SPC AC 270603

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0103 AM CDT Tue Apr 27 2021

   Valid 281200Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE SOUTHERN PLAINS...

   ...SUMMARY...
   Isolated severe thunderstorms are possible from the southern Plains
   through the Middle Mississippi Valley on Wednesday.

   ...Synopsis...
   Southern-stream upper trough is forecast to move eastward throughout
   the day, ending the period extended from southwest KS through
   northern Mexico and phased with a northern-stream upper trough. This
   evolution will allow strong southwesterly flow aloft to persist over
   northern Mexico and the southern Plains while also expanding
   northeastward through the Mid MS Valley and into the middle OH
   Valley.

   Surface pattern early Wednesday morning will feature a cold front
   extending from a low over southern Lower MI southwestward to another
   low over western KS early Wednesday morning. A dryline is forecast
   to extend from the western KS low southward into southwest TX. This
   cold front will move southeastward throughout the day while
   gradually overtaking the dryline from northwest to southeast. 

   A broad warm sector will exist ahead of this front from the southern
   Plains through the OH Valley. Elevated thunderstorms will likely be
   ongoing early Wednesday morning from western/central OK through the
   Ozark Plateau. This cluster of storms is forecast to gradually move
   northeastward into the Mid MS Valley and eventually into middle OH
   Valley. Occasionally strong updrafts may produce hail and/or
   damaging wind gusts throughout the day.

   ...Southern Plains...
   Early morning storms over OK and north-central TX are expected to
   gradually move northeastward throughout the day. Destabilization is
   anticipated in the wake of these storms, generally from western OK
   southward into southwest TX, ahead of the dryline. As such,
   additional storms are expected to develop, with an initial
   supercellular storm mode likely. Mid-level lapse rates will be
   modest, but still supportive of moderate instability as a result of
   moist low-levels. Strength of the surface winds will largely be tied
   the strength of a surface low moving northeastward into southwest OK
   ahead of the cold front. However, the strength of this low varies
   within the guidance, leading to uncertainty regarding the strength
   of the southerly surface winds. Even so, the presence of this low
   suggests a localized corridor of southerly (and perhaps
   southeasterly) surface winds from northwest TX into far southwest
   OK, resulting a locally higher tornado threat.

   Afternoon thunderstorm initiation is also anticipated across the
   higher terrain of northern Mexico, with the storms then tracking
   eastward/northeastward into the TX Hill Country. Moderate to strong
   buoyancy and strong vertical suggest will support severe
   thunderstorms capable of all severe hazards, including very large
   hail. 

   ...Lower Great Lakes Region into central PA/NY...
   Moderate westerly flow aloft is forecast to extend from the Lower
   Great Lakes into southern New England throughout the day. A subtle
   shortwave trough embedded within this belt of westerly flow will
   move into the region during the afternoon. Modest low-level moisture
   and diurnal heating will likely result in air mass destabilization
   ahead of this shortwave, promoting thunderstorm development. The
   moderate westerly flow aloft coupled with steep low-level lapse
   rates will contribute to the potential for a few stronger gusts with
   the more robust storms. Some large hail is possible as well.

   ..Southern MO/Far Southern IL/Western KY/Southern IN...
   Recent guidance has shown a trend toward more forward propagation of
   the thunderstorm cluster moving through the region during the late
   afternoon. This is likely a result of stronger flow aloft edging
   into the region. However, given the mesoscale nature of this
   transition, uncertainty regarding the linear transition is high,
   precluding introduction of greater probabilities with this outlook.

   ..Mosier.. 04/27/2021

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