re: Tenth Anniversary of the 2011 Super Tornado Outbreak

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— AN OUTBREAK OF TORNADOES AND DAMAGING WINDS IS EXPECTED TODAY
THROUGH EVENING OVER PORTIONS OF NRN MS…NRN AL…TN AND KY. —

…LOWER MS VALLEY INTO TN/OH VALLEYS…
A COMPLICATED FORECAST SCENARIO WILL UNFOLD TODAY…BEGINNING RATHER
EARLY IN THE PERIOD OVER SRN AND ERN AR AND NRN LA. MODELS ARE IN
RELATIVELY GOOD AGREEMENT WITH THE PLACEMENT OF THE SURFACE LOW OVER
AR BETWEEN 12-15Z…THEN DEVELOPING IT NEWD ACROSS TN/KY BY 00Z.
NUMEROUS THUNDERSTORMS WILL BE ONGOING…AND SOME WILL BE
SEVERE…FROM NRN LA/SRN AR EWD ACROSS NRN MS…WRN AND MIDDLE TN
AND KY. DESPITE TIME OF DAY…PRESENCE OF RICH LOW LEVEL MOISTURE
WILL RESULT IN LITTLE CAPPING AND SURFACE BASED EFFECTIVE PARCELS.
THUS…A TORNADO AND DAMAGING WIND THREAT WILL BE PRESENT WITH THIS
ACTIVITY.

SOUTH OF THE ONGOING ACTIVITY…STRONG DAYTIME HEATING WILL OCCUR.
WITH BOUNDARY LAYER DEWPOINTS IN THE LOW 70S F…THIS WILL CREATE
STRONG TO EXTREME INSTABILITY OVER MS AND AL NWD TO THE ONGOING
CONVECTION. A CAPPING INVERSION WILL LIKELY KEEP MUCH OF THE WARM
SECTOR S OF THE EARLY STORMS CAPPED…ALLOWING A BUILD UP OF
INSTABILITY TO BE RELEASED WHEN LARGE SCALE FORCING ARRIVES DURING
THE AFTERNOON ALONG THE COLD FRONT AND ESPECIALLY NEAR THE
FRONT/OUTFLOW BOUNDARY INTERSECTION. THE RESULT WILL BE FAST MOVING
TORNADIC SUPERCELLS CAPABLE OF STRONG TO VIOLENT TORNADOES. GIVEN
THE FAST STORM MOTIONS AND RELATIVELY WIDE WARM SECTOR…SOME
TORNADOES COULD BE LONG LIVED. THE GREATEST THREAT FOR THESE TYPES
OF STORMS WILL BE OVER THE HIGH RISK AREA…OVER NRN MS AND
AL…INTO EXTREME NWRN GA AND SRN MIDDLE TN. THIS ACTIVITY WILL
PERSIST TO THE E…BUT CELLS MAY EVENTUALLY MERGE INTO MORE OF A
QLCS STRUCTURE CAPABLE OF PARTICULARLY DAMAGING WINDS AND A FEW
TORNADOES.

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SPC AC 271629 DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1129 AM CDT WED APR 27 2011 VALID 271630Z - 281200Z ...THERE IS A HIGH RISK OF SVR TSTMS ACROSS PARTS OF MS...AL...TN AND GA...... ...THERE IS A MDT RISK OF SVR TSTMS ACROSS PARTS OF MS...AL...TN...KY...WRN VA...WRN NC...WRN SC...NRN GA... ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE MID-MS VALLEY...CNTRL TO ERN GULF COAST STATES...TN VALLEY...OH VALLEY...CNTRL TO SRN APPALACHIAN MTNS...CAROLINAS...MID-ATLANTIC AND NERN STATES... ...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. ..BROYLES/GARNER.. 04/27/2011

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