• MESO: Mesoscale Discussion 1279

    From COD Weather Processor@1:2320/105 to wx-storm@lists.illinois.edu on Wed Jul 14 12:02:30 2021
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    ACUS11 KWNS 141202
    SPC MCD 141202=20

    Mesoscale Discussion 1279
    NWS Storm Prediction Center Norman OK
    0702 AM CDT Wed Jul 14 2021

    Areas affected...northern Iowa...southern Minnesota...southwest
    Wisconsin...and far northwest Illinois

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

    Valid 141202Z - 141430Z

    Probability of Watch Issuance...60 percent

    SUMMARY...Ongoing showers and thunderstorms will begin to pose a
    sporadic and episodic threat for marginally severe hail and gusty
    thunderstorm winds this morning. As further destabilization occurs
    later this morning, a more robust severe potential should
    materialize. A watch may be needed at some point later this morning
    across portions of the area.

    DISCUSSION...Widespread showers and thunderstorms associated with a
    weak cyclonic perturbation in the mid-level flow continue to move
    east across the central Plains this morning. The strongest
    convection is organizing on the leading edge of the precipitation
    shield, where a seasonably strong 850-mb jet has focused the
    strongest low-level theta-e advection this morning. Furthermore,
    despite being near the daily minimum in instability, most-unstable
    CAPE in excess of 2000 J/kg persists this morning across northwest
    Iowa ahead of this organizing convection thanks in part to the
    combination of sustained low-level theta-e advection and 700-500 mb
    lapse rates between 7 and 8 C/km. This instability is more than
    sufficient to sustain the ongoing convection through the morning
    hours, especially given the background large-scale ascent associated
    with the mid-level cyclonic perturbation. Small hail and gusty winds
    are possible with the present convection.=20

    Thunderstorm intensity should slowly increase through the morning as
    diurnal heating commences, slowly eroding the residual nocturnal
    boundary layer, and convection organizes into a mesoscale convective
    system (MCS). This erosion of the nocturnal boundary layer should
    allow for elevated convection to transition to surface-based or near-surface-based convection through the morning, increasing the
    potential for strong thunderstorm downdrafts to reach the surface.=20

    Two potential limiting factors for a more robust and widespread
    severe threat will be the lack of a widespread, higher-end CAPE
    reservoir downstream of the ongoing convection and modest deep-layer
    shear. With time, deep-layer shear should increase across the
    region, aiding the organization of convection into an MCS. Until
    then, severe potential should be more sporadic, primarily being
    driven by storm-scale processes. Later, despite differences in
    intensity and location, high-resolution numerical guidance depicts
    development of a belt of enhanced low-level flow as the MCS
    organization increases. Development of this feature would act to
    enhance corridors of severe wind potential.

    It may become necessary for a watch to be needed at some point this
    morning across portions of the highlighted area. However,
    uncertainties associated with how and when ongoing and developing
    convection transitions from elevated to surface based or
    near-surface based are too large to pinpoint when and where a watch
    would be needed. As such, conditions will continue to be monitored
    through the morning.

    ..Marsh/Hart.. 07/14/2021

    ...Please see https://urldefense.com/v3/__http://www.spc.noaa.gov__;!!DZ3fj= g!oiCJPh9s1_byekQeoZP8Ok9NJHpyoUhxKSt37XBFuzTSIBiGaXBpz8nUXGsXOOZWktPvcISv$=
    for graphic product...


    LAT...LON 43459587 43779531 44649193 44839042 44238938 43238926
    42318967 41889115 41759368 41769548 42699608 43459587=20

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