• MESO: Mesoscale Discussion 1789

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Wed Sep 29 00:01:52 2021
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    ACUS11 KWNS 290001
    SPC MCD 290001=20

    Mesoscale Discussion 1789
    NWS Storm Prediction Center Norman OK
    0701 PM CDT Tue Sep 28 2021

    Areas affected...Portions of south Texas

    Concerning...Severe potential...Watch unlikely=20

    Valid 290001Z - 290130Z

    Probability of Watch Issuance...5 percent

    SUMMARY...Isolated strong/severe storms ongoing near the Mexican
    border and far south Texas may produce a few severe wind or hail
    reports over the next several hours. Storm coverage is expected to
    gradually decrease with the loss of diurnal heating suggesting a
    watch is not needed.

    DISCUSSION...On the southern fringes of a weak upper low observed
    from regional WV imagery across the southern Rockies, isolated to
    widely scattered thunderstorms are ongoing across portions of
    northern Mexico and southern Texas. Surface obs and SPC mesoanalysis
    show these storms are ongoing in a warm, moist and unstable
    environment with 2000-2500 J/kg of MLCAPE and 2-6km lapse rates of
    7-8 C/km. Vertical shear remains weak with only 25-30 kts of
    deep-layer shear in place owing to relatively weak mid-level flow.
    The favorable buoyancy/shear profiles should be sufficient for a few
    organized multicells and perhaps a weak supercell.

    As ongoing storms move to the east northeast with time, occasional strengthening is possible, with a risk for damaging downburst winds
    or hail. Recent radar trends have shown this with two lead storms
    over Zavala and Frio counties over the last half hour. The
    downstream environment should remain favorable for a lower-end
    severe risk for at least a couple more hours. After sunset,
    increasing nocturnal inhibition and the movement of the main
    synoptic ascent from the upper low away from convection should
    result in a gradual downtrend in coverage/intensity. Storms may
    linger for a few hours after dark across the southern parts of the
    discussion area, closer to the coast, where a greater reservoir of warm/unstable air exists. However, weak vertical shear suggests
    little organization potential and overall confidence in convective
    evolution is low. Given the limited spatial/temporal threat window
    for wind and hail, a weather watch is unlikely.

    ..Lyons/Grams.. 09/29/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!vOgW_mf4-exB3wdfEqquUUOXPrY0RamEsHsQAqasy8LJcuWOMELR_YpdcqBmljnB9rdsJog3$=
    for graphic product...


    LAT...LON 29080079 29460052 29710014 29899944 29899854 29269720
    29039714 28729709 28359717 27989743 27769818 27769828
    27829910 27879942 27889997 29080079=20

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