SPC AC 181940

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0240 PM CDT Fri Apr 18 2025

   Valid 182000Z - 191200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE SOUTHERN
   PLAINS NORTHEASTWARD TO THE UPPER GREAT LAKES AREA...

   ...SUMMARY...
   Scattered severe thunderstorms are possible from parts of the
   southern Plains northeastward through the mid Mississippi Valley and
   southwestern Great Lakes, mainly from late afternoon into early
   Saturday morning.  Large to very large hail and severe wind gusts
   should be the main hazards, especially across the southern Plains
   where storms will be more concentrated.  A few tornadoes also appear
   possible.

   ...Discussion...
   Aside from some minor trimming of the MRGL risk area across southern
   Wisconsin, and from the western Oklahoma vicinity into southern
   Kansas, no changes appear necessary at this time.

   Extensive cloud cover along the frontal zone -- from the southern
   Plains to Wisconsin -- continues to modulate heating/destabilization
   in the near-frontal warm sector.  These effects include a
   persistent, stout cap across the southern Plains area early this
   afternoon (per the 18Z OUN RAOB).

   Still, storm development is expected late this afternoon, as
   reflected in previous forecasts, with all-hazards severe potential 
   with more vigorous convection.

   ..Goss.. 04/18/2025

   .PREV DISCUSSION... /ISSUED 1133 AM CDT Fri Apr 18 2025/

   ...Southern Plains to the Ozarks...
   Strong southwesterly mid to high-level flow will extend from the
   southern High Plains northeastward into the Great Lakes, downstream
   of a mid- to upper-level low over the Four Corners states.  A cold
   front was analyzed from the Low Rolling Plains northeastward through
   central OK late this morning.  This portion of the front will
   continue southeastward into the I-44 corridor but decelerate towards
   late afternoon.  A dryline extends southward from the front over the
   TX Big Country.  

   Despite widespread mid/high-level cloudiness, filtered heating of a
   moist/destabilizing airmass will likely result in a weakened cap by
   late afternoon.  Initial supercell development should pose a threat
   for large to very large hail, potentially up to 2-3 inches in
   diameter.  Model forecast hodographs and ample PW suggest a strong
   tendency towards upscale growth into an HP supercell and linear
   segment this evening from south-central OK northeastward towards the
   western part of the Ozark Plateau.  Coincident with this expected
   convective mode evolution, an increase in the severe/damaging wind
   threat is anticipated.  The threat for a few tornadoes remains
   apparent, especially across parts of OK along/south of the front, as
   favorable low-level shear will be present this evening where
   surface-based supercells and mesovortex potential may still exist.

   ...Lower MI to Mid Mississippi Valley...
   In wake of weakened thunderstorm activity across Lower MI, a warm
   sector will attempt to spread northward and destabilize
   across parts of the mid MS Valley and southern/central Great Lakes
   this afternoon.  An upper trough will only slowly encroach on the
   Upper Midwest through late tonight.  As a result, southwesterly flow
   aloft will be maintained and limited southeastward progression of a
   cold front is forecast.  

   Model guidance continues to indicate surface-based thunderstorm
   development will be inhibited until peak heating, when frontal
   forcing and gradual erosion of the cap leads to isolated storms
   developing from MO northeastward towards the southwest Great Lakes. 
   Uncertainty remains regarding timing, overall thunderstorm coverage,
   and the exact evolution of this activity.  Nonetheless, strong shear
   through a deep layer and adequate buoyancy (due in part to steep
   mid-level lapse rates), will support organized thunderstorms.  The
   stronger thunderstorms will probably pose a risk for severe hail and
   damaging winds.  This severe threat may persist for much of the
   evening, and perhaps continuing into the early overnight hours.

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