Replies to post #138043 on Tornado Alley (PROG)

[F6]

Apr 27, 2011 1300 UTC Day 1 Convective Outlook

[to repeat: F6 comment -- this is a scary setup; things could get really ugly -- today's a good day to try and make sure that anyone, any family and friends, in or near the High or Moderate Risk will not procrastinate and will instead promptly seek shelter if and as any threatening weather approaches from midday today through tonight]




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.


Probability of damaging thunderstorm winds or wind gusts of 50 knots or higher within 25 miles of a point.
Hatched Area: 10% or greater probability of wind gusts 65 knots or greater within 25 miles of a point.


Probability of one inch diameter hail or larger within 25 miles of a point.
Hatched Area: 10% or greater probability of two inch diameter hail or larger within 25 miles of a point.

Forecast Discussion

    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 
     
    CLICK TO GET WUUS01 PTSDY1 PRODUCT 
  
    NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z

http://www.spc.noaa.gov/products/outlook/day1otlk.html