ZCZC SPCSWOD48 ALL
   ACUS48 KWNS 210900
   SPC AC 210900

   Day 4-8 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0400 AM CDT Wed Apr 21 2021

   Valid 241200Z - 291200Z

   ...DISCUSSION...
   There is good agreement in medium-range guidance that an upper
   trough will continue eastward across the lower MS Valley and
   Southeast on Day 4/Saturday. A warm front should lift northward
   through the day across parts of central AL/GA and into SC as a
   surface low develops eastward across the lower OH Valley to the
   central Appalachians. There is some concern that a strong low-level
   jet will shift quickly eastwards across these areas, possibly
   outrunning the better low-level moisture return. Boundary-layer
   winds veered to nearly westerly should occur as the low-level jet
   core develops eastward. Still, the increasing low-level moisture,
   strong deep-layer shear, and some eastward advection of steepened
   mid-level lapse rates should overlap along/south of the warm front.
   Any thunderstorms that can form in this regime and remain surface
   based will pose a threat for all severe hazards. Additional severe
   convection may develop Saturday afternoon along an eastward-moving
   cold front. Convection will likely be ongoing Saturday morning given
   the strong ascent/isentropic lift associated with the low-level jet,
   and these morning thunderstorms may slow the northward progress of
   the warm front. Regardless, given latest trends in guidance
   suggesting a farther north position of the warm front by Saturday
   afternoon, have expanded the 15% severe area to include more of
   central AL/GA and southern SC.

   The upper trough should continue eastward into the western Atlantic
   on Day 5/Sunday, with rich low-level moisture confined to parts of
   the FL Peninsula. Given minimal large-scale ascent forecast over
   this region, overall severe potential ahead of a cold front appears
   too low to include any severe probabilities. Medium-range guidance
   remains in fairly good agreement that another large-scale upper
   trough will advance eastward from the western CONUS to the Plains in
   the Day 6/Monday to Day 7/Tuesday time frame. The past several runs
   of the deterministic ECMWF show a very favorable setup for organized
   severe thunderstorms capable of producing all severe hazards across
   parts of central TX into OK and southern KS, with moderate to strong
   instability forecast to overlap rich low-level moisture in the
   presence of strong deep-layer shear. There has been a consistent
   enough signal in guidance suggesting a substantial severe threat may
   develop across these areas next Tuesday to introduce 15% severe
   probabilities. Greater probabilities may be needed if current model
   trends continue. Finally, some severe threat may continue on Day
   8/Wednesday across parts of the lower/mid MS Valley as the upper
   trough moves eastward. But, there is too much uncertainty regarding
   the evolution of convection on Tuesday and the quality of low-level
   moisture return with eastward extent to include a 15% severe area
   for next Wednesday.

   ..Gleason.. 04/21/2021

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