SPC AC 151256

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0656 AM CST Mon Feb 15 2021

   Valid 151300Z - 161200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
   THE FLORIDA PANHANDLE...NORTHERN FLORIDA...EXTREME SOUTHEASTERN
   ALABAMA...SOUTHERN GEORGIA...AND IMMEDIATE COASTAL EASTERN SOUTH
   CAROLINA AND SOUTHERN NORTH CAROLINA...

   ...SUMMARY...
   Severe thunderstorms are possible from the Florida Panhandle into
   southern Georgia through the evening, and over the coastal Carolinas
   overnight. Damaging gusts and a couple tornadoes will be the main
   concerns.

   ...Synopsis...
   A mid/upper-level longwave trough will persist over the central
   CONUS through the period, regulating a large belt of cyclonic flow
   now covering the Rockies and Plains States.  The most important/
   embedded perturbation influencing sensible weather will continue to
   be a synoptic trough -- now evident in moisture-channel imagery from
   south-central KS across western OK, central TX and Coahuila.  This
   feature will eject northeastward across the mid Mississippi and
   lower Ohio Valleys through the period, while deamplifying. By 12Z
   tomorrow, the southern rim of the associated 500-mb vorticity field
   should be crossing central portions of MS/AL.

   At the surface, the associated frontal-wave low was analyzed over
   the Gulf approximately east of CRP and south of LCH, with cold front
   southwestward over east-central MX.  A warm front was drawn from the
   low eastward across the northern Gulf near buoy 42365 then eastward
   over central FL, where it was quasistationary.  The low is forecast
   to move across the Mississippi River mouth around 15Z then inland
   near PNS about 18Z, when a trough will extend northward to another
   weak low in the cool air over northern AL.  The lows should combine
   and reach extreme eastern TN near TYS by 00Z, with occluded/cold
   front southward to near TLH then southwestward over the east-central
   Gulf.  The warm front should move northward over the central/eastern
   FL Panhandle, extreme southeastern AL and southern Ga before being
   overtaken by the cold front.  The northeastern limb of the warm
   front also should get very close to the SC/NC coast overnight.

   ...Coastal northeast Gulf...
   A band of scattered thunderstorms is forecast to evolve through the
   remainder of the morning and midday over the Gulf, near the surface
   low and trailing cold front, and extending poleward of the low on an
   elevated version of the frontal zone toward the MS/AL coastlines. 
   This activity should move northeastward across the Gulf Coast from
   there eastward through the day.  As it does so, encountering
   progressively more surface-based parcels near and south of the warm
   front, the associated severe threat will spread obliquely inland
   across the FL Panhandle and southeastern AL this afternoon, before
   reaching southern GA late afternoon and evening.  Potential for
   isolated, marginally severe hail will extend somewhat more inland
   than the damaging-gust and tornado threats, with suitably moist
   elevated inflow parcels located under colder air aloft and favorable
   deep shear.

   Along and south of the warm front, the buoyancy/shear parameter
   space will be favorable for supercells and bowing segments within
   the main convective band, and isolated supercells cannot be ruled
   out to its east.  Mid-60s to low-70s F surface dew points are
   expected in that regime, supporting 1000-1500 J/kg MLCAPE near the
   coast, decreasing northward to less than 500 J/kg just north of the
   warm front.  Expect strengthening mid/upper winds supporting
   50-65-kt southwesterly effective-shear vectors, oriented just
   slightly rightward of the main corridor of convective forcing.  The
   southern fringe of a 45-60-kt LLJ will contribute to mildly curved
   but long hodographs with 200-300 J/kg effective SRH -- greatest
   along the warm front.

   ...Coastal Carolinas...
   Elevated thunderstorms should spread northeastward over interior
   areas tonight, as a northward extension of the Gulf regime. A
   separate, prefrontal low-level convergence zone is forecast to
   support increasing thunderstorms tonight, initially from roughly 
   the GGE-ILM corridor southward across Atlantic waters, the northern
   portion of which should affect coastal areas from there past HSE
   through the end of the period.  A damaging-gust threat may develop
   very near the coastline, where boundary-layer instability may be at
   least marginally favorable, with tornado potential from any mature
   supercell that can cross the coast.  Farther northwestward, isolated
   strong-severe gusts may penetrate the near-surface stable layer;
   that potential will lessen with inland extent.

   The northwestern margin of the most substantial severe potential,
   relative to the coastline, may be just a few miles either way. 
   Planar guidance across this region, from a preponderance of
   operational models and ensemble (HREF, SREF, GEFS) members, now
   indicates MLCAPE developing somewhat inland, with 250-500 J/kg
   values possible, and 1000 J/kg just offshore.  Buoyancy will
   increase with proximity to the Gulf Stream, where vertical
   heat/moisture fluxes will provide the most sustained airmass
   modification.  Scrutiny of forecast soundings from some of them
   still show shallow near-surface stable layers at the bottom of the
   lifted layer yielding surface- based effective-inflow parcels. 
   Vertical shear will be favorable, with 45-55-kt effective-shear
   magnitudes and effective SRH around 200-300 J/kg.

   ..Edwards/Smith.. 02/15/2021

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