Apr 27, 2011 2000 UTC Day 1 Convective Outlook
Updated: Wed Apr 27 19:16:08 UTC 2011
Probabilistic to Categorical Outlook Conversion Table
Public Severe Weather Outlook

The SPC is forecasting ...outbreak of tornadoes and severe thunderstorms expected over parts of the tennessee valley...southern appalachian mountains and southern ohio valley this afternoon through tonight....   Please read the latest public statement about this event.

Categorical Graphic
20110427 2000 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20110427 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
20110427 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
20110427 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.
   SPC AC 271911
   
   DAY 1 CONVECTIVE OUTLOOK  
   NWS STORM PREDICTION CENTER NORMAN OK
   0211 PM CDT WED APR 27 2011
   
   VALID 272000Z - 281200Z
   
   ...THERE IS A HIGH RISK OF SVR TSTMS LATE THIS AFTN AND EVE ACROSS
   PARTS OF NORTHERN AND EASTERN MISSISSIPPI...CENTRAL AND NORTHERN
   ALABAMA...NORTHWEST GEORGIA AND SOUTHERN TENNESSEE...
   
   ...THERE IS A MDT RISK OF SVR TSTMS SURROUNDING THE HIGH RISK
   AREA....ACROSS MUCH OF THE REMAINDER OF MISSISSIPPI...ALABAMA AND
   NORTHERN GEORGIA...THE WESTERN
   CAROLINAS...TENNESSEE...KENTUCKY...SOUTHEASTERN
   INDIANA...OHIO...PARTS OF WESTERN PENNSYLVANIA AND WEST VIRGINIA...
   
   ...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
   AREA...ACROSS MUCH OF THE EASTERN U.S....
   
   ...20Z OUTLOOK UPDATE...
   
   ...A DANGEROUS OUTBREAK OF SEVERE THUNDERSTORM AND TORNADOES APPEARS
   UNDERWAY...AND CONFIDENCE IS INCREASING THAT THIS WILL EXPAND
   NORTHWARD FROM THE GULF STATES THROUGH MUCH OF THE OHIO VALLEY LATE
   THIS AFTERNOON AND EVENING...
   
   REMNANTS OF THE MORNING MESOSCALE CONVECTIVE SYSTEM AND ITS
   ASSOCIATED SURFACE COLD POOL...JUST EAST OF UPPER PORTIONS OF THE
   LOWER MISSISSIPPI VALLEY INTO THE TENNESSEE VALLEY...REMAIN A
   COMPLICATING FACTOR CONCERNING CONVECTIVE POTENTIAL.  HOWEVER...
   LATEST OBSERVATIONAL DATA SUGGEST THAT THE COLD POOL MAY BE IN THE
   PROCESS OF WEAKENING.  AND BOUNDARY LAYER MOISTENING ASSOCIATED WITH
   THE PRECEDING RAINFALL...COUPLED WITH LATE AFTERNOON INSOLATION AND
   MODIFYING LOW-LEVEL THERMODYNAMIC PROFILES ASSOCIATED WITH STRONG
   DYNAMIC FORCING FOR UPWARD VERTICAL MOTION...PROBABLY WILL
   CONTRIBUTE TO SUFFICIENT DESTABILIZATION TO JUSTIFY EXPANDING
   MODERATE RISK SEVERE PROBABILITIES NORTHWESTWARD AND NORTHWARD
   THROUGH MUCH OF THE REMAINDER OF THE OHIO VALLEY.
   
   MODEL GUIDANCE...INCLUDING THE LATEST RUC AND 12Z ECMWF...INDICATE
   THAT A DEEP SURFACE CYCLONE WILL DEEPEN FURTHER AS IT MIGRATES
   NORTHWARD OUT OF THE OZARK PLATEAU...ACROSS THE LOWER OHIO VALLEY
   ...INTO THE LOWER GREAT LAKES REGION LATE THIS AFTERNOON AND
   TONIGHT.  THIS WILL ACCOMPANY A VIGOROUS MID-LEVEL SHORT WAVE
   IMPULSE...WHICH IS PROGGED TO CONTRIBUTE TO STRONG MID-LEVEL HEIGHT
   FALLS CENTERED ALONG THE OHIO RIVER...FROM NEAR ITS CONFLUENCE WITH
   THE MISSISSIPPI RIVER INTO THE CINCINNATI OH AREA BETWEEN 28/00-06Z.
    A 50-70 KT 850 MB JET APPEAR LIKELY TO DEVELOP  NORTHWARD TO THE
   WEST OF THE APPALACHIANS...AS A 100 KT 500 MB JET STREAK NOSES
   NORTHEASTWARD ACROSS KENTUCKY AND TENNESSEE INTO OHIO BY LATE THIS
   EVENING...CONTRIBUTING TO STRONG VERTICAL SHEAR SUPPORTIVE OF BOTH
   LONG-LIVED SUPERCELLS AND ORGANIZED STORM CLUSTERS.  GIVEN AT LEAST
   WEAK BOUNDARY LAYER DESTABILIZATION... STRONG TORNADOES APPEAR
   POSSIBLE...IN ADDITION TO THE RISK FOR STRONG /STRAIGHT LINE/
   DAMAGING CONVECTIVE GUSTS.
   
