SPC AC 211257

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0657 AM CST MON DEC 21 2015

   VALID 211300Z - 221200Z

   ...THERE IS A MRGL RISK OF SVR TSTMS PORTIONS SRN LA TO WRN FL
   PANHANDLE...

   ...SUMMARY...
   ISOLATED STRONG TO SEVERE THUNDERSTORMS ARE POSSIBLE ACROSS PARTS OF
   THE CENTRAL GULF COAST REGION.

   ...SYNOPSIS...
   IN MID/UPPER LEVELS...BROAD AREA OF CYCLONIC FLOW WILL COVER MOST OF
   CONUS FOR MOST OF PERIOD.  ONE EMBEDDED SHORTWAVE TROUGH --
   CURRENTLY LOCATED OVER NRN MO WITH ELEVATED TSTMS TO ITS E OVER NERN
   MO/IL -- IS FCST TO DEAMPLIFY AND EJECT NEWD.  BY 00Z THIS FEATURE
   SHOULD REACH NWRN NY..THEN CROSS MAINE IN MUCH-WEAKENED FORM AROUND
   06Z.  GEN TSTM POTENTIAL SHOULD LINGER AHEAD OF THIS FEATURE THROUGH
   MIDDAY...BEFORE IT WEAKENS AND OUTRUNS SUPPORTIVE LOW-LEVEL WAA/
   MOISTURE-TRANSPORT REGIME. 

   UPSTREAM PERTURBATION -- NOW EVIDENT IN MOISTURE-CHANNEL IMAGERY
   OVER NERN NM -- IS FCST TO PIVOT ENEWD OVER SRN/CENTRAL HIGH PLAINS
   TODAY...PHASING WITH SMALLER TROUGH NOW OVER WRN WY.  COMBINED
   TROUGH THEN SHOULD REACH FROM SERN SD TO E-CENTRAL OK BY 00Z. 
   SIMILARLY TO FOREGOING TROUGH...IT WILL THEN ACCELERATE ENEWD TO
   INDIANA BY 12Z.  

   AT SFC...FRONTAL-WAVE LOW ASSOCIATED WITH LEADING SHORTWAVE TROUGH
   WAS EVIDENT OVER ERN IA...WITH COLD FRONT SWD ACROSS SERN KS AND
   CENTRAL OK.  BY 00Z...LOW SHOULD REACH WRN LOWER MI AREA...WITH
   FRONT SWWD ACROSS SWRN IL...NWRN AR AND CENTRAL TX.  WITH LOSS OF
   SUBSTANTIAL UPPER SUPPORT...FRONT SHOULD DECELERATE AND DECAY
   TONIGHT...FROM ERN LOWER MI ACROSS AR TO S-CENTRAL TX.  MARINE/WARM
   FRONT WAS ANALYZED ACROSS NRN GULF...FROM LOW NEAR CRP NEWD ALONG
   MIDDLE TX COAST THEN EWD OVER OUTER SHELF WATERS OFF LA...SEWD TO FL
   EVERGLADES.  THIS BAROCLINIC ZONE WILL MOVE NWD GRADUALLY AND
   ERRATICALLY THROUGH PERIOD...MODULATED ON MESOSCALE BY CONVECTIVE
   PROCESSES/OUTFLOW. 

   ...CENTRAL GULF COAST REGION...
   WIDELY SCATTERED TSTMS IN CLUSTERS AND BKN BANDS ARE EXPECTED TO
   FORM THIS AFTERNOON AND EVENING OVER PARTS OF LA AND MS...OFFERING
   POTENTIAL FOR ISOLATED DAMAGING GUSTS.  SUPERCELLS ARE POSSIBLE...
   AND MRGL TORNADO RISK EXISTS AS WELL.  THIS REGIME WILL SHIFT EWD
   ACROSS LOWER DELTA AND GULF COASTAL PLAIN OVERNIGHT...REACHING
   EXTREME SRN AL AND PORTIONS FL PANHANDLE. 

