SPC AC 211934

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0234 PM CDT Fri Mar 21 2025

   Valid 212000Z - 221200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Severe thunderstorms are not forecast through tonight.

   ...20Z Update...
   Prior forecast remains largely intact. Only change is to trim the
   thunder area over southeast SD/far southwest MN in the wake/north of
   the passing mid-level shortwave impulse in eastern NE.

   ..Grams.. 03/21/2025

   .PREV DISCUSSION... /ISSUED 1124 AM CDT Fri Mar 21 2025/

   ...Iowa Vicinity...

   An upper shortwave trough will move across the Midwest today and
   tonight, with increasing deep-layer west/northwest flow associated
   with this feature overspreading the region. Boundary layer moisture
   will remain scant ahead of an east/southeast moving cold front
   sweeping across the Lower MO/Mid-MS Valley this afternoon and
   evening. Nevertheless, very cold temperatures aloft (near -25 C at
   500 mb, as sampled by 12z regional balloon observations), will
   support steep midlevel lapse rates. Additionally, surface heating
   into the mid 50s to low 60s will support steepening low-level lapse
   rates. These steep lapse rates amid cold temperatures aloft should
   be sufficient for SBCAPE values up to 300 J/kg. As such, isolated
   high-based, low-topped thunderstorms are expected this afternoon and
   evening from the Mid-MO Valley into the Mid-MS Valley. Given the dry
   boundary layer and near 30-40 kt flow between 850-700 mb, convection
   may locally enhance already gusty surface winds, and a few stronger
   gusts near 40-45 kt may occur, especially across southeast Iowa into
   northwest IL. A couple instances of small hail/graupel also could
   accompany this activity.

   ...Pacific Northwest/Northern Rockies...

   An upper shortwave trough will move across the region tonight.
   Cooling aloft will support steepening lapse rates and sufficient
   destabilization to support isolated lightning flashes.

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