Dec 21, 2013 1630 UTC Day 1 Convective Outlook
Updated: Sat Dec 21 16:28:45 UTC 2013 (20131221 1630Z Day 1 shapefile | 20131221 1630Z Day 1 KML)
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...severe thunderstorms expected over parts of the lower mississippi and tennessee valleys today through tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

Categorical Graphic
20131221 1630 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20131221 1630 UTC Day 1 Tornado Probabilities Graphic
Probability of a tornado within 25 miles of a point.
Hatched Area: 10% or greater probability of EF2 - EF5 tornadoes within 25 miles of a point.
Probabilistic Damaging Wind Graphic
20131221 1630 UTC Day 1 Damaging Wind Probabilities Graphic
Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% of greater probability of wind gusts 65 knots or greater within 25 miles of a point.
Probabilistic Large Hail Graphic
20131221 1630 UTC Day 1 Large Hail Probabilities Graphic
Probability of hail 1" or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of hail 2" or larger within 25 miles of a point.
Risk Area (sq. mi.) Area Pop. Some Larger Population Centers in Risk Area
MODERATE 90,576 6,760,499 Nashville, TN...Jackson, MS...Clarksville, TN...Murfreesboro, TN...Jackson, TN...
SLIGHT 289,934 39,464,228 Houston, TX...San Antonio, TX...Indianapolis, IN...Columbus, OH...Memphis, TN...
   SPC AC 211624

   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.

   ..MEAD/GARNER.. 12/21/2013

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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z