• MESO: Mesoscale Discussion 0692

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Thu May 20 18:59:31 2021
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    ACUS11 KWNS 201859
    SPC MCD 201858=20

    Mesoscale Discussion 0692
    NWS Storm Prediction Center Norman OK
    0158 PM CDT Thu May 20 2021

    Areas affected...Portions of eastern WY...southeastern MT...the
    western NE Panhandle...and western SD

    Concerning...Severe potential...Watch possible=20

    Valid 201858Z - 202130Z

    Probability of Watch Issuance...40 percent

    SUMMARY...An isolated threat for marginally severe hail and
    strong/gusty winds should gradually increase this afternoon. Watch
    issuance is possible.

    DISCUSSION...Visible satellite imagery shows convection is deepening
    over and just downstream of the higher terrain in north-central CO
    and central/eastern WY, with a couple of lightning flashes recently
    noted. Stronger mid-level winds associated with a large, closed
    upper low centered over the Pacific Northwest are expected to
    generally remain displaced to the west of the northern/central High
    Plains. But, around 20-30 kt of south-southwesterly flow at 500 mb
    should be present by late afternoon. Weak low-level
    south-southeasterly winds have transported modest boundary-layer
    moisture to the lee of the CO Front Range and Laramie/Bighorn
    Mountains in WY, with surface dewpoints generally ranging from the
    mid 40s to low 50s. Steep low and mid-level lapse rates are present
    across this region, which combined with the limited low-level
    moisture and strong diurnal heating should support MLCAPE generally
    in the 500-1500 J/kg range by late afternoon.

    Current expectations are for storms to gradually move off the higher
    terrain and into parts of the High Plains through the remainder of
    the afternoon and continuing into the early evening. Both the
    limited low-level moisture and rather weak mid-level flow are
    concerns for overall storm intensity and organization. Still, about
    25-30 kt of effective bulk shear is forecast across the warm sector
    by most short-term guidance, which will probably be enough for some
    convective organization. Storms may have a tendency to cluster as
    they move northeastward with time, especially in eastern WY and
    southeastern MT. Isolated strong to severe wind gusts will be a
    possibility if this occurs, as the very well mixed boundary layer
    will promote efficient downdraft accelerations. Marginally severe
    hail may also occur with any initially discrete storms. At this
    point, the need for a Severe Thunderstorm Watch this afternoon
    remains unclear, but observational trends will be monitored for
    potential storm clustering which may locally increase the severe
    wind threat.

    ..Gleason/Kerr.. 05/20/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!rZQ-OHoRT6xq30zS0xUJq-W7m1KAjrYSjjI_suk9KLWosdFpyj-OOMTbI9fFl4ZNjhspXpLn$=
    for graphic product...


    LAT...LON 45120677 45870625 46350519 46240463 45910404 45420334
    43810303 41380274 41160294 41020337 40960398 41000462
    41100501 42550534 43810610 44410673 45120677=20

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