SPC AC 071255

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0655 AM CST Wed Feb 07 2018

   Valid 071300Z - 081200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   THE CENTRAL TO EASTERN GULF COASTAL REGION...

   ...SUMMARY...
   Thunderstorms may produce isolated severe weather today over parts
   of the central to eastern Gulf coastal region.

   ...Synopsis...
   The mid/upper-level longwave trough over eastern North America will
   maintain broadly cyclonic flow over most of the U.S. east of the
   Rockies.  A well-defined/embedded shortwave trough -- now evident in
   moisture channel imagery over southeastern NM and far west TX --
   will move eastward across much of TX and begin to elongate/weaken
   through 00Z.  By 12Z, the associated 500-mb vorticity field will
   have stretched considerably, extending from northern GA to the TX
   coastal plain, leaving the attendant shortwave trough in an even
   more weakened/positively tilted state.

   At the surface, an elongated area of low pressure was evident along
   a cold-frontal zone from WV to eastern KY, with the cold front
   extending southwestward over central MS, southwestern LA and deep
   south TX.  By 00Z the consolidated surface low should reach southern
   New England, with cold front to the western carolinas, western FL
   Panhandle and north-central/western Gulf.  By 12Z, the front should
   clear all the mainland except for central/southern FL.  A diffuse
   marine/warm front, representing the equatorward part of the
   transition from a well-modified Gulf boundary layer to a residual,
   partly modified, continental/polar air mass, was analyzed from
   south-central AL across the western FL Panhandle and eastern Gulf to
   south FL.  This baroclinic zone will move slowly east-northeastward
   over southern/eastern AL, southwestern GA and the western FL
   Panhandle today, before being overtaken by the cold front and the
   prefrontal part of a convective band.

   ...Central/eastern Gulf coastal region...
   Scattered thunderstorms should continue to shift eastward across the
   region through the daytime hours, in a generally southwest/
   northeast-oriented band.  This convection (especially the
   middle-southern part) may offer isolated damaging gusts and the
   potential for a tornado.  The outlook has been adjusted southward in
   deference both to ongoing convective trends (timed faster than in
   earlier progs) and some potential for isolated daytime severe in the
   warm sector, nearer to the coast.

   The ongoing band of convection is located close to the cold front
   over much of its MS/LA extent, and is elevated ahead of the cold
   front and north of the marine front farther northeast.  The
   foregoing warm sector is expected to destabilize diurnally, from a
   combination of warm advection and erratic insolation beneath broken
   cloud cover, with greater buoyancy just inland from the coast. 
   Modified forecast soundings suggest low-level moisture will offset
   modest lapse rates aloft enough for relatively uninhibited, surface-
   based buoyancy in the warm sector. MLCAPE should rise into the
   500-1000 J/kg range generally south of the MGM-CSG corridor, ahead
   of the cold front, and dropping off considerably farther north due
   to thicker cloud cover, lower antecedent theta-e along/north of the
   marine front, and less time for advective warming.  Deep shear will
   increase northward toward more stable air, with 40-50-kt effective-
   shear magnitudes common near the coast.  In the warm sector,
   effective SRH should peak this morning, increasing northeastward
   across the diffuse marine front, but also, slowly decreasing with
   time.  Still, a few supercells are possible, either embedded in the
   main band or forming gradually over low-level convergence lines in
   the near-coastal warm sector.

   Continued veering of low-level flow, as the surface cyclone and
   related isallobaric forcing eject away from the region -- will lead
   to reduced convergence and low-level shear from late afternoon into
   evening along the front, as well as in the warm sector.  That
   process also will overlap with the onset of diabatic surface cooling
   as boundary-layer lapse rates decline from their afternoon peak. 
   Convective coverage and remaining severe potential accordingly
   should diminish markedly during that time frame.

   ..Edwards/Smith.. 02/07/2018

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