• MESO: Mesoscale Discussion 1478

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Wed Aug 11 16:49:04 2021
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    ACUS11 KWNS 111648
    SPC MCD 111648=20

    Mesoscale Discussion 1478
    NWS Storm Prediction Center Norman OK
    1148 AM CDT Wed Aug 11 2021

    Areas affected...Much of central/eastern WI and parts of far
    northeastern IA...southeastern MN...and the U.P. of MI

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

    Valid 111648Z - 111845Z

    Probability of Watch Issuance...95 percent

    SUMMARY...The threat for several tornadoes, very large hail, and
    significant severe wind gusts will likely increase this afternoon.
    Tornado Watch issuance will be needed within an hour.

    DISCUSSION...Recent visible satellite imagery shows convection is
    beginning to develop along and ahead of a surface cold front across
    northern and central WI. Pronounced large-scale forcing for ascent
    associated with an upper trough across the Upper Midwest will likely
    aid the development of at least scattered severe storms this
    afternoon across much of WI. The airmass south of the front is
    strongly unstable, with very steep mid-level lapse rates present
    atop a rather moist low-level airmass. Any remaining convective
    inhibition should dissipate within the next hour or two with
    continued robust diurnal heating. MLCAPE will likely reach
    3500-4500+ J/kg by peak afternoon heating across this region, and
    enhanced mid-level west-southwesterly winds will support 50+ kt of
    effective bulk shear across the warm sector. Current expectations
    are for supercells to form initially, posing a threat for both very
    large (2+ inch) hail and tornadoes. This tornado threat will be
    aided by the continued presence of a 30-40 kt southwesterly
    low-level jet, which should support 0-1 km SRH of 100-200 m2/s2 and
    low-level updraft rotation. There is some signal in short-term
    guidance that these supercells may try to grow upscale into an MCS
    at some point this afternoon. If this were to occur, then numerous severe/damaging wind gusts, some of which could be significant,
    would be likely given the large buoyancy available. Regardless of
    eventual storm evolution, the increasing threat for supercells this
    afternoon will require Tornado Watch issuance within an hour.

    ..Gleason/Thompson.. 08/11/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!q1PIgYsG49j8S7QKCqhbsX0JEMiqBmgxR7IS0ZFy1Xg3P2jV5HV_DjbEn5cc-4wHvMBns-v5$=
    for graphic product...


    LAT...LON 43238756 42528773 42528925 42589046 42789105 43099137
    43859143 45218958 45478856 45548744 45368680 44648711
    43868744 43238756=20

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