SPC AC 180535

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1235 AM CDT Fri Apr 18 2025

   Valid 181200Z - 191200Z

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

   ...SUMMARY...
   Scattered severe thunderstorms are possible from parts of the
   southern Plains northeastward toward the upper Great Lakes from late
   afternoon into early Saturday morning. Large hail and isolated
   severe gusts will be the main hazards, especially across the
   southern Plains where storms will be more concentrated.

   ...Discussion...

   Strong 500mb speed max is currently digging south across the
   northern Inter-Mountain Region. This feature will encourage a
   pronounced upper trough to evolve over the southwestern U.S. by the
   end of the period. A broad zone of strong mid-level flow will extend
   across northern Mexico-southern Plains-Great Lakes as height falls
   gradually spread into this corridor of the CONUS. As this trough
   deepens into lower latitudes, surface pressures will rise in the lee
   of the Rockies over the Plains, and a sharp cold font will surge
   into western IA-southern KS-OK Panhandle by the start of the period.
   This boundary will likely serve as a focus for robust convection
   through the period, and it should extend from eastern WI-western
   MO-northern TX Panhandle by early evening.

   Latest model guidance does not allow the upper Midwest/Great Lakes
   region to destabilize appreciably as buoyancy will be somewhat
   limited across this region. Even so, strong flow/shear warrant some
   concern for convective organization, and some risk for wind/hail
   will accompany frontal/pre-frontal storms as the boundary advances
   east through the period.

   Of more concern is the air mass across the southern Plains. Strong
   boundary-layer heating is expected across the High Plains from
   eastern NM into northwest TX, just south of the front. The northeast
   extent of this steep low-level lapse rate plume will likely extend
   into central OK. Latest model guidance suggests strong SBCAPE will
   evolve east of the dryline, and south of the front. As temperatures
   warm through the mid 80s convective temperatures will be breached
   along the dryline, likely by 21z. Forecast soundings support this
   and supercells are expected. This activity should grow upscale and
   spread northeast along the frontal zone. A considerably amount of
   post-frontal elevated convection is ultimately expected, and steep
   mid-level lapse rates favor hail with the elevated storms. While the
   frontal zone will prove sharp, with significant undercutting of
   updrafts, there is some risk for tornadoes prior to this occurring.
   Latest NAM guidance has 500mb temperature cooling to -15C at FSI by
   04z, thus very large hail can be expected with supercells. Cold
   front will gradually sag southeast during the overnight hours, but
   an extended convective event is likely from western OK into the
   Mid-MS Valley due to the evolution of the longer-wave pattern.

   ..Darrow/Wendt.. 04/18/2025

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