SPC AC 181957

   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0257 PM CDT SUN AUG 18 2013

   VALID 182000Z - 191200Z

   ...THERE IS A SLGT RISK OF SVR TSTMS INTO THIS EVENING FOR PARTS OF
   THE NRN PLAINS...

   ...NRN PLAINS THIS AFTERNOON/EVENING...
   WATER VAPOR IMAGERY SHOWED A PROGRESSIVE SHORTWAVE TROUGH MOVING
   THROUGH ERN MT/FAR WRN ND EARLY THIS AFTERNOON.  HEIGHT FALLS WITH
   THIS FEATURE...ALTHOUGH WEAK...WILL SPREAD ACROSS CENTRAL/ERN
   ND...NERN SD INTO MN THROUGH THIS EVENING.  OBJECTIVE ANALYSES
   INDICATED THE WARM SECTOR HAS BECOME MODERATELY UNSTABLE /MLCAPE
   1500-2500 J PER KG/...ALONG AND E-S OF A FRONT WHICH EXTENDED FROM
   NERN ND THROUGH SOUTH CENTRAL ND TO NWRN SD.  CLOUDS FROM EARLIER
   CONVECTION HAVE CONTINUED TO TEMPER SURFACE HEATING AND HAVE
   PRECLUDED A GREATER REDUCTION IN INHIBITION.  IT/S LIKELY THAT
   ASCENT ATTENDANT TO THE SHORTWAVE TROUGH WILL BE NEEDED FOR STORMS
   TO FORM ALONG THE FRONT LATE THIS AFTERNOON/EVENING.  ONCE THAT
   OCCURS...THEN THE AVAILABLE CAPE/SHEAR PARAMETER SPACE WILL SUPPORT
   ORGANIZED STRONG TO SEVERE STORMS. THUS...NO CHANGES WILL BE MADE TO
   THE SLIGHT RISK AND SEE TEXT AREA IN THE NRN PLAINS. 

   ...ERN AL TO WRN/CENTRAL GA...
   POTENTIAL EXISTS FOR STORMS TO PRODUCE AN ISOLATED THREAT FOR STRONG
   WIND GUSTS INTO ERN AL AND WRN/CENTRAL GA.  THE LOW SEVERE WIND
   PROBABILITIES HAVE BEEN EXPANDED TO INCLUDE MORE OF ERN AL AND
   WRN/CENTRAL GA.  FOR ADDITIONAL MESOSCALE DETAILS...REFER TO SPC
   MESOSCALE DISCUSSION 1736.

   ..PETERS.. 08/18/2013

   .PREV DISCUSSION... /ISSUED 1126 AM CDT SUN AUG 18 2013/

   ...NRN PLAINS THIS AFTERNOON/EVENING...
   MORNING WATER VAPOR IMAGERY AND UPPER AIR ANALYSES REVEAL A SERIES
   OF MIDLEVEL SHORTWAVE TROUGHS MOVING EWD OVER THE NRN PLAINS.  THE
   PRIMARY WAVE IS NOW LOCATED OVER CENTRAL MT AND WILL CROSS ND BY
   THIS EVENING...WHILE SMALLER-SCALE WAVES ARE MOVING EWD FROM NE WY
   AND OVER SRN MANITOBA.  EACH WAVE IS ASSOCIATED WITH ONGOING
   CONVECTION...THE STRONGEST OF WHICH IS WITH THE MANITOBA TROUGH AND
   COULD BRUSH NW MN THIS AFTERNOON.  OTHERWISE...THE WEAK ELEVATED
   CONVECTION MOVING OVER NW SD AND NE MT WILL LIKELY PERSIST FOR
   SEVERAL MORE HOURS WHILE SPREADING EWD OVER NW ND...AND ALONG THE
   WRN/CENTRAL BORDER OF THE DAKOTAS.

   CLOUDS WITH THE ELEVATED CONVECTION WILL SLOW SURFACE HEATING IN
   WHAT IS EXPECTED TO BE THE MOST UNSTABLE PORTION OF THE WARM SECTOR
   FROM WRN SD INTO SRN ND.  12Z SOUNDINGS FROM THIS AREA ALSO REVEALED
   A WARM ELEVATED MIXED LAYER WITH A STRONG CAP...SUCH THAT AFTERNOON
   SURFACE TEMPERATURES WILL NEED TO REACH THE MID 90S TO LARGELY
   REMOVE CONVECTIVE INHIBITION.  THUS...THERE IS SOME UNCERTAINTY
   REGARDING STORM COVERAGE THIS AFTERNOON ALONG THE NE-SW ORIENTED
   SURFACE TROUGH FROM NW SD INTO SE ND.  ASSUMING SURFACE HEATING AND
   LARGER-SCALE ASCENT ARE SUFFICIENT FOR CONVECTIVE
   INITIATION...INVERTED-V PROFILES WITH MODEST UNIDIRECTIONAL WLY
   SHEAR WILL SUPPORT MULTICELL CLUSTERS CAPABLE OF PRODUCING DAMAGING
   GUSTS.  SOMEWHAT STRONGER BUOYANCY AND VERTICAL SHEAR ARE EXPECTED
   INTO NW SD /IN THE WAKE OF THE MORNING CONVECTION/...SUCH THAT
   PROFILES MAY SUPPORT A SUPERCELL OR TWO...WITH AN ATTENDANT RISK FOR
   LARGE HAIL GIVEN STEEP MIDLEVEL LAPSE RATES.  THE DAMAGING
   WIND/LARGE HAIL THREAT WILL BEGIN TO DIMINISH BY LATE EVENING AS THE
   BOUNDARY LAYER COOLS AND CONVECTIVE INHIBITION STRENGTHENS.

   OTHERWISE...A FEW HIGH-BASED STORMS WILL BE POSSIBLE FARTHER S ALONG
   THE SURFACE LEE TROUGH THIS AFTERNOON FROM THE NEB PANHANDLE INTO NE
   CO.  ISOLATED STRONG/DAMAGING OUTFLOW GUSTS AND MARGINALLY SEVERE
   HAIL WILL BE POSSIBLE FOR A FEW HOURS LATE THIS AFTERNOON/EVENING.

   ...N CENTRAL/NE GULF COAST TODAY...
   A WEAK SURFACE LOW...ASSOCIATED WITH THE MIDLEVEL TROUGH OVER THE MS
   VALLEY AND A SURFACE BAROCLINIC ZONE FROM THE SW TO N CENTRAL GULF
   OF MEXICO...WILL DRIFT NWD FROM SE LA TO SE MS.  A FEW ROTATING
   STORMS HAVE BEEN OBSERVED NEAR THE COAST AND OFFSHORE THROUGH THE
   MORNING...THOUGH SUPERCELL STRUCTURES HAVE TENDED TO WEAKEN IN THE
   PAST COUPLE OF HOURS.  LIKEWISE...VWP/S FROM MOB AND EVX BOTH SHOW
   GRADUAL WEAKENING OF THE LOW-LEVEL SHEAR WITH TIME...AND THIS TREND
   WILL CONTINUE AS THE UNSTABLE WARM SECTOR MAKES ONLY SLOW INLAND
   PROGRESS.  OVERALL...THE RISK FOR A BRIEF TORNADO MAY HAVE ALREADY
   PEAKED THIS MORNING...WHILE ISOLATED MARGINALLY DAMAGING GUSTS WILL
   STILL BE POSSIBLE THIS AFTERNOON ACROSS SRN AL AND THE FL
   PANHANDLE/SW GA.

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