SPC AC 210501

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1101 PM CST Thu Dec 20 2018

   Valid 211200Z - 221200Z

   ...NO SEVERE THUNDERSTORM AREAS FORECAST...

   ...SUMMARY...
   Isolated thunderstorms will be possible today from the Appalachian
   Mountains to part of the Mid-Atlantic States and across
   north-central Florida, while some isolated storms are expected late
   this afternoon and evening across eastern New England.

   ...Synopsis...
   Very strong 12-hour 500-mb height falls will spread from the
   Southeast States to off the Mid-Atlantic Coast through New England
   to southern Quebec this forecast period, as a full-latitude trough
   lifts northeastward across the eastern United States.  This
   evolution will occur as a 120kt+ 500-mb jet translates into the
   downstream side of the trough and off the Atlantic coast during the
   overnight hours, while upstream flow across the contiguous U.S.
   become quasi-zonal.  At the surface, a warm front will shift
   poleward across New England today, with a lead cold front then
   advancing east across this region.  A secondary cold front will
   shift east across the eastern states, and should extend from western
   New England to the southern extent of the FL Peninsula by 12Z
   Saturday.

   ...Central Appalachians and Piedmont to MD/Delmarva region...
   The greatest potential for destabilization today should be along and
   east of the Piedmont region of the Carolinas and VA where surface
   heating with residual dew points in the upper 40s to middle 50s are
   expected.  This combined with cooling 500-mb temperature to at or
   below -20 C will steepen lapse rates with buoyancy becoming deep
   enough for lightning production, as updrafts develop along the
   secondary cold front.  Despite the potential for low-topped storms
   to occur and spread to the east/northeast, stronger winds
   aloft/shear may be displaced east of the colder 500-mb
   temperatures/stronger destabilization.  This offset in what could be
   a favorable CAPE/shear parameter space precludes the introduction of
   severe-weather probabilities with this outlook.

   ...Eastern New England...
   Although a modified warm sector will advance northward across
   eastern New England into this afternoon, limited poleward moisture
   return and weak lapse rates will preclude greater destabilization
   this forecast period.  The strongest height falls are expected to
   spread across New England from late afternoon into the evening. 
   This combined with low-level warm advection should result in
   sufficient elevated buoyancy, becoming deep enough for some
   thunderstorm development.

   ..Peters/Wendt.. 12/21/2018

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