• MESO: Mesoscale Discussion 0696

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Fri May 21 20:55:09 2021
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    ACUS11 KWNS 212055
    SPC MCD 212054=20

    Mesoscale Discussion 0696
    NWS Storm Prediction Center Norman OK
    0354 PM CDT Fri May 21 2021

    Areas affected...Portions of northeastern CO...the NE
    Panhandle...and southwestern SD

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

    Valid 212054Z - 212330Z

    Probability of Watch Issuance...60 percent

    SUMMARY...The potential for large hail, strong/gusty winds, and
    perhaps a couple of tornadoes should increase late this afternoon.
    Watch issuance is possible.

    DISCUSSION...20Z surface observations show a cold front extending
    from central into southwestern SD, the NE Panhandle, and parts of
    northeastern CO. Strong boundary-layer heating has occurred so far
    this afternoon to the south of this front and east of a surface lee trough/dryline in eastern CO, with surface temperatures generally
    warming into the upper 70s to mid 80s. Low-level moisture still
    remains somewhat limited, but surface dewpoints are mostly holding
    in the mid to upper 50s. The presence of steep mid-level lapse rates
    across the warm sector is contributing to 1000-1500 J/kg of MLCAPE
    per latest mesoanalysis estimates. A large, closed upper low remains
    centered over CA and the Great Basin, with a fairly broad swath of
    enhanced south-southwesterly mid-level winds extending from the
    Southwest across the central Rockies and adjacent High Plains.

    As convective inhibition continues to weaken, current expectations
    are for isolated storms to develop by 22-23Z (4-5 PM MDT) along both
    the cold front and dryline as a subtle vorticity maximum ejects
    northeastward over the central Rockies/High Plains. 40-50+ kt of
    effective bulk shear will likely support supercells initially, with
    mainly a large hail threat given the steep mid-level lapse rates.
    There may be some potential for very large hail (2+ inch) for a
    brief period before storms begin to interact and merge with one
    another along the cold front. A south-southeasterly low-level jet is
    forecast to strengthen to around 40-45 kt by early this evening, and
    low-level shear will increase as well south of the front. A couple
    of tornadoes appear possible in this regime, but there is
    uncertainty regarding dominant storm mode (supercells vs clusters)
    by this evening as the low-level shear strengthens. Strong/gusty
    winds will also be possible if storms grow upscale into one or more
    small clusters. Given the increasing severe potential anticipated
    over the next couple of hours, watch issuance will probably be

    ..Gleason/Guyer.. 05/21/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!peqmoDbOLq3mIw1MX6NOlBBy97MzJiCLBRe4SJdnC4jE04HKIvqxuXQtPpjy3JzYb9_N92WE$=
    for graphic product...


    LAT...LON 40020212 39510259 39340310 39660349 40320388 41370393
    42280357 43220257 43570185 43550126 43080124 42000194
    40990210 40020212=20

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