Apr 26, 2011 1300 UTC Day 1 Convective Outlook
Updated: Tue Apr 26 12:39:04 UTC 2011
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...outbreak of severe storms and tornadoes expected over parts of the southern plains and lower mississippi valley this evening and overnight....   Please read the latest public statement about this event.

Categorical Graphic
20110426 1300 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20110426 1300 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
20110426 1300 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
20110426 1300 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.
   SPC AC 261234
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0734 AM CDT TUE APR 26 2011
   
   VALID 261300Z - 271200Z
   
   ...THERE IS A HIGH RISK OF SVR TSTMS LATE THIS AFTERNOON INTO
   TONIGHT FOR NE TX...FAR SE OK...EXTREME NW LA...AND THE SRN HALF OF
   AR...
   
   ...THERE IS A MDT RISK OF SVR TSTMS SURROUNDING THE HIGH RISK...FROM
   NE TX/SE OK TO THE CONFLUENCE OF THE MS/OH RIVERS...
   
   ...THERE IS A SLGT RISK OF SVR TSTMS IN A BROAD SWATH FROM THE SRN
   PLAINS EWD/NEWD TO THE MS/OH VALLEYS AND THE LOWER GREAT LAKES...
   
   ...DANGEROUS TORNADO AND SEVERE THUNDERSTORM OUTBREAK EXPECTED LATE
   THIS AFTERNOON INTO TONIGHT FROM NE TX ENEWD TO THE MS RIVER...
   
   ...SYNOPSIS...
   AN INITIAL SHORTWAVE TROUGH OVER MO/IL WILL EJECT NEWD TOWARD LOWER
   MI TODAY...WHILE AN INTENSE UPSTREAM JET STREAK PROGRESSES ESEWD TO
   THE SRN PLAINS BY TONIGHT.  MORNING SUBSIDENCE OVER AR /IN THE WAKE
   OF THE DEPARTING WAVE/ WILL BE REPLACED BY ASCENT BEGINNING ALONG
   THE RED RIVER VALLEY OF N TX/SRN OK LATER THIS AFTERNOON...SPREADING
   EWD THIS EVENING INTO SE OK/NE TX AND SRN AR OVERNIGHT.  A STRONG
   SURFACE CYCLONE WILL DEVELOP EWD ALONG THIS SAME CORRIDOR IN
   RESPONSE TO STRONG UPPER DIVERGENCE IN THE LEFT EXIT REGION OF THE
   90-100 KT MIDLEVEL JET.  THE CYCLOGENESIS WILL ALSO PROMOTE THE
   DEVELOPMENT OF A STRENGTHENING SLY/SSWLY LLJ FROM THE ARKLATEX TO
   THE LOWER MS VALLEY OVERNIGHT.  MEANWHILE...RICH BOUNDARY LAYER
   MOISTURE ACROSS E TX...BENEATH A PLUME OF STEEP MIDLEVEL LAPSE RATES
   ORIGINATING TO THE W...WILL PROVIDE MODERATE-STRONG WARM SECTOR
   INSTABILITY IN ADVANCE OF THE DEVELOPING CYCLONE.  THE NET RESULT
   WILL BE A STRONGLY SHEARED AND UNSTABLE WARM SECTOR LATE THIS
   AFTERNOON INTO TONIGHT...WHICH APPEARS SUPPORTIVE OF A DANGEROUS
   TORNADO AND SEVERE THUNDERSTORM OUTBREAK.
   
   ...S/SE AR INTO NW MS TODAY...
   INITIAL WARM ADVECTION STORMS WILL FORM DURING THE DAY ALONG AND N
   OF THE OUTFLOW/WARM FRONT ACROSS S/SE AR AND NW MS...AS LOW-LEVEL
   WAA INCREASES WITH A DEVELOPING LLJ.  A FEW SUPERCELLS ARE EXPECTED
   ALONG THE WARM FRONT...WHERE MODERATE INSTABILITY AND EFFECTIVE SRH
   AOA 200 M2/S2 WILL BE SUPPORTIVE OF A FEW TORNADOES...IN ADDITION TO
   LARGE HAIL AND DAMAGING WINDS.
   
