May 21, 2013 2000 UTC Day 1 Convective Outlook
Updated: Tue May 21 20:01:43 UTC 2013
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...severe thunderstorms expected over parts of the southern plains and arklatex this afternoon and tonight....   Please read the latest public statement about this event.  View What is a Watch? clip.

Categorical Graphic
20130521 2000 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20130521 2000 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
20130521 2000 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
20130521 2000 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 77,615 10,122,086 Dallas, TX...Ft. Worth, TX...Arlington, TX...Plano, TX...Garland, TX...
SLIGHT 394,675 59,307,405 San Antonio, TX...Austin, TX...Columbus, OH...Detroit, MI...Memphis, TN...
   SPC AC 211958
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0258 PM CDT TUE MAY 21 2013
   
   VALID 212000Z - 221200Z
   
   ...THERE IS A MDT RISK OF SVR TSTMS OVER NORTH CENTRAL AND NERN
   TX...SERN OK...SRN AR...AND NRN LA...
   
   ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SRN PLAINS ACROSS THE
   TN VALLEY...LOWER GREAT LAKES...AND THE NERN STATES...
   
   ...SRN PLAINS INTO THE TN VALLEY...
   A SERIES OF LINEAR CONVECTIVE SYSTEMS ARE MOVING EWD FROM SERN
   OK/NORTH CENTRAL TX INTO AR AND KY/TN...IN ADVANCE OF AN UPPER
   VORTICITY MAX/SHORT WAVE TROUGH LOCATED OVER THE TX PANHANDLE. 
   STRONG DIABATIC HEATING HAS OCCURRED SOUTH OF A CONVECTIVE BOUNDARY
   ARCING FROM EAST CENTRAL AR ACROSS SRN AR INTO NORTH CENTRAL
   TX...AND WITH STEEP LAPSE RATES /REF 18Z FWD AND 19Z SHV SPECIAL
   RAOBS/ IN PLACE COUPLED WITH SURFACE DEW POINTS AROUND 70F...THE
   ENVIRONMENT HAS BECOME VERY UNSTABLE WITH SBCAPE NEAR 3500 J/KG. 
   VISIBLE IMAGERY SHOWS CLOUD STREETS OVER ERN TX AND LA FEEDING NWD
   TOWARD THE THERMAL BOUNDARY INDICATING A FAVORABLE MOISTURE
   TRANSPORT INTO THE AREA.  LOW-LEVEL CONVERGENCE IS FOCUSED IN THREE
   AREAS:  1) NEAR A SURFACE LOW OVER THE METROPLEX...2) ALONG THE COLD
   FRONT EXTENDING SWWD INTO THE HILL COUNTRY...AND 3) ALONG THE
   EAST/WEST CONVECTIVE BOUNDARY ACROSS NERN TX/SWRN AR.  STORMS ARE
   EXPECTED TO INTENSIFY THE REMAINDER OF THIS AFTERNOON INTO THE
   EVENING AS THE UPPER SYSTEM AND SURFACE LOW MOVE EWD...WITH
   POTENTIAL FOR SUPERCELLS AND BOWING LINE SEGMENTS.  12Z NSSL WRF-ARW
   AND RECENT HRRR MODELS INDICATE DEVELOPMENT OF LONG-LIVED BOWING
   QLCS/S MOVING EWD ACROSS NERN TX INTO SRN AR AND NRN LA...AND
   ENVIRONMENT APPEARS SUPPORTIVE OF THIS EVENT. THUS...WILL MAINTAIN
   POTENTIAL FOR VERY LARGE HAIL AND SEVERAL TORNADOES /INCLUDING ONE
   OR TWO STRONG TORNADOES/ ASSOCIATED PRIMARILY WITH ANY SUPERCELLS
   THAT FORM...ALONG WITH  WIDESPREAD SIGNIFICANT WIND DAMAGE GIVEN THE
   POTENTIAL FOR A PROGRESSIVE BOWING CONVECTIVE SYSTEM TO FORM WITHIN
   THE NEXT 1-2 HRS.
   
   ELSEWHERE FROM THE MID-SOUTH INTO THE TN VALLEY...ADDITIONAL LINEAR
   CONVECTIVE SYSTEMS OVER ERN AR AND SRN KY/MIDDLE TN ARE LIKELY TO
   PROGRESS EWD/NEWD OVER THE NEXT SEVERAL HOURS...WHILE NEW
   STRONG/SEVERE STORMS HAVE DEVELOPED ALONG THE TRAILING OUTFLOW
   BOUNDARY OVER FAR NRN MS.  GIVEN THE DEGREE OF INSTABILITY WITH CAPE
   OF 2000-3000 J/KG AND DEEP LAYER SHEAR OF 30-40 KT...A THREAT FOR
   DAMAGING WINDS AND HAIL WILL CONTINUE WITH THESE STORMS.
   
