SPC AC 241249

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0649 AM CST Thu Dec 24 2020

   Valid 241300Z - 251200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE EASTERN CAROLINAS AND SOUTHEASTERN VIRGINIA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
   FLORIDA PANHANDLE...AND SURROUNDING THE "ENHANCED" AREA...

   ...SUMMARY...
   The main threat for severe thunderstorms will be across portions of
   the Carolinas and Mid-Atlantic region this afternoon and evening,
   with tornadoes and damaging gusts possible.

   ...Synopsis...
   In mid/upper levels, broadly cyclonic flow will shift eastward from
   the central to eastern CONUS through the period, in step with a
   high-amplitude synoptic trough.  As one associated shortwave eject
   northeastward across the upper Great Lakes, an upstream perturbation
   -- now evident in moisture-channel imagery over the MB/ON border --
   will dig southeastward.  Another perturbation initially over central
   TX will pivot eastward to the southern Appalachians.  As this
   occurs, a closed 500-mb cyclone should develop over the lower OH
   Valley region during the 00-06Z time frame, its center moving
   eastward past SDF before 12Z tomorrow. 

   At the surface, the primary, occluded synoptic low over the eastern
   Lake Superior region will fill and eject northeastward across
   northern ON.  The associated cold front -- initially drawn across
   parts of OH, eastern KY/TN, AL, to near the Mississippi River mouth,
   will move eastward across the southern/central Appalachians through
   this evening.  As the mid/upper cyclone closes to the west, a weak,
   closed area of low pressure over the eastern AL/northwestern GA
   region should move up the front to the eastern WV/southwestern PA
   corridor by 00Z, becoming the primary low over NY tonight and
   reaching southern QC by 12Z.  By then, the trailing cold front will
   reach eastern PA, then offshore from the remainder of the CONUS
   Atlantic Coast, having crossed south FL around 06Z. 

   ...Eastern/central Carolinas and vicinity...
   Widely scattered to scattered thunderstorms are expected to cross
   this region today into this evening in two primary regimes:
   1.  Ahead of the cold front, moving northward to north-northeastward
   from a favorably moistening/destabilizing marine boundary-layer
   region over the Atlantic.  This convection will cross lesser but
   still sufficient/surface-based buoyancy across the eastern
   Carolinas, as the warm sector modifies and becomes more unstable,
   amidst strengthening deep shear.  Warm-sector dew points in the 60s
   will offset modest midlevel lapse rates enough to contribute to
   MLCAPE generally in the 500-1000 J/kg range from eastern NC across
   the southern sounds/Outer Banks of NC, locally higher near the
   southern NC coastline.  Discrete supercells are possible, offering
   tornadoes and isolated damaging wind.  Then...
   2.  A near-frontal band of thunderstorms will sweep eastward to
   east-northeastward over the region, presenting a more-dominant wind
   threat, but with some line-embedded/QLCS tornado threat as well.  

   Height falls and tightening height/thermal gradients aloft are
   expected to spread over the region near and just ahead of the front
   throughout the day, as the synoptic trough deepens to the west.  A
   65-75 kt LLJ will develop and shift across this region, leading to
   uncommonly similar, 40-50-kt magnitudes of shear vectors in the 0-1,
   0-3, and effective layers. Despite the nearly unidirectional,
   southerly low/middle-level flow, even very slight hodograph
   curvature contributes to 200-350 J/kg effective SRH in forecast
   soundings.  

   The air mass will become still more stable with northward extent
   past a diffuse warm front that will move northward over the
   Delmarva/Mid-Atlantic this evening and tonight.  However, at least a
   marginal wind and tornado threat appears to be justified, given the
   strength of the ambient deep-layer flow and at least weak
   instability near the surface.

   ...FL/GA...
   See the remainder of tornado watch 517, mesoscale-discussion updates
   for that, and separate mesoscale discussion 1372 for more details on
   the severe threat with ongoing convection in the near-frontal squall
   line and warm sector over the FL Panhandle region.
     
   The main belt of convection will sweep eastward across the remainder
   of the outlook area through this evening, presenting a threat for
   sporadic damaging gusts and a tornado or two.  Moisture and buoyancy
   will continue to be greatest over open Gulf waters near the Loop
   Current, where marine thermal/moisture fluxes optimize boundary-
   layer theta-e.  Still, at least marginally favorable thermodynamic
   conditions will exist in a northward-narrowing corridor ahead of the
   main convective line, as partly modified parcels advect from the
   Gulf.  This air mass, characterized by modest lapse rates but also
   minimal MLCINH, also may support maturation of cells developing in
   the warm sector before they cross into more-stable air.

   ..Edwards/Smith.. 12/24/2020

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