Apr 27, 2011 1300 UTC Day 1 Convective Outlook
Updated: Wed Apr 27 13:00:10 UTC 2011
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...outbreak of severe thunderstorms and tornadoes expected over parts of the tennessee valley and southeast us this afternoon and tonight....   Please read the latest public statement about this event.

Categorical Graphic
20110427 1300 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20110427 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
20110427 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
20110427 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 271255
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0755 AM CDT WED APR 27 2011
   
   VALID 271300Z - 281200Z
   
   ...THERE IS A HIGH RISK OF SVR TSTMS TODAY INTO EARLY TONIGHT FOR NE
   MS...CENTRAL/NRN AL...NW GA...SE TN...
   
   ...THERE IS A MDT RISK OF SVR TSTMS SURROUNDING THE HIGH RISK...FROM
   NRN/CENTRAL MS TO SRN AL...NW GA...MIDDLE/ERN TN...WRN SC...WRN
   NC...WRN VA...AND ERN KY...
   
   ...THERE IS A SLGT RISK OF SVR TSTMS FROM THE N CENTRAL GULF COAST
   TO THE OH VALLEY/APPALACHIANS AND NY...
   
   --DANGEROUS TORNADO OUTBREAK IS EXPECTED FROM LATE MORNING INTO LATE
   EVENING ACROSS THE TN VALLEY REGION--
   
   ...SYNOPSIS...
   AN INTENSE MIDLEVEL SHORTWAVE TROUGH AND ASSOCIATED 100 KT 500 MB
   JET WILL PIVOT EWD FROM TX/OK THIS MORNING TO LOWER MS/TN VALLEY
   REGION BY THIS EVENING...AND CONTINUE NEWD TO THE CENTRAL
   APPALACHIANS OVERNIGHT.  THE PRIMARY SURFACE CYCLONE WILL DEVELOP
   NEWD FROM WRN AR THIS MORNING TO WRN KY/SRN INDIANA BY THIS
   EVENING...AND THEN CONTINUE NEWD TO THE LOWER GREAT LAKES OVERNIGHT.
   A TRAILING COLD FRONT AND REMNANT DRYLINE FEATURE WILL SWEEP EWD
   ACROSS THE ARKLAMISS THIS AFTERNOON...PROVIDING A FOCUS FOR SEVERE
   STORM DEVELOPMENT.  GIVEN THE PRESENCE OF VERY STRONG WINDS
   THROUGHOUT THE TROPOSPHERE AND AN INFLUX OF RICH MOISTURE FROM THE
   GULF OF MEXICO...THE ENVIRONMENT WILL BE FAVORABLE FOR A DANGEROUS
   OUTBREAK OF FAST-MOVING TORNADIC SUPERCELLS AND EMBEDDED BOW ECHOES
   TODAY INTO EARLY TONIGHT.
   
   ...MS/AL/TN/KY/GA AREA THROUGH TONIGHT...
   THE REMNANTS OF AN INTENSE OVERNIGHT MCS /INCLUDING EMBEDDED
   SUPERCELLS AND BOWS/ IS WEAKENING OVER NE AL AND MIDDLE TN AS OF
   SUNRISE.  OTHER ELEVATED THUNDERSTORM DEVELOPMENT IS UNDERWAY TO THE
   W FROM NE LA INTO SE AR/NRN MS.  SOME FORM OF THIS ELEVATED
   CONVECTION WILL LIKELY PERSIST THROUGH THE DAY WHILE SPREADING ENEWD
   OVER NRN MS INTO WRN/MIDDLE TN.  S OF THIS ACTIVITY AND IN THE WAKE
   OF THE EARLIER MS/AL MCS...IT APPEARS THAT THE WARM SECTOR WILL
   RECOVER QUICKLY DURING THE MORNING/EARLY AFTERNOON IN RESPONSE TO
   CLOUD BREAKS AND STRONG LOW-LEVEL SLY FLOW.  A RESERVOIR OF 70-72 F
   BOUNDARY LAYER DEWPOINTS FROM SRN LA TO SRN AL WILL SPREAD NWD IN
   THE WAKE OF THE MORNING STORMS...BENEATH THE REMNANTS OF STEEP
   MIDLEVEL LAPSE RATE PLUME SPREADING EWD FROM TX/LA.  SURFACE HEATING
   WITHIN THE MOIST WARM SECTOR WILL BOOST MLCAPE VALUES TO 2500-4000
   J/KG ALONG AND S OF THE REMNANT OUTFLOW BOUNDARY...AND REDUCE
   CONVECTIVE INHIBITION BY ABOUT MIDDAY.  THIS WILL ALLOW THE
   DEVELOPMENT OF SCATTERED-NUMEROUS WARM SECTOR SUPERCELLS ALONG
   CONFLUENCE BANDS E OF THE COLD FRONT/DRYLINE BY EARLY AFTERNOON.
   
   THE VERTICAL SHEAR ENVIRONMENT WILL BECOME VERY FAVORABLE FOR
   TORNADIC SUPERCELLS...CHARACTERIZED BY LONG/CURVED HODOGRAPHS WITH
   EFFECTIVE BULK SHEAR IN EXCESS OF 70 KT AND EFFECTIVE SRH OF 300-600
   M2/S2 IN THE UNSTABLE WARM SECTOR.  GIVEN THE EXPECTATION OF SEVERAL
   HOURS OF DISCRETE SUPERCELL DEVELOPMENT IN THE WAKE OF THE MORNING
   STORMS...A VERY MOIST BOUNDARY LAYER...THE STRONG INSTABILITY...AND
   INTENSE VERTICAL SHEAR...THE SCENARIO APPEARS FAVORABLE FOR MULTIPLE
   LONG-TRACK SUPERCELLS WITH THE POTENTIAL TO PRODUCE INTENSE/DAMAGING
   TORNADOES IN THE HIGH RISK AREA.
   
   ...AR NEWD TO THE LOWER OH VALLEY TODAY...
   ALONG THE PATH OF THE MIDLEVEL TROUGH AND SURFACE CYCLONE...THERE
   WILL BE A FOCUS FOR ADDITIONAL SEVERE THUNDERSTORM DEVELOPMENT LATER
   TODAY.  THE DEGREE OF INSTABILITY IS IN QUESTION GIVEN THE IMPACTS
   OF OVERNIGHT AND ONGOING CONVECTION FARTHER TO THE SE.  STILL...ANY
   CLOUD BREAKS WILL ALLOW DESTABILIZATION BENEATH COOL MIDLEVEL
   TEMPERATURES...IN AN ENVIRONMENT OF STRONG DEEP-LAYER SPEED SHEAR. 
   DAMAGING WINDS/HAIL AND A COUPLE OF TORNADOES WILL BE POSSIBLE WITH
   ANY CLUSTERS IMMEDIATELY IN ADVANCE OF THE SURFACE LOW.
   
   ...UPPER OH VALLEY TONIGHT...
   THE PRIMARY SURFACE CYCLONE AND STRONG KINEMATIC PROFILES ARE
   EXPECTED TO REACH THE UPPER OH VALLEY EARLY TONIGHT.  THE MAGNITUDE
   OF WARM SECTOR INSTABILITY IS IN QUESTION GIVEN WIDESPREAD
   CONVECTION ONGOING AND EXPECTED TO THE SOUTH.  HOWEVER...VERY STRONG
   LOW-LEVEL SHEAR IN A MOIST ENVIRONMENT COULD SUPPORT A FEW TORNADOES
   WITH EMBEDDED BOWS AND SUPERCELL STRUCTURES.
   
   ..THOMPSON/COHEN.. 04/27/2011
   
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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z