   ...ARKLATEX/SE OK LATE THIS AFTERNOON INTO EARLY TONIGHT...
   OVERNIGHT CONVECTION THAT MOVED OVER AR/NW MS LEFT AN E-W OUTFLOW
   BOUNDARY THAT WILL BECOME THE EFFECTIVE WARM FRONT TODAY.  FARTHER
   S...A WEAKENING MCS OVER SRN LA/MS HAS GENERATED A COLD POOL ACROSS
   CENTRAL LA/E TX...BUT AN UNIMPEDED FEED OF LOW 70S BOUNDARY LAYER
   DEWPOINTS REMAINS TO THE W OF THIS CONVECTION ACROSS SE TX.  THE SE
   TX MOISTURE WILL SURGE NWD TODAY AS THE SRN LA STORMS
   DISSIPATE...WHILE STEEP MIDLEVEL LAPSE RATES WILL CONTINUE TO
   OVERSPREAD THE WARM SECTOR FROM THE W.  THESE PROCESSES...COMBINED
   WITH DAYTIME HEATING...WILL CONTRIBUTE TO AFTERNOON MLCAPE VALUES OF
   2500-3500 J/KG IN THE NE TX WARM SECTOR...WITH SOMEWHAT LESSER
   INSTABILITY FARTHER E ACROSS NRN LA/SRN AR...AND FARTHER N INTO SE
   OK.
   
   SUPERCELLS ARE EXPECTED TO INITIATE BY MID-LATE AFTERNOON ACROSS
   N/NE TX...ALONG THE WARM FRONT AND DRYLINE TO THE IMMEDIATE E/SE OF
   THE SURFACE LOW...AND SPREAD EWD TOWARD SW AR/NW LA THROUGH TONIGHT.
   THE STRONG INSTABILITY...EFFECTIVE BULK SHEAR IN EXCESS OF 60
   KT...EFFECTIVE SRH OF 300-500 M2/S2...AND A SUFFICIENTLY MOIST
   BOUNDARY LAYER WILL BE FAVORABLE FOR LONG-TRACK/DAMAGING TORNADOES
   WITH DISCRETE AND CLUSTER SUPERCELLS INTO THE LATE EVENING/EARLY
   NIGHTTIME HOURS.  VERY LARGE HAIL AND DAMAGING WINDS WILL ALSO BE
   LIKELY WITH THE INITIAL SUPERCELLS.
   
   ...NRN LA/SE AR/NW MS/WRN TN TONIGHT...
   AS THE STRONG SYNOPTIC CYCLONE PROGRESSES EWD AND INTERACTS WITH THE
   UNSTABLE WARM SECTOR...INTENSE CONVECTION IS LIKELY THROUGH THE
   OVERNIGHT HOURS.  INITIAL TORNADIC SUPERCELLS IN NE TX/SW AR AREA
   SHOULD EVENTUALLY GROW UPSCALE INTO ONE OR MORE INTENSE
   MCS/S...WHICH COULD EVOLVE INTO BOW ECHOES WITH THE POTENTIAL FOR
   WIDESPREAD/INTENSE DAMAGING WINDS...AS WELL AS TORNADOES.  THIS
   ACTIVITY SHOULD PROGRESS QUICKLY EWD/NEWD TO THE MS RIVER LATE
   TONIGHT AS THE SYNOPTIC WAVE BEGINS TO EJECT ENEWD. 
   
   ...OH VALLEY REGION THROUGH EARLY TONIGHT...
   THE EJECTING LEAD SHORTWAVE TROUGH WILL PROVIDE A BROAD SWATH OF
   MODERATE-STRONG SWLY MIDLEVEL FLOW...ABOVE RESIDUAL BOUNDARY LAYER
   DEWPOINTS IN THE UPPER 50S TO LOWER 60S.  THE LINGERING CLOUDS AND
   RESIDUAL OUTFLOW FROM PRIOR CONVECTION MAKE THE CONVECTIVE EVOLUTION
   SOMEWHAT UNCERTAIN TODAY ACROSS THE OH VALLEY REGION...THOUGH
   SURFACE-BASED INSTABILITY WILL BE SUFFICIENT FOR A FEW SUPERCELLS
   AND/OR BOWING SEGMENTS WITH A RISK FOR A COUPLE OF
   TORNADOES...DAMAGING WINDS...AND ISOLATED LARGE HAIL.
   
   ..THOMPSON/COHEN.. 04/26/2011
   
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