SPC AC 041254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0654 AM CST Fri Jan 04 2019

   Valid 041300Z - 051200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   SOUTHERN GEORGIA...NORTHWESTERN FLORIDA AND EASTERN NORTH
   CAROLINA...

   ...SUMMARY...
   Isolated strong-severe cells are possible within a band of showers
   and thunderstorms today over parts of Florida and Georgia, and
   tonight over eastern North Carolina.

   ...Synopsis...
   A split-flow pattern remains over the CONUS in mid/upper levels, the
   main overland perturbation being a compact, well-defined cyclone now
   located over the Arklatex region and moving into the Mid-South.  The
   500-mb low is forecast to pivot east-northeastward across the
   Tennessee Valley region this afternoon into early evening, ending up
   over central/western VA by 12Z, with slight filling of central
   height, but nonetheless forcing a well-defined area of height falls
   and DCVA in its eastern semicircle.

   At the surface, 11Z analysis showed a low over western TN roughly
   between MEM-MKL, with occluded front to north-central AL and cold
   front southward from there across the western FL Panhandle, to the
   north-central/southwestern Gulf.  A diffuse/synoptic warm front was
   evident over central GA and the extreme southern SC coast, then
   offshore eastern SC/southern NC.  A secondary/marine warm front was
   evident in weakening form over the northeastern Gulf, south of
   Apalachee Bay and west of the Peninsula on shelf waters.

   The initial surface low is expected to occlude/fill further and
   follow beneath or just ahead of the mid/upper cyclone track toward
   WV this evening.  Meanwhile the cold front will advance eastward
   over FL, GA and the Carolinas.  A newer/triple-point low should form
   late tonight over VA and move toward Chesapeake Bay, with cold front
   arching southeastward offshore from the Carolinas, GA, and most or
   all the FL Peninsula by the end of the period.

   ...FL Gulf Coast...southwestern GA...
   A narrow band of mostly showers, with thunderstorms near the
   immediate coast and sporadic/embedded cyclonic-shear areas and small
   bow/LEWP features well inland, continues to move eastward into the
   eastern Panhandle and southwestern GA.  A brief tornado or damaging
   gust cannot be ruled out the next couple hours.  See SPC mesoscale
   discussion 8 for details on the near-term scenario.

   A narrow, marginal plume of surface-based buoyancy has shifted
   inland ahead of this feature, associated with surface dew points
   mainly upper 60s F, with MLCAPE 100-500 J/kg and areas of 200-300
   effective SRH.  This regime will shift eastward across the outlook
   area through the remainder of the morning, then onshore northwest/
   west-central FL this afternoon.  Though low/middle-level lapse rates
   will remain weak, the modest buoyancy and strong low-level/
   convective-scale lift may support a continued coherent quasi-linear
   band of strong convection with isolated embedded thunder, strong/
   damaging gusts, and marginal tornado risk.  Veering preconvective
   boundary-layer flow with time is expected mid/late afternoon,
   reducing low-level shear, hodograph size and convergence, with
   inland extent.  The convective band also will encounter more-stable
   trajectories over south-central/southeastern GA, and with eastward
   extent from northwestern FL and the eastern Panhandle across the
   northern peninsula.

   ...Eastern NC...
   One or two bands of thunderstorms should cross this region this
   evening and overnight.  Isolated damaging gusts are possible, and a
   brief tornado cannot be ruled out.  The leading activity likely
   would represent a northward extension of the activity now over parts
   of GA and FL Panhandle, with additional convection possible closer
   to the cold front.  Potential with trailing convection is
   conditional on mesoscale processes, primarily recovery behind the
   initial activity.

   Mass response to the approaching, still-strong cyclone aloft will
   result in strengthening deep-layer shear and low-level hodographs,
   with a 50-60-kt LLJ expected to develop over the outlook area prior
   to most of the convection.  As is most often the case in wintertime,
   the predominant modulator for severe-storm potential will be
   instability.  The most favorably buoyant low-level airmass will
   remain offshore, over the Gulf Stream and perhaps some shelf waters.
   However, a southerly boundary-layer flow component will lead to
   inland advection of a partially marine-modified, but also perhaps
   precip-affected, marine layer.  Despite weak midlevel lapse rates,
   this may support patches or narrow corridors of 200-500 J/kg MLCAPE
   near the coast, decreasing northward/inland ahead of the cold front.

   ..Edwards/Mosier.. 01/04/2019

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