SWODY1
SPC AC 211940
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0140 PM CST WED DEC 21 2011
VALID 212000Z - 221200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER A SMALL PORTION OF THE
UPPER OH VALLEY...
ONLY MINOR CHANGES WERE MADE TO THE ONGOING OUTLOOK. INSTABILITY
HAS REMAINED VERY WEAK ACROSS OH/WV/PA THIS AFTERNOON WITH DEWPOINTS
ONLY IN THE LOWER 50S. HOWEVER...PARTIAL SUNSHINE HAS STEEPENED LOW
LEVEL LAPSE RATES IN A REGION OF INTENSE LARGE SCALE FORCING FOR
ASCENT. A SMALL CLUSTER OF CONVECTION OVER SOUTHEAST OH RECENTLY
PRODUCED A LIGHTNING STRIKE. GIVEN THE VERY STRONG WIND FIELDS
/60-80 KNOTS AT MOST LEVELS/ HAVE OPTED TO MAINTAIN A SMALL SLIGHT
RISK OVER THIS AREA IN THE EVENT THAT THE MORE INTENSE CONVECTIVE
ELEMENTS CAN TRANSFER THAT MOMENTUM TO THE SURFACE. THE THREAT
SHOULD DIMINISH RAPIDLY AFTER SUNSET.
..HART.. 12/21/2011
.PREV DISCUSSION... /ISSUED 1026 AM CST WED DEC 21 2011/
...SYNOPSIS...
VORTICITY MAXIMUM OVER IL/IND WILL CONTINUE EWD ACROSS THE OH VALLEY
AND LOWER GREAT LAKES TO OFF THE NEW ENGLAND COAST BY THU MORNING.
MEANWHILE...UPSTREAM LOW OVER NWRN MEXICO WILL DE-AMPLIFY WHILE
TRANSLATING EWD ACROSS NRN MEXICO AND EVENTUALLY INTO THE SRN PLAINS
BY 22/12Z. IN THE LOW LEVELS...SURFACE LOW ASSOCIATED WITH THE
FORMER MIDLEVEL SYSTEM OVER NWRN OH WILL CONCURRENTLY DEVELOP ENEWD
WITH NRN EXTENSION OF ATTENDANT COLD FRONT PROGRESSING THROUGH THE
UPPER OH VALLEY TO OFF THE NEW ENGLAND AND MID ATLANTIC COASTS LATE
IN THE PERIOD. FARTHER SW...EXPECT LOW-LEVEL BAROCLINIC ZONE TO
STALL ACROSS THE SRN APPALACHIANS WITH TRAILING SEGMENT OF FRONT
OVER THE LOWER MS VALLEY AND NