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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