   ..WEISS.. 05/21/2013
   
   .PREV DISCUSSION... /ISSUED 1128 AM CDT TUE MAY 21 2013/
   
   ...SUBSTANTIAL SEVERE WEATHER EXPECTED ACROSS N-CNTRL TX TO THE
   ARKLATEX THIS AFTERNOON AND EVENING...
   
   ..SRN PLAINS TO LOWER MS VALLEY...
   SIGNIFICANT SEVERE WEATHER EPISODE INITIALLY UNFOLDING ACROSS THE
   RED RIVER WITH SUPERCELLS PRODUCING LARGE HAIL ALONG AND N OF THE
   COMPOSITE OUTFLOW/COLD FRONT PUSHING SWD FROM THE BIG COUNTRY TO FAR
   SERN OK AHEAD OF A WELL-DEFINED SHORTWAVE TROUGH OVER THE SRN HIGH
   PLAINS. AIR MASS S OF THE BOUNDARY IS BECOMING STRONGLY UNSTABLE AS
   GRADUAL SURFACE HEATING CONTINUES AMIDST LOWEST 100 MB MEAN MIXING
   RATIOS OF 14-17 G/KG AND 700-500 MB LAPSE RATES OF 8 TO 9.5 DEG C/KM
   SAMPLED IN ALL REGIONAL 12Z RAOBS. AS MLCIN IS MINIMIZED ALONG THE
   DRYLINE MIXING EWD ACROSS THE EDWARDS PLATEAU...ADDITIONAL SUPERCELL
   DEVELOPMENT IS ANTICIPATED WITH SWD EXTENT.
   
   A BELT OF 50-60 KT MID-LEVEL SWLYS ATTENDANT TO THE COMPACT
   SHORTWAVE TROUGH WILL PROMOTE RISKS FOR ALL SEVERE TYPES /SOME OF
   WHICH WILL BE SIGNIFICANT/. MORNING CONVECTION-ALLOWING WRF AND HRRR
   SIMULATIONS ARE INSISTENT THAT UPSCALE GROWTH WILL OCCUR THROUGHOUT
   THE AFTERNOON. GIVEN THE DEGREE OF INSTABILITY...BUOYANCY AND
   SHEAR...SETUP MAY YIELD A DERECHO ACCELERATING EWD TOWARDS THE
   ARKLATEX REGION BY EARLY EVENING. HERE TOO...THE STRONGEST LOW-LEVEL
   S/SWLYS WILL REMAIN...PROMOTING EMBEDDED MESOVORTICES CAPABLE OF
   PRODUCING A COUPLE OF SIGNIFICANT TORNADOES. OTHERWISE...POTENTIAL
   DOES EXIST FOR WIDESPREAD/DESTRUCTIVE DAMAGING WIND GUSTS.
   
   ...TN VALLEY TO THE LOWER GREAT LAKES/NORTHEAST...
   A BROAD AREA OF ORGANIZED SEVERE POTENTIAL IS EXPECTED ALONG THE ERN
   EXTENT OF THE ELEVATED MIXED LAYER PLUME SAMPLED IN 12Z RAOBS. WITH
   ROBUST HEATING...MODERATE TO STRONG INSTABILITY IS ANTICIPATED...BUT
   MOST OF THE REGION WILL REMAIN ON THE FRINGE OF SUFFICIENT
   DEEP-LAYER SHEAR FOR SUPERCELLS. THREE ZONES OF FOCUS FOR SUSTAINED
   CONVECTION ARE APPARENT. ONE WILL BE DOWNSTREAM OF AN ONGOING
   CLUSTER ACROSS WRN TN. THE OTHERS SHOULD BE ALONG A QUASI-STATIONARY
   FRONT BISECTING NY INTO SRN NEW ENGLAND AND PERHAPS IN ASSOCIATION
   WITH A WEAK MCV MOVING INTO LOWER MI. MULTICELL CLUSTERS AND A FEW
   SUPERCELLS WILL BE PRIMARILY CAPABLE OF ISOLATED DAMAGING WINDS AND
   SEVERE HAIL.
   
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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z