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Dec 21, 2013 1630 UTC Day 1 Convective Outlook

December 21, 2013

Dec 21, 2013 1630 UTC Day 1 Convective Outlook

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DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1024 AM CST SAT DEC 21 2013

VALID 211630Z – 221200Z

…THERE IS A MDT RISK OF SVR TSTMS ACROSS PARTS OF LA…MS…AR…AL…TN…AND KY…

…THERE IS A SLGT RISK OF SVR TSTMS FROM THE OH VALLEY TO GULF COAST…

…SYNOPSIS…

AN AMPLIFIED…MULTI-STREAM FLOW REGIME WILL PERSIST THROUGH THE D1 PERIOD WITH THE PRIMARY FEATURE OF INTEREST BEING A SRN-STREAM SHORT-WAVE TROUGH AND ASSOCIATED 90-100 KT MIDLEVEL JET STREAK WHICH WILL EJECT NEWD FROM THE SRN HIGH PLAINS INTO THE MID MS VALLEY BY 22/12Z. THIS SYSTEM WILL ASSUME A NEGATIVE TILT WITH A CORRIDOR OF 500-MB HEIGHT FALLS EXCEEDING 50-100 M/12 HR SPREADING THROUGH THE SRN PLAINS INTO OZARKS AND MID/LOWER MS VALLEYS.

IN THE LOW LEVELS…A WAVY COLD FRONT WAS ANALYZED FROM THE LOWER GREAT LAKES SWWD THROUGH ERN AR INTO SERN TX AS OF MID MORNING. MODEL GUIDANCE CONTINUES TO INDICATE THE DEVELOPMENT OF A FRONTAL WAVE –FOSTERED BY THE SHORT-WAVE TROUGH– OVER NERN TX LATE THIS MORNING WITH THIS FEATURE TRACKING NEWD INTO THE OH VALLEY TONIGHT. AHEAD OF THIS LOW…EXPECT THE SURFACE FRONT TO STALL. WITH IT/S PASSAGE…THE FRONT SHOULD BEGIN TO ACCELERATE SEWD THROUGH THE LOWER MS VALLEY.

–A SEVERE WEATHER OUTBREAK IS POSSIBLE TODAY WITH THE POTENTIAL FOR WIDESPREAD DAMAGING WINDS AND SEVERAL TORNADOES…SOME OF WHICH COULD BE SIGNIFICANT–

…OH VALLEY TO GULF COAST…

12Z OBSERVED SOUNDINGS SHOW THE PRESENCE OF AN UNSEASONABLY MOIST AIR MASS WITHIN THE PRE-FRONTAL WARM SECTOR WHERE BOUNDARY LAYER DEWPOINTS WERE IN THE 60S WITH LOWEST 100-MB MEAN-MIXING RATIOS OF 12.5-14.5 G/KG. A RESIDUAL EML OVER TX –REF. 12Z CRP/BRO SOUNDINGS– MAY BOOST INSTABILITY TODAY EWD INTO THE LOWER MS VALLEY. BUT IN GENERAL…WEAK LAPSE RATES AND WIDESPREAD CLOUDS WILL LIMIT MLCAPE VALUES TO LESS THAN 1000 J/KG.

GRADUAL BOUNDARY LAYER WARMING AND CONTINUED MOISTENING ACROSS THE WARM SECTOR WILL COMBINE WITH INCREASING HEIGHT FALLS ASSOCIATED WITH THE SHORT-WAVE TROUGH TO SUPPORT AN INCREASE IN STORM COVERAGE AND INTENSITY THIS MORNING ALONG AND AHEAD OF THE SYNOPTIC COLD FRONT AND EVOLVING SURFACE LOW. GIVEN THE STRONG LOW-LEVEL AND DEEP-LAYER SHEAR PRESENT…ACTIVITY WILL EVOLVE INTO A MIXTURE OF SUPERCELLS AND SHORT-SEGMENT BOWING LEWP STRUCTURES BEFORE TRANSITIONING TO A CONTINUOUS LINEAR MCS AFTER 22/00Z.

THE GREATEST THREAT FOR TORNADOES –SOME SIGNIFICANT– IS EXPECTED TO EVOLVE THIS AFTERNOON FROM SRN/ERN PARTS OF AR SWD INTO LA AND EWD INTO WRN TN AND MS WHERE THE ISALLOBARIC RESPONSE TO THE MIGRATORY SURFACE LOW WILL ENHANCE LOW-LEVEL CONVERGENCE AND VERTICAL SHEAR. HERE…FORECAST HODOGRAPHS SHOW 50-70 KT OF EFFECTIVE BULK SHEAR WITH EFFECTIVE SRH VALUES OF 300-600 M2/S2. OTHERWISE…EXPECT DAMAGING WIND POTENTIAL TO INCREASE THROUGH THE PEAK OF THE DIURNAL HEATING CYCLE –AND WITH THE CONSOLIDATION OF STORMS INTO A QLCS– FROM THE LOWER OH VALLEY SWD INTO THE LOWER MS VALLEY. THIS DAMAGING WIND THREAT –ALONG WITH A RISK FOR A FEW TORNADOES– WILL PERSIST OVERNIGHT…SPREADING EWD THROUGH THE OH AND TN VALLEYS…AS WELL AS THE CNTRL GULF COAST STATES.

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