   CONSIDERABLE CLOUD COVER AND NON-SVR CONVECTION IS EVIDENT ATTM OVER
   NWRN GULF...OFFSHORE TX COAST AND APCHG COASTAL SWRN LA...WITHIN
   PERSISTENT BELT OF LOW-LEVEL WAA AND WEAK CONVERGENCE ABOVE SFC. 
   CONVECTION SHOULD CONTINUE TO FORM IN THIS REGIME AND MOVE/EXPAND
   NEWD THROUGHOUT TODAY AND INTO THIS EVENING.  THIS WILL OCCUR AS
   PRECONVECTIVE AIR MASS ALONG/AHEAD OF CONVERGENCE/CONFLUENCE LINES
   GRADUALLY ENCOUNTER WEAK BUT ADEQUATE SFC-BASED EFFECTIVE INFLOW
   PARCELS...PRODUCED THROUGH COMBINATION OF WARM/MOIST ADVECTION AND
   MUTED DIABATIC HEATING.  WEAK LAPSE RATES ALOFT WILL KEEP CAPE
   DENSITY LOW...BUT SUFFICIENTLY DEEP BUOYANT PROFILES AND INCREASING
   NEAR-SFC THETAE SHOULD YIELD 250-1000 J/KG MLCAPE IN RELATIVELY
   UNDISTURBED WARM SECTOR NEAR DIFFUSE MARINE FRONT.  ENLARGED LOW-
   LEVEL HODOGRAPHS WITH 200-250 J/KG SRH AND 30-KT SHEAR-VECTOR
   MAGNITUDE THROUGH 0-3 KM LAYER ARE POSSIBLE...SUPPORTING SOME
   SUPERCELL THREAT.

   OVERNIGHT ACROSS COASTAL AL AND FL PANHANDLE...FCST
   SOUNDINGS...ALONG WITH KINEMATIC PROGS FROM SEVERAL OPERATIONAL AND
   HI-RES/CONVECTION-ALLOWING MODELS/ENSEMBLES...INDICATE SFC WINDS
   WILL VEER SOMEWHAT WITH TIME BUT STILL MAINTAIN SUBSTANTIAL SLY
   COMPONENT.  THIS...ALONG WITH PRESENCE OF SECONDARY LOW-LEVEL JETLET
   WITH PEAK 850-MB WINDS AROUND 35 KT...INDICATES SUFFICIENT LOW-LEVEL
   SHEAR TO SUPPORT SOME SUPERCELL RISK.  0-1-KM SRH 150-250 J/KG IS
   POSSIBLE WHERE NRN FRINGE OF 300-800 J/KG MLCAPE FIELD SHIFTS
   SLIGHTLY INLAND.  AS SUCH...OUTLOOK HAS BEEN EXTENDED EWD TO CENTRAL
   PORTIONS FL PANHANDLE FOR TONIGHT. 

   FARTHER W...IN TERMS OF BOTH LOW-LEVEL SHEAR AND BUOYANCY...
   BOUNDARY-LAYER ENVIRONMENT MAY APPEAR FAVORABLE FOR SVR OVER PARTS
   OF SE TX THIS AFTN FOLLOWING PASSAGE OF ELEVATED CONVECTION. 
   HOWEVER...STG CAPPING IS EVIDENT IN AVAILABLE 12Z COASTAL RAOBS...AS
   WELL AS SOUNDINGS UPSTREAM WITHIN EML.  THIS CAPPING LAYER IS FCST
   TO PERSIST THROUGH TODAY WITH ONLY MINIMAL REDUCTION FROM BELOW...
   GIVEN ABSENCE OF SUBSTANTIAL PERTURBATIONS EVIDENT IN UPSTREAM
   SUBTROPICAL-JET BRANCH...PER MOISTURE-CHANNEL IMAGERY.  AS SUCH...
   SVR RISK APPEARS TOO CONDITIONAL ATTM TO INCLUDE IN OUTLOOK W OF
   CENTRAL LA.  POTENTIAL FOR SBCINH TO BE OVERCOME BY LOW-LEVEL LIFT
   AND THETAE ADVECTION WILL INCREASE WITH EWD EXTENT IN LA.

   ..EDWARDS/COOK.. 12/21/2015

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