SPC AC 231749

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1149 AM CST Wed Dec 23 2020

   Valid 241200Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS FAR
   NORTHEASTERN SOUTH CAROLINA...EASTERN NORTH CAROLINA...AND
   SOUTHEASTERN VIRGINIA...

   ...SUMMARY...
   Severe thunderstorms are expected across the Carolinas and Mid
   Atlantic region Thursday afternoon and evening. A few severe
   thunderstorms are also possible across portions of southeast
   Alabama, southwest Georgia, and Florida Thursday.

   ...Synopsis...
   A strong upper trough with an expansive area of surrounding/cyclonic
   flow will shift steadily eastward into the eastern half of the U.S.
   Thursday, with a closed low gradually evolving across the
   Midwest/Ohio Valley.  Very strong flow aloft will accompany this
   system, including mid-level south-southwesterly flow in excess of
   100 kt which will spread across the Appalachians into the East Coast
   States during the period and an 80 to 90 kt southerly low-level jet
   from the Carolinas into New England.

   At the surface, a deep low initially near Lake Superior is progged
   to fill gradually as it moves northeast across Ontario, to be
   replaced by a deepening frontal low which is forecast to develop
   over the Carolinas during the afternoon and shift quickly northward
   to the Lower Great Lakes region by the end of the period.  A
   strong/trailing cold front will sweep eastward across the
   Appalachians during the afternoon, and then off the middle and
   southern Atlantic Coast overnight.

   ...The Carolinas to the Mid Atlantic region...
   As a strong cold front crosses the mountains during the afternoon,
   pre-frontal low-level warm/moist advection will occur east of the
   Appalachian crest.  However, weak lapse rates and very limited
   potential for any heating will hinder CAPE development across the
   warm sector.  Still, sufficient destabilization for thunderstorm
   development is expected, and thus expect an increase in convective
   coverage and intensity across the region through the afternoon and
   into the evening.  

   CAM runs continue to indicate that pre-frontal storms will affect
   the eastern North Carolina and southeastern Virginia vicinity, in
   response to strong low-level warm advection.  Particularly during
   the late afternoon, as the upper system advances, evolution of a few
   of these storms into supercells appears likely.  Given
   intense/veering flow with height spreading across the area, shear
   will be quite favorable for both mid-level and low-level rotation. 
   As such, potential for tornadoes with this pre-frontal convection
   appears sufficient to upgrade to ENH risk/10% tornado probability.

   Later, a well-defined band of frontal convection will spread across
   the eastern seaboard.  Given the very strong flow through the lower
   troposphere, locally damaging winds are expected along the
   convective line, along with the possibility for brief QLCS-type
   tornadoes.  Risk will likely continue through the evening, with the
   convective band expected to eventually clear the coast near or
   shortly after midnight.

   ...Southeastern AL/southern Ga/FL vicinity...
   Strong thunderstorms will likely be ongoing at the start of the
   period, along an advancing cold front.  Modest instability/weak
   lapse rates ahead of the boundary will likely limit storm intensity,
   as storms progress eastward.  Still, strong/unidirectional
   southwesterly flow aloft is expected across the region, on the
   southeastern periphery of the strong upper system.  This suggests
   potential for local updraft enhancement -- and associated risk for
   locally damaging wind gusts, and possibly a brief tornado.  Storms
   will spread across central and eastern Georgia and into the Florida
   Peninsula through late afternoon/early evening, with any severe risk
   likely to diminish after dark.

   ..Goss.. 12/23/2020

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