   ..KERR.. 04/27/2011
   
   .PREV DISCUSSION... /ISSUED 1129 AM CDT WED APR 27 2011/
   
   ...MAJOR TORNADO OUTBREAK EXPECTED THIS AFTERNOON AND EVENING ACROSS
   THE TN VALLEY AND SRN APPALACHIAN MTNS EXTENDING NWD INTO THE SRN OH
   VALLEY...
   
   ...TN VALLEY/SRN APPALACHIAN MTNS AND SRN OH VALLEY...
   A VIGOROUS UPPER-LEVEL TROUGH WILL MOVE INTO THE OZARKS TODAY AS A
   POWERFUL MID-LEVEL JET ROUNDS THE BASE OF THE TROUGH AND NOSES INTO
   THE OH AND TN VALLEYS. AT THE SFC... A LOW WILL MOVE QUICKLY NEWD
   ACROSS THE MID-MS VALLEY AS LOW-LEVEL MOISTURE IS TRANSPORTED NNEWD
   ACROSS THE OH AND TN VALLEYS THIS AFTERNOON. MODERATE
   DESTABILIZATION IS ALREADY IN PLACE ACROSS THE LOWER MS VALLEY AND
   THIS AIR MASS SHOULD MOVE NEWD INTO THE MODERATE AND HIGH RISK AREAS
   BY LATE THIS AFTERNOON. THE WARM AND MOIST AIR MASS ALONG WITH
   STRONG SHEAR PROFILES IN THE LOW TO MID-LEVELS WILL BE VERY
   FAVORABLE FOR SEVERE THUNDERSTORMS AND TORNADOES THIS AFTERNOON AND
   EVENING. THIS SYSTEM IS VERY ORGANIZED AND CONFIDENCE IS HIGH
   REGARDING THE POTENTIAL FOR A MAJOR TORNADO OUTBREAK THIS AFTERNOON
   AND EVENING ACROSS THE CNTRL GULF COAST STATES AND TN VALLEY
   EXTENDING NWD INTO THE SRN OH VALLEY.
   
   CONCERNING THE CURRENT SETUP...A LARGE MCS IS ONGOING ACROSS THE TN
   VALLEY. THIS CONVECTIVE SYSTEM IS TIMED TO MOVE INTO ERN TN AND NW
   GA BY 21Z. MESOANALYSIS SHOWS A RESERVOIR OF MODERATE INSTABILITY
   FROM LA EXTENDING EWD ACROSS SCNTRL MS WHERE MLCAPE VALUES ARE
   ESTIMATED IN THE 2000 TO 3000 J/KG RANGE. THIS MODERATE INSTABILITY
   SHOULD BE TRANSPORTED NEWD INTO THE HIGH RISK AREA THIS AFTERNOON AS
   THE MCS EXITS THE REGION TO THE EAST. AS THE THERMODYNAMIC SETUP
   IMPROVES...THE WIND SHEAR ENVIRONMENT WILL ALSO RAPIDLY BECOME MORE
   FAVORABLE AS THE NOSE OF A 80 TO 100 KT MID-LEVEL JET EJECTS EWD
   ACROSS THE REGION AND INTERACTS WITH A WELL-DEVELOPED LOW-LEVEL JET.
   THIS WILL CREATE LARGE LOOPING HODOGRAPHS FROM ERN MS AND NCNTRL AL
   NWD INTO SRN TN. 0-3 KM STORM RELATIVE HELICITIES IN THE 450 TO 600
   M2/S2 RANGE SHOULD BE VERY FAVORABLE FOR TORNADIC SUPERCELLS. THE
   CURRENT THINKING IS THAT THE GREATEST TORNADO POTENTIAL WILL EXIST
   LATE THIS AFTERNOON AND EARLY THIS EVENING FROM COLUMBUS MS EWD TO
   BIRMINGHAM AND HUNTSVILLE AL. THIS AREA IS CURRENTLY SOUTH OF THE
   ONGOING MCS WHERE DESTABILIZATION SHOULD BE MOST FAVORABLE. AS
   SUPERCELLS INITIATE IN ERN MS AND TRACK ENEWD ACROSS THE HIGH RISK
   AREA LATE THIS AFTERNOON...A TORNADO OUTBREAK IS EXPECTED. A FEW
   LONG-TRACK STRONG TORNADOES AND POSSIBLY A VIOLENT TORNADO OR TWO
   MAY OCCUR AS THE LOW-LEVEL JET STRENGTHENS EARLY THIS EVENING. HAVE
   ADDED A 45 PERCENT SIG-TOR AREA WHERE CONFIDENCE IS THE HIGHEST
   CONCERNING THE POTENTIAL FOR LONG-TRACK TORNADOES.  LARGE HAIL AND
   WIND DAMAGE WILL ALSO BE LIKELY WITH SUPERCELLS THAT DEVELOP.
   
