• HVYRAIN: Excessive Rainfall Discussion...corrected

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Wed May 5 00:59:41 2021
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    FOUS30 KWBC 050059

    Excessive Rainfall Discussion...Corrected
    NWS Weather Prediction Center College Park MD
    858 PM EDT Tue May 04 2021

    Day 1
    Valid 01Z Wed May 05 2021 - 12Z Wed May 05 2021


    0100 UTC Update...

    Continued to make adjustments to the Day 1 ERO based on the latest
    mesoanalysis and convective trends/evolution. This includes
    cutting back significantly the western peripheries of the outlook
    areas across the Deep South as the airmass continues to stabilize
    behind the QLCS.


    0900 UTC discussion...

    ...Deep South into the Tennessee Valley...=20
    The thermodynamic setup ahead of the approaching upper shortwave
    trough remains favorable for heavy rain according to the latest
    guidance, including low level moisture flux standardized anomalies
    (3+ sigmas above the mean) and IVT values (at over above 1000
    kg/(m*s)). Area of low pressure at the surface traversing the
    northern TN and southern OH Valleys will likely result in multiple
    heavy rainfall axes -- one farther north across much of TN into
    southern KY associated with the frontal wave (deep-layer synoptic
    support), where the instability will be sufficient (ML CAPE
    1000-2000 J/kg) to support 1-1.5+ inch/hr rainfall rates. Lower
    FFG values farther north also enhance the potential for flash
    flooding, as depicted with the latest HREF exceedance
    Farther south (Deep South/just inland of the central Gulf coast),
    2000-4000 J/kg of ML CAPE partially induced by an eroding
    mid-level capping inversion along with precipitable water values
    ~2" and convergent 850 hPa flow of 35-45 knots will allow for
    hourly totals to 2.5" within the stronger, more organized
    convection. Strong southwesterly mean flow should keep activity
    fairly progressive until the front lays a bit more toward the
    coast Tuesday night which will orient it more toward the mean flow
    and could allow some repeating cells, which is shown most vividly
    on the 00z FV3CAM. In terms of pinpointing an area of greatest
    risk, while spread continues across MS and AL, the best guess as
    to the area of higher impact currently is across southern MS and
    southern AL in the middle of the expected instability pool, so per
    coordination with the JAN/Jackson MS, BMX/Birmingham AL, and
    MOB/Mobile AL, a Moderate Risk was added. More than a few pieces
    of guidance (though usually high biased) indicate local amounts of 5-7"...believe local 3-6" amounts are most plausible considering
    the frontal progression for much of the period. Higher potential
    is theoretically possible closer to the Gulf Coast, depending upon
    how mesoscale forcing/outflow boundary movement evolves Tuesday
    night into Wednesday morning. If convective trends or the
    guidance follows suit and trends southward later today, a
    southward shift or expansion of the MDT risk is possible.

    ...Upper Ohio Valley...=20
    The broad poleward flow of Gulf moisture brings 1.5 inch PWs
    across the Midwest/Ohio Valley into PA and southeast NY which is 2
    to 2.5 standard deviations above normal. Some instability,
    occasionally over 1000 J/kg, will be enhanced by right entrance
    jet forcing to allow localized heavy rain over what has been near
    normal rainfall over the past two weeks. Therefore, a Marginal
    Risk is maintained from KY through western PA into far western NY,
    and expanded eastward to encompass heavy rain signals seen in the
    guidance across northeast PA and in the vicinity of New York City.


    Day 2
    Valid 12Z Wed May 05 2021 - 12Z Thu May 06 2021


    21Z update... No changes were needed to the inherited Marginal
    Risk area for this period. Still expecting low/mid-level winds to
    transport deep moisture nearly parallel to the west-east oriented
    frontal boundary along the Gulf Coast, capable of producing
    periods of intense rainfall. Given recent rains over the area and
    lowered FFG/increased soil saturation, there will be an increased
    threat for localized flooding concerns.


    Previous discussion... The cold front becomes more west to east
    oriented over the central Gulf Coast Wednesday, allowing low-level
    inflow to be come nearly parallel to the front and mean 850-500 mb
    flow. This brings an enhanced potential for cell training within a
    favorable thermodynamic environment (PWs 1.75-2.00
    inches...mixed-layer CAPES 1500-2500+ j/kg). With greater
    confidence of rainfall along the central Gulf Coast from NOLA east
    along the FL/GA line, shrank the Marginal Risk south a bit from
    central GA and removed from SC which have been dry over the past


    Day 3
    Valid 12Z Thu May 06 2021 - 12Z Fri May 07 2021

    The probability of rainfall exceeding flash flood guidance is less
    than 5 percent.


    Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!rj5BtM6iNX7LQZbXw4Be6unzMufZrXXXbfIrFn0eZbb4= HGaNOQX92d6cJFKDytjxYb_Tn_ol$=20
    Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!rj5BtM6iNX7LQZbXw4Be6unzMufZrXXXbfIrFn0eZbb4= HGaNOQX92d6cJFKDytjxYbgzj1Li$=20
    Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!rj5BtM6iNX7LQZbXw4Be6unzMufZrXXXbfIrFn0eZbb4= HGaNOQX92d6cJFKDytjxYRSFsbMc$=20


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