Epic Outbreak of Tornadoes Across the Southeast on 27-28 April 2011 Impacts Northeast Georgia and Western North Carolina

SPC AC 261730
   
DAY 2 CONVECTIVE OUTLOOK  
NWS STORM PREDICTION CENTER NORMAN OK
1230 PM CDT TUE APR 26 2011
   
VALID 271200Z - 281200Z
   
...THERE IS A MDT RISK OF SVR TSTMS WEDNESDAY ACROSS PARTS OF
EASTERN INDIANA...MUCH OF OHIO...WESTERN PENNSYLVANIA...WEST
VIRGINIA...MUCH OF KENTUCKY AND TENNESSEE...NORTHEAST
GEORGIA...CENTRAL AND NORTHERN ALABAMA AND MISSISSIPPI...
   
...THERE IS A SLGT RISK OF SVR TSTMS SURROUNDING THE MODERATE RISK
AREA ACROSS MUCH OF THE EASTERN U.S....
   
...SYNOPSIS...
...OUTBREAK OF SEVERE THUNDERSTORMS POSSIBLE...
MODELS INDICATE THAT THE LARGE UPPER TROUGH...NOW EVOLVING OVER THE
CENTRAL U.S...WILL SLOWLY LIFT ACROSS AND EAST OF THE MISSISSIPPI
VALLEY DURING THIS PERIOD.  AS IT DOES...IT APPEARS LIKELY TO TAKE
ON AN INCREASING NEGATIVE TILT...AS A STRONG SHORT WAVE IMPULSE
LIFTS OUT OF ITS BASE...AND GRADUALLY PIVOTS AROUND THE EASTERN
PERIPHERY OF A DEVELOPING CYCLONIC CIRCULATION OVER THE UPPER
MISSISSIPPI VALLEY/GREAT LAKES REGION.  THIS SHOULD BE ACCOMPANIED
BY STRENGTHENING CYCLONIC MID/UPPER FLOW FIELDS IN THE WARM SECTOR
OF A SURFACE CYCLONE PROGGED TO DEEPEN AS IT MIGRATES NORTHEAST OF
THE OZARK PLATEAU TOWARD THE GREAT LAKES.  THIS PROBABLY WILL
INCLUDE A 90-100 KT 500 MB JET CORE NOSING ACROSS THE TENNESSEE AND
OHIO VALLEYS BY LATE AFTERNOON.
   
VARIABILITY EXISTS AMONG THE MODELS CONCERNING THE DETAILS OF THIS
CYCLOGENESIS...PERHAPS IN PART DUE TO THE INFLUENCE OF CONVECTION
...WHICH COULD BE EXTENSIVE AND ONGOING AT 12Z WEDNESDAY.  HIGH
RESOLUTION MODEL DATA IS STRONGLY SUGGESTIVE THAT A SIGNIFICANT
CLUSTER OF STORMS COULD BE IN PROGRESS WEDNESDAY MORNING EAST OF THE
MIDDLE AND LOWER MISSISSIPPI VALLEY.  IF THIS VERIFIES...IT IS
UNCERTAIN WHAT INFLUENCE IT WILL HAVE ON SUBSEQUENT BOUNDARY LAYER
DESTABILIZATION ACROSS THE OHIO/TENNESSEE VALLEY REGION.  THIS
UNCERTAINTY PRECLUDES AN OUTLOOK UPGRADE TO HIGH RISK PROBABILITIES
AT THE PRESENT TIME.  HOWEVER...MOST OTHER INDICATIONS ARE STRONGLY
SUGGESTIVE THAT A MAJOR OUTBREAK OF SEVERE THUNDERSTORMS COMMENCING
TONIGHT ACROSS PARTS OF THE OZARK PLATEAU/LOWER MISSISSIPPI VALLEY
WILL CONTINUE AND EXPAND NORTHWARD AND EASTWARD TOWARD THE
APPALACHIANS DURING THE DAY WEDNESDAY.  THIS WILL INCLUDE THE
POTENTIAL FOR LONG-LIVED SUPERCELLS CAPABLE OF STRONG
TORNADOES...AND ORGANIZED STORM CLUSTERS CAPABLE OF STRONG DAMAGING
WIND GUSTS.
   
...EASTERN U.S...
IN AREAS NOT IMPACTED SIGNIFICANTLY BY EARLY MORNING CONVECTION...
PRE-FRONTAL BOUNDARY LAYER MOISTURE OVER A BROAD AREA EAST OF THE
MISSISSIPPI VALLEY INTO THE APPALACHIANS...AND PERHAPS EAST OF THE
APPALACHIANS...WILL BE SUPPORTIVE OF CAPE ON THE ORDER OF 1000 TO
2000 J/KG WITH SURFACE HEATING.  IN THE PRESENCE OF WIND PROFILES
CHARACTERIZED BY STRONG DEEP LAYER SHEAR...AND LARGE CLOCKWISE
CURVED LOW-LEVEL HODOGRAPHS BENEATH A BROAD 50-70 KT SOUTHERLY
LOW-LEVEL JET...A SIGNIFICANT TORNADIC SUPERCELL THREAT COULD BE
ONGOING OR RAPIDLY DEVELOP BY LATE MORNING ACROSS PARTS OF
NORTHEASTERN MISSISSIPPI/NORTHERN ALABAMA THROUGH WESTERN/MIDDLE
TENNESSEE AND WESTERN/CENTRAL KENTUCKY.  HOWEVER...THIS WILL BE
LARGELY CONTINGENT ON EVOLUTION OF OVERNIGHT CONVECTION.  IF AN
LARGE CLUSTER OF STORMS DEVELOPS AS SOME GUIDANCE SUGGESTS...AN
EXPANSIVE SURFACE COLD POOL COULD SLOW DESTABILIZATION AND MITIGATE
THE SEVERE POTENTIAL.
   
REGARDLESS...SUFFICIENT WEAKENING OF THE CONVECTION AND THE
ASSOCIATED COLD POOL IS EXPECTED TO OCCUR BY THE AFTERNOON TO
SUPPORT WEAK TO MODERATE DESTABILIZATION IN THE PRESENCE OF
STRENGTHENING LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION.  AND
THIS PROBABLY WILL BE SUFFICIENT TO SUPPORT CONVECTIVE
DEVELOPMENT...INCLUDING TORNADIC SUPERCELLS...IN THE WARM SECTOR
...AND ALONG AND AHEAD OF THE COLD FRONT TRAILING SOUTHWARD FROM THE
SURFACE LOW.  AS THIS FRONT ADVANCES TOWARD THE APPALACHIANS
...UPSCALE CONVECTIVE GROWTH INTO AN EXTENSIVE SQUALL LINE COULD
OCCUR BY EARLY WEDNESDAY EVENING...BEFORE SPREADING EAST OF THE
APPALACHIANS OVERNIGHT.
   
..KERR.. 04/26/2011
   
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