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Moderate Risk extended way north 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