SPC AC 131256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0656 AM CST Sat Feb 13 2021

   Valid 131300Z - 141200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
   CENTRAL/SOUTHWESTERN FLORIDA...

   ...SUMMARY...
   Strong to marginally severe thunderstorms are possible today through
   tonight across parts of central and southwestern Florida.

   ...Synopsis...
   In mid/upper levels, a high-latitude blocking pattern will start to 
   break down as 1) a high now over northern Yukon redevelops
   southeastward across the Northwest Territories and 2) a long-lived
   cyclone over southern AB moves slowly eastward to northwestern ON. 
   As this occurs, a speed max and associated shortwave trough --
   initially near the CA/OR coastline -- will dig southeastward in a
   strongly difluent upper pattern across the Great Basin today.  This
   perturbation will expand into a synoptic-scale trough by the end of
   the period, from the Four Corners region southward across
   northwestern MX.  

   In turn, the mid/upper trough now over southern NM and northwestern
   MX -- with embedded 500-mb low apparent over Chihuahua -- should
   pivot across northeastern MX and south TX through the period.  By
   12Z, this trough should reach the middle TX Coast and western Gulf,
   while weakening rapidly amidst a broadening cyclonic-flow field
   related to the expansive upstream troughing.  Downstream, a more-
   subtle shortwave trough -- now evident in moisture-channel imagery
   over the western Gulf -- will eject east-northeastward over the
   central/northeastern Gulf, weakening substantially by the time ot
   reaches FL around the end of the period. 

   At the surface, a quasistationary to northward-drifting frontal zone
   was drawn across north-central FL just south of a DAB-LEE-CGC line,
   and is being overtaken from west-east by a convective complex moving
   off the Gulf.  A frontal-wave low was drawn behind the convection
   near buoy 42039, with slow-moving but strong cold front
   southwestward across the north-central/west-central Gulf.  This low
   should weaken over time, with a new one forming tonight over the
   central Gulf along the front, and ahead of the ejecting
   southern-stream shortwave trough.

   ...FL...
   A loosely organized band of thunderstorms over coastal areas of
   west-central FL -- and adjacent Gulf waters -- will proceed
   east-northeastward across the peninsula through the rest of the
   morning, offering the threat for isolated strong-severe gusts. A
   brief tornado cannot be ruled out near and just south of the frontal
   zone, where low-level SRH/vorticity will be relatively maximized
   based on the modified TBW wind/thermal RAOB. See SPC mesoscale
   discussion 73 for near-term details.  

   This activity has been very poorly progged by most 00Z convection-
   allowing guidance including HREF members, as well as HRRR runs from
   then through 10Z, when finally forced by reflectivity input to
   depict (underdone) convection initially.  The atmosphere clearly is
   not obedient to the models.  Observationally, based on satellite and
   radar trends, associated cloud cover and precip should impede
   diurnal destabilization over northern and parts of central FL,
   effectively shunting the baroclinic zone southward on the mesoscale
   and casting increasing doubt on severe potential with northern
   extent.  

   An additional round or two of strong/isolated severe convection,
   with potential for embedded supercells/bow structures, is possible
   this afternoon and/or tonight over parts of central/southern FL.
   Some of this activity will interact with the boundary reinforced/
   relocated southward by morning convection, while also accessing
   less-disturbed warm-sector inflow parcels.  In the warm sector over
   west-central/southwestern FL late tonight, forecast soundings
   indicate that rich boundary-layer moisture will offset slightly
   warming midlevel temperatures (around 500 mb) enough to yield
   500-1000 J/kg MLCAPE, though low-level shear/hodographs will
   diminish with southward extent.  In association with such
   convection, a relatively concentrated corridor of potential may
   develop within the broader marginal area; however, mesoscale
   uncertainties preclude assigning greater unconditional probabilities
   at this time.

   ..Edwards/Smith.. 02/13/2021

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