SPC AC 281245

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0745 AM CDT Wed Oct 28 2020

   Valid 281300Z - 291200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   EXTREME SOUTHEASTERN LA...SOUTHEASTERN MISSISSIPPI...SOUTHERN
   ALABAMA...AND THE WESTERN FLORIDA PANHANDLE...

   ...SUMMARY...
   Hurricane Zeta may spawn a few tornadoes later today through tonight
   over parts of the central Gulf Coast States.

   ...Synopsis...
   In mid/upper levels, a strong cyclone is forecast to eject
   east-northeastward from its current central position over
   southeastern NM, with the 500-mb low over the CDS area by 00Z, and
   reaching southeastern OK by 12Z.

   At the surface, 11Z analysis showed a weak low near DUJ with arctic
   cold front southwestward across eastern portions of KY/TN, coming
   quasistationary over northeastern or central MS.  The boundary then
   curved across southwestern LA and the waters offshore from BRO.  As
   the mid/upper cyclone proceeds across the southern Plains, and
   shortly behind the passage of Zeta across the central/north-central
   Gulf, the boundary will move eastward across the northwestern Gulf
   as a cold front.  Portions well inland over MS/TN may move slightly
   northwestward before Zeta passes those latitudes.

   ...Southeastern LA to western GA and FL Panhandle...
   The NHC forecast depicts Hurricane Zeta's center moving ashore
   southeastern LA late this afternoon.  That will be preceded from
   midday through afternoon by increasingly favorable low-level winds
   near the coast, enlarging hodographs and SRH values.  The favorable
   kinematic field will spread northeastward across the outlook area
   this evening and tonight as Zeta rapidly translates across southern
   MS and AL to northwest GA.  The circulation will become embedded in
   a regime of increasing ambient deep shear, ahead of the ejecting
   southern Plains cold-core cyclone.  The main concerns for the
   tornado threat instead are related to instability and convective
   coverage/mode.

   Convection not part of Zeta's circulation envelope has been noted
   during the past several hours, moving north-northwestward off the
   Gulf to between GPT-MOB and over portions of southeastern MS.  A few
   cells therein exhibited horizontal cyclonic shear despite 1) the
   lack of substantial apparent surface boundaries, and 2) the modest
   to weak SRH apparent in 06Z RAOBs and VAD wind profiles for observed
   cell motions.  Ambient hodographs have increased slightly since,
   based on the 12Z LIX RAOB, and as they continue to grow today, any
   such convection remaining in the area later this morning, or forming
   over the Gulf in a weak-CINH setting as broader low-level
   convergence increases ahead of Zeta, may pose a marginal tornado
   threat before Zeta's directly related outer bands begin to affect
   the coastal areas this afternoon.

   A favorably high-theta-e air mass, with low/mid-70s F surface dew
   points, has advected inland across parts of the central Gulf Coast
   and lowest Delta regions, gradually diffusing an old frontal zone 
   that had been in place for a few days, and effectively displacing it
   inland in the direction of the arctic front described above.  That
   process will remain incomplete, however, and both surface theta-e
   and SBCAPE will continue to diminish northward through AL/GA from
   the optimal CAPE/shear parameter space forecast to affect the
   5%/"Slight" area.  With its expected acceleration and fast forward
   speed, Zeta should outrun the greatest buoyancy inland tonight.  As
   that occurs, the tornado threat will ramp down gradually, but still
   be nonzero well inland, across the "Marginal" area, where a tornado
   cannot be ruled out in what still will be a large-hodograph
   environment.

   ..Edwards/Mosier.. 10/28/2020

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