SPC AC 201255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0755 AM CDT Wed Oct 20 2021

   Valid 201300Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE UPPER
   MIDWEST...

   ...SUMMARY...
   Thunderstorms with isolated severe hail are possible today across
   northern Iowa and southern Minnesota, primarily this morning into
   early/mid-afternoon.

   ...Midwest including northern Iowa/southern Minnesota...
   Immediately ahead of an eastward-moving upper low, early morning
   convection has exhibited a modest uptick in coverage and intensity
   in the pre-dawn hours across far northeast Nebraska and northwest
   Iowa into far southwest Minnesota. For additional short-term
   details, see Mesoscale Discussion 1877. Steep mid-level lapse rates
   (12Z Omaha observed sounding) and sufficient elevated instability
   (upwards of 1000 J/kg MUCAPE) may support some storms capable of
   hail today as this activity spreads northeastward across northern
   Iowa/southern Minnesota.

   Farther south across the middle Mississippi Valley, substantial
   convective inhibition will likely inhibit surface-based storms along
   and ahead of the cold front, although moistening near 850 mb may
   support some slightly elevated convection. The potential for
   severe-caliber hail/wind is expected to remain very limited today.

   ...Lower Michigan and far northern Indiana/northwest Ohio...
   Thunderstorms are expected to develop this evening into the
   overnight, particularly across Lower Michigan. This convection will
   be rooted atop a stable boundary, with modest elevated instability
   (MUCAPE generally 500-750 J/kg) supportive of thunderstorms. This
   could include some potential for hail, but current thinking is that
   the risk for severe-caliber hailstones should remain low given the
   modest overall buoyancy.

   ...Middle Gulf Coast...
   Relatively rich low-level moisture will stream northward into
   portions of Louisiana and southern Mississippi/southwest Alabama
   during the day today, along the western periphery of a surface ridge
   over the Southeast CONUS. As this occurs, moderate buoyancy will
   support scattered thunderstorms near the coast this morning, with
   convection expected to spread further inland with time by afternoon.
   While weakly veering wind profiles may favor some organization with
   the strongest storms, poor mid-level lapse rates and generally
   modest low/mid-level flow should tend to limit the severe risk in
   this area, although a few instances of small hail and gusty winds
   cannot be ruled out.

   ..Guyer/Leitman.. 10/20/2021

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