Gulf of Mexico outer rain
    bands will increasingly impact the west coast of FL and eventually
    the FL Panhandle. While these bands will initially be quick
    moving, we may see some brief training later tonight, with 3-6"
    rainfall amounts possible. Still a bit unclear whether these
    totals stay just offshore or get into the western FL coast, but
    enough of a risk to maintain the Slight risk area. The areas near
    Apalachicola in the Fl panhandle will also likely become a focus
    for increasing convective coverage overnight. Expect the flash
    flood risk to increase here as well.

    Chenard


    Day 2
    Valid 12Z Thu Sep 26 2024 - 12Z Fri Sep 27 2024

    ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL IN PORTIONS OF THE
    SOUTHERN APPALACHIANS AND NORTHEAST GEORGIA...

    ..LIFE-THREATENING FLASH FLOODING LIKELY OVER THE SOUTHERN
    APPALACHIANS AND PORTIONS OF THE SOUTHEAST..

    The extensive period of heavy rainfall during the D1 period will
    continue through D2 across portions of the interior Southeast U.S
    with a primary focus across the Southern Appalachians where the
    back end of our predecessor rainfall event (PRE) will still be
    occurring. Further to the south, all eyes turn to Hurricane Helene
    as the storm continues to move quickly through the Eastern Gulf
    within the weakness created by the trough to the north. The
    northern extent of Helene's tropical moisture will begin nosing
    into the Gulf coast with PWAT anomalies steadily rising from 1-2
    deviations above normal to as high as 4 deviations above normal by
    the time it makes landfall on Thursday evening. Despite a fast
    forward propagation, copious amounts of rainfall will impact the
    Florida Panhandle over to the Big Bend with the feeder bands on the
    eastern flank of the cyclone impacting the coastal areas of the
    Florida Peninsula. Rainfall amounts between Panama City to
    Tallahassee will reach between 8-12" with local maxima of up to 15"
    possible within the axis of Helene's landfall as the storm moves
    almost due north, perpendicular to the coastal plain. For this
    reason, a High Risk was added to portions of the Florida
    Panhandle/Big Bend, including the Tallahassee area, as well as
    across Apalachicola.

    By Thursday night into Friday morning, Helene will continue making
    steady progress northward with a large rain shield capable of
    dropping 3-6" of rainfall within the core of the cyclone in short
    periods of time. A swath of 6+ inch totals will progress northward,
    eventually cascading into the Atlanta metro and surrounding
    locales generating a higher threat of flash flood potential within
    the urban corridor. Areas in the path of Helene will undoubtedly
    see significant flood potential with small rivers and streams
    likely becoming inundated very quickly to exacerbate the flood
    threat within the rural areas of Georgia. The recent HREF blended
    mean QPF footprint is robust for the period with a large areal
    extent of 5+ inches with the secondary maxima positioned across
    Northeast GA where the next round of significant rains will
    approach with Helene's core making an approach.

    The situation across the Southern Appalachians is becoming
    increasingly severe with regards to the ongoing flash flood threats
    from the PRE and eventually with the approach of Helene's core
    as the cyclone moves up through Western GA. This will place the
    eastern quadrant of the cyclone across Central and Northern GA with
    a strong, prevailing east southeast flow expected to bank against
    the terrain situated over North GA through the Western Carolinas, a
    mechanism for maximizing ascent within a deep tropical moisture
    plume that will enhance rainfall rates for several hours moving
    through late Thursday night into Friday morning. The 12z HREF mean
    QPF output continued to signal for prospects of 3-6+ inches within
    a short time frame as the core of higher PWATs advect into the
    region and douse the Southern Appalachians in another round of
    significant rainfall with rates generally steady between 1-2"/hr
    with some small probabilities (10-15%) for hourly rates pushing
    close to 3"/hr in the stronger convective cores embedded in the
    precip shield. The HREF neighborhood probability for >5" has peaked
    at 90+% for the Southern Appalachian front, a signal reserved for
    only the most impactful events, correlating well with the EAS
    signal of >90% as well for at least 3". The combination of multiple
    periods of heavy rainfall with elevated rates likely thanks to
    Helene will produce significant, life- threatening flash flooding
    across the area over Northeastern GA through the Blue Ridge of SC
    and NC, including areas in and around Asheville and to the west of
    Greenville- Spartanburg in Upstate SC. Several landslides due to
    the prolific rainfall are very likely with a history of these types
    of events producing catastrophic damage from both flooding and the
    prospects of land and mudslides within the complex terrain. For
    these purposes, and in coordination with the GSP and FFC WFO's, the
    High Risk was maintained and expanded further southwest to just
    northeast of the Atlanta metro to account for the most significant
    signals for flash flood prospects during the period.

    Considering the scope of Helene's size, the flash flood threat will
    not be relegated to a small corridor like we see in some tropical
    events. The trough to the west will play a pivotal role in the
    steering mechanism for Helene as the storm approaches the latitude
    of North GA, eventually pulling the cyclone more northwest towards
    the back end of the forecast period. Due to the progression, there
    is a growing consensus within the deterministic of the heavy rain
    threat also shifting into Northern AL with the area in and around
    Huntsville to the TN state line becoming more favorable for flash
    flooding potential within the western flank of the cyclone. The
    signal is not as prolific as the Southern Apps or where Helene
    makes landfall, but the probability and QPF fields certainly
    depict a significant amount of rain (3-5") within a time frame
    generally between 4-8 hrs prior to the end of the forecast period.
    This swath will extend across Eastern AL and Western GA up through
    Northern AL, following the western periphery of the cyclone as it
    moves north then northwest. The heavy rain will expand into
    Southern TN before the end of the forecast period with more rain
    anticipated in the Tennessee Valley after D2. Because of the
    growing consensus in the heavy rainfall across the above areas, the
    MDT risk was expanded further west and northwest into Alabama and
    over the southern fringes of TN.

    A large SLGT and MRGL risk will encompass a large area of the
    Southeast and Southern Mid Atlantic as well as the outer bands of
    Helene are forecast to impact the region with locally heavy
    rainfall possible as is customary with these types of tropical
    evolutions.

    Kleebauer


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