SPC AC 201254

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0654 AM CST Sun Dec 20 2020

   Valid 201300Z - 211200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ALONG PARTS OF
   THE COASTAL WESTERN FLORIDA PENINSULA...

   ...SUMMARY...
   Isolated severe thunderstorms may affect parts of the coastal
   western Florida Peninsula late this afternoon and evening.

   ...Synopsis...
   In mid/upper levels, broadly cyclonic flow will cover much of the
   central and eastern CONUS this period, with several embedded
   shortwave troughs.  Among them, a leading perturbation -- initially
   located over the southern Appalachians to western FL Panhandle, will
   weaken and eject east-northeastward away from the ongoing convective
   regime extending southwest/northeast across the Gulf.  A stronger
   shortwave trough -- apparent in moisture-channel imagery from
   central KS to west TX -- is forecast to dig southeastward through
   the period, reaching portions of the Mid-South and LA around 00Z,
   and extending from the Carolinas to northern FL around 12Z tomorrow.

   At the surface, an elongated area of low pressure was analyzed from
   near the mouth of the Mississippi River eastward across shelf waters
   south of PNS.  A warm front was drawn southeastward across the
   eastern Gulf and Keys, and a cold front southwestward to the west-
   central Gulf Coast of MX.  The warm front will move eastward
   diffusely across the northeastern Gulf and into parts of west-
   central FL this evening, while the cold front crosses the central/
   northeastern Gulf.  This front may catch up to a currently primary/
   prefrontal band of broken convection extending northeast-southwest
   across much of the southwestern/central Gulf.  The front should
   cross northwestern FL this evening and tonight, reaching east-
   central/southwestern FL by 12Z.

   ...FL Gulf Coast...
   Thunderstorms from the Gulf may cross parts of the FL coastline from
   midday through this evening.  Marginal threat for a tornado and/or
   damaging gusts is apparent for a short-lived, late-afternoon/evening
   episode over parts of coastal west-central FL.

   The north and east sides of a marginally thermodynamically favorable
   warm sector will brush across the southward-extending AAF area of
   the southeastern FL Panhandle today, and just onshore the west-
   central FL Peninsular coastline late afternoon into evening.  A
   strong storm cannot be ruled out in the small notch of warm sector
   briefly passing over the AAF area; however, flow is progged to veer
   in the warm sector as the low passes and before cold frontal
   passage, while destabilization is limited and uncertain due to
   ongoing/foregoing antecedent precip.  Potential in between the two
   areas -- around northern Apalachee Bay and northern parts of the FL
   coastal bend, still appears lower and too conditional for an outlook
   area. 

   Farther south, a window of opportunity onshore may develop near the
   warm-frontal zone, where flow veers enough to advect better-modified
   marine air inland, but before too much westerly near-surface
   component reduces lift, shear and storm-relative flow in the
   boundary layer.  Upper-60s F surface dew points will represent
   enough boundary-layer moisture to offset modest midlevel lapse
   rates, for supporting a deep buoyant profile (but with low CAPE
   density).  Time series of forecast soundings suggest potential for
   500-1000 J/kg MLCAPE along the immediate coastline area and 40-50 kt
   effective-shear magnitudes, with hodographs supporting 150-300 J/kg
   effective SRH during the early part of the most-buoyant period,
   prior to the further veering.  The main uncertainty is how much
   mature prefrontal convection can cross this environment while it
   lasts.  As convection reaches the coast farther southward past about
   the FMY area, both deep shear and low-level hodographs will be
   shrunken relative to near Tampa Bay.

   ..Edwards/Smith.. 12/20/2020

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