SPC AC 161557

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1057 AM CDT Thu Sep 16 2021

   Valid 161630Z - 171200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF MN
   SOUTHWARD INTO EASTERN SD AND NORTHERN NE...

   ...SUMMARY...
   Severe storms will be possible late this afternoon and evening
   particularly across Minnesota into southeast South Dakota and parts
   of Nebraska.

   ...MN...
   Relatively fast zonal flow extends from the northern Rockies across
   the northern Plains today, with multiple fast-moving shortwave
   troughs embedded in this flow.  A surface low over SD will migrate
   eastward into MN later today, with an associated cold front
   providing the focus for scattered thunderstorms.  Activity will
   begin by mid/late afternoon over northern and central MN, where
   strong heating is occurring.  Forecast soundings show relatively
   steep mid-level lapse rates and MLCAPE values of 1500-2000 J/kg in
   this area.  Vertical shear profiles are also quite strong,
   supportive of supercells capable of large hail, damaging winds, and
   a few tornadoes.  Initial storms may be discrete for a few hours
   before linear forcing mechanism and merging cold pools result in a
   squall line.  Storms are expected to weaken this evening after dark
   as instability diminishes over northeast MN.

   ...SD/NE...
   Full sunshine is present today over most of SD/NE, leading to rapid
   heating and destabilization.  A cold front will sag into this region
   this evening, providing the focus for scattered thunderstorms. 
   Model guidance suggests that most of the storms will be behind the
   surface front and somewhat elevated.  Nevertheless, hail and gusty
   winds may occur in the stronger cells for a few hours.

   ..Hart/Goss.. 09/16/2021

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