• MESO: Mesoscale Discussion 1812

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Sat Oct 9 17:04:49 2021
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    ACUS11 KWNS 091704
    SPC MCD 091704=20

    Mesoscale Discussion 1812
    NWS Storm Prediction Center Norman OK
    1204 PM CDT Sat Oct 09 2021

    Areas affected...Parts of central/eastern South Dakota and adjacent
    portions of western Minnesota

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

    Valid 091704Z - 091930Z

    Probability of Watch Issuance...80 percent

    SUMMARY...Increasing thunderstorm development, perhaps including a
    few supercells with primarily a risk for severe hail, particularly
    across and northeast of the Huron and Mitchell areas, by the 2-4 PM
    time frame.

    DISCUSSION...Considerable convective development, with increasing
    embedded thunderstorm activity, is ongoing in a corridor
    near/southeast of the Black Hills into portions of the upper Red
    River Valley. This is near the nose of a strong southwesterly
    mid/upper jet, still mostly over the southern Great Basin, but with
    one significant emerging speed maximum now progressing through the
    Nebraska Panhandle vicinity.

    Forcing with this speed maximum appears to be contributing to
    recently increasing thunderstorms across and northeast of the Pine
    Ridge and Rosebud Reservation vicinities, where 16Z surface
    observations suggest that there may also be a developing surface
    low. Models indicate that increasingly divergent upper flow will
    contribute to lower/mid tropospheric cyclogenesis today through
    tonight, with more rapid deepening forecast east/north of the Huron
    and Mitchell vicinities into the upper Red River Valley after 00Z
    this evening.

    In the nearer term, southeasterly near-surface winds are
    contributing to gradual boundary-layer moistening to the
    east/northeast of the developing low, across southeastern into
    central South Dakota, where dew points are now rising into the lower
    60s. It is not clear that moisture will increase much further, but,
    coupled with mid-level cooling accompanying the emerging speed
    maximum, thermodynamic profiles may become increasing conducive to
    severe hail.

    Based on recent model output and observational data, it appears that
    this may become focused across and northeast of the Huron/Mitchell
    vicinity as early as 19-21z. Thereafter, a continuing gradual
    increase in forcing for ascent and southeasterly inflow of
    destabilizing boundary-layer air probably will support further
    upscale growth, intensification and organization in the presence of
    strong deep-layer shear. This may include a couple of supercells,
    with perhaps low-level hodographs becoming supportive of a risk for
    tornadoes, though stronger south/southeasterly 850 mb jet
    strengthening is generally not forecast until after dark across
    western Minnesota.

    ..Kerr/Hart.. 10/09/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!s2nJJ-x2sYoFgI0mphibFt4hCoY6dvo4xyO-7nVCbJaeVQsowf5X2eUvgSh7dWaROueZ7FHm$=
    for graphic product...


    LAT...LON 44169962 45079868 45469668 44579601 43659715 43599913

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