   FURTHER TO THE NORTH ACROSS MIDDLE TN IN WCNTRL KY...THE ONGOING MCS
   IS HAMPERING DESTABILIZATION. HOWEVER...AS THE MID-LEVEL JET NOSES
   INTO THE REGION FROM THE WEST...THE MODERATE INSTABILITY IN NRN LA
   AND SRN AR IS EXPECTED TO ADVECT QUICKLY NNEWD ACROSS WCNTRL TN AND
   WCNTRL KY BETWEEN THE EXITING MCS AND A CLUSTER OF STORMS IN NW AR
   AND SRN MO. THIS SHOULD ALLOW THE MOIST AXIS TO BE LOCATED FROM WEST
   OF NASHVILLE NEWD TO NEAR LOUISVILLE WHERE NUMEROUS SFC-BASED
   THUNDERSTORMS ARE EXPECTED TO INITIATE LATE THIS AFTERNOON. THE
   LOW-LEVEL JET AND MID-LEVEL JET ARE FORECAST TO BE JUXTAPOSED ACROSS
   THIS CORRIDOR SUGGESTING SHEAR PROFILES WILL BE VERY FAVORABLE FOR
   SUPERCELLS AND TORNADOES EVEN IF DESTABILIZATION IS NOT AS GREAT AS
   IS EXPECTED. THE CURRENT THINKING IS THAT A BROKEN LINE OF TORNADIC
   SUPERCELLS WILL INITIATE ALONG THE WRN EDGE OF THE MODERATE RISK
   AREA AND TRACK EWD ACROSS CNTRL KY AND MIDDLE TN EARLY THIS EVENING.
   THE LOW-LEVEL SHEAR ENVIRONMENT WILL BE VERY FAVORABLE FOR TORNADOES
   AND A FEW LONG-TRACK STRONG TORNADOES MAY OCCUR. THE MODERATE RISK
   AREA HAS BEEN EXTENDED WWD ACROSS PARTS OF MIDDLE TN AND CNTRL KY TO
   ACCOUNT FOR THIS POSSIBILITY. LARGE HAIL AND WIND DAMAGE WILL ALSO
   BE POSSIBLE WITH SUPERCELLS THAT DEVELOP.
   
   AS A BROKEN LINE OF TORNADIC SUPERCELLS MOVES EWD ACROSS THE HIGH
   AND MODERATE RISK AREAS THIS EVENING...A TRANSITION TO MORE OF A
   WIND DAMAGE THREAT MAY OCCUR AS THE LINE CONGEALS AND BOWING
   LINE-SEGMENTS DEVELOP. AN ENHANCED WIND DAMAGE THREAT SHOULD EXIST
   AS FAR EAST AS WRN VA ...WRN NC AND WRN SC DURING THE LATE EVENING
   HOURS WHEN A LARGE MCS MOVES INTO THE SRN APPALACHIAN MTNS. THE
   TORNADO THREAT MAY SHIFT SWD LATE THIS EVENING INTO NRN GA WHERE THE
   STORMS MAY TEND TO REMAIN DISCRETE.
   
   ...CNTRL TO NRN APPALACHIAN MTNS/CAROLINAS/ERN GULF COAST STATES...
   THE UPPER-LEVEL TROUGH OVER THE CNTRL STATES WILL MOVE INTO THE MS
   VALLEY THIS EVENING AS A POWERFUL MID-LEVEL JET EJECTS NEWD OUT OF
   THE BASE OF THE TROUGH. THE EXIT REGION OF THE MID-LEVEL JET WILL
   MOVE INTO THE CNTRL APPALACHIAN MTNS AND ERN GREAT LAKES WHERE DEEP
   LAYER SHEAR PROFILES WILL BECOME MORE FAVORABLE FOR SEVERE STORMS
   TONIGHT. ALONG AND TO THE EAST OF THE JET AXIS...A LINE OF STRONG
   THUNDERSTORMS IS EXPECTED TO BE LOCATED FROM ERN OH EXTENDING SWD
   INTO FAR WRN SC AROUND 06Z. THIS LINE SHOULD MOVE EWD ACROSS
   NY...PA...VA...NC AND SC WHERE A WIND DAMAGE THREAT SHOULD EXIST DUE
   TO STRONG LOW-LEVEL FLOW. AN ENHANCED WIND DAMAGE THREAT COULD EXIST
   ACROSS VA AND WRN CAROLINAS WHERE THE MODELS DEVELOP MODERATE
   INSTABILITY AFTER 06Z.
   
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