• MESO: Mesoscale Discussion 1768

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Mon Sep 20 17:39:01 2021
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    ACUS11 KWNS 201738
    SPC MCD 201738=20

    Mesoscale Discussion 1768
    NWS Storm Prediction Center Norman OK
    1238 PM CDT Mon Sep 20 2021

    Areas affected...portions of eastern Minnesota...western
    Wisconsin...central into eastern Iowa

    Concerning...Severe potential...Watch likely=20

    Valid 201738Z - 201945Z

    Probability of Watch Issuance...80 percent

    SUMMARY...The severe threat is expected to gradually increase over
    the next several hours across portions of the Mid to Upper
    Mississippi Valley. Damaging gusts and large hail will be the
    primary severe threats and a Severe Thunderstorm Watch will likely
    be needed.

    DISCUSSION...A surface cold front, coupled with the approaching
    mid-level trough and associated deep-layer ascent, is gradually
    progressing eastward towards an open warm sector with 75-80F surface temperatures (given adequate diurnal heating) and 65-70F dewpoints.
    Additional surface heating and convergence along the front will
    support a gradual increase in convective development and
    intensification through the afternoon.=20

    Given largely unidirectional tropospheric flow/vertical shear and
    limited forcing for ascent in the warm sector, organized linear
    storm modes along/immediately ahead of the cold front are expected.
    However, brief instances of transient low-level rotation are
    possible given modest surface-700 mb directional shear evident in
    RAP forecast soundings. Despite modest (i.e. 6-6.5 C/km) low and
    mid-level lapse rates, 500-1000 J/kg MLCAPE is expected given the aforementioned surface temperatures/dewpoints. The stronger line
    segments will be capable of producing damaging wind gusts and
    perhaps large hail, especially for updrafts acquiring any
    appreciable rotation. A Severe Thunderstorm Watch will likely be
    needed within the next couple of hours to address the severe threat.

    ..Squitieri/Hart.. 09/20/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!odxrVTBXnDAsaV4CNmwqR5KX89ELVT7TvYi-um5LP10dzZ5bLuY-zplQHRgy8qWAIWSNNhIk$=
    for graphic product...


    LAT...LON 41129180 40539267 40449321 40379389 40509475 40789481
    41219464 43499376 44899280 44799219 44589159 44319107
    43929073 43339057 42779063 42259088 41579147 41129180=20

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