SPC AC 081251

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0651 AM CST Wed Dec 08 2021

   Valid 081300Z - 091200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM
   NORTHWESTERN FLORIDA AND THE EASTERN FLORIDA PANHANDLE TO EXTREME
   SOUTHERN SOUTH CAROLINA...

   ...SUMMARY...
   Isolated damaging to marginally severe gusts or a brief tornado may
   occur today from northwestern Florida and the eastern Florida
   Panhandle to extreme southern South Carolina.

   ...Synopsis...
   The mid/upper-tropospheric pattern will be characterized by split
   flow over the West Coast, and a progressive synoptic pattern
   downstream.  Moisture-channel imagery indicates two primary cyclonic
   circulations/vorticity maxima along a positively tilted, southern-
   stream trough:  the first near LAS, the second along 30N west of
   northern Baja.  The northern one will be entrained into the
   midlatitude westerlies move over portions of the central/southern
   Plains as an open-wave trough through the period, while the southern
   one moves generally slowly southward across open Pacific waters.  As
   that occurs, a downstream synoptic trough -- initially extending
   from northern ON to the lower Mississippi Valley -- will pivot
   eastward and become more negatively tilted.  By 00Z the trough
   should extend from James Bay across PA to the Carolinas, moving
   offshore all but eastern New England by the end of the period. 

   At the surface, 11Z analysis showed a quasistationary frontal zone
   from southern SC through a weak low near MCN to near MOB, west-
   southwestward through a weak frontal-wave low over the Gulf south of
   GLS, then south-southwestward to south of BRO.  A remnant marine
   warm front was evident off the west coast of the FL Peninsula,
   becoming diffuse northward and inland over south-central GA. 
   Cyclogenesis is expected well offshore from the Mid-Atlantic today
   as the mid/upper trough approaches, with the resulting low deepening
   and ejecting northeastward across ocean waters east of New England. 
   As that occurs, the main/northern frontal zone will move
   southeastward, with the resulting cold front reaching southeastern
   GA and the eastern FL Panhandle by 00Z.  The overland segment of the
   front should move through most of central FL overnight, before
   decelerating substantially and weakening into early day 2. 

   ...Southeast...
   Isolated to widely scattered thunderstorms are possible today,
   predominantly in a southwest/northeast-aligned band of convection
   arising from low-level lift near the front.  Loosely organized
   multicell clusters and at least transient supercell potential
   (either within the main band or moving off the Gulf) exists,
   suggesting a marginal wind/tornado threat. 

   South-southwest to southwest winds should be maintained at the
   surface, given the ambient synoptic-scale isallobaric response to
   the Atlantic cyclogenesis.  This will have two main effects for
   convective/severe potential:
   1.  Moisture/theta-e advection from a fairly well-modified
   maritime/tropical boundary layer over the northeastern Gulf, across
   shelf waters and over the southeastern GA/northwestern FL region. 
   This should combine with limited diurnal heating (after low clouds
   and fog erode) to offset weak mid/upper lapse rates enough for a
   prefrontal corridor of 500-1000 J/kg MLCAPE. 
   2.  Tempering of bulk shear due to lack of more veering with height.
   Still, sufficient speeds aloft will be present to support
   effective-shear magnitudes of 40-50 kt, with slight low-level
   hodograph curvature.

   Convective coverage/intensity should lessen late this afternoon into
   evening over northern FL as activity encounters weaker instability,
   and large-scale lift diminishes along/ahead of the front.

   ..Edwards/Goss.. 12/08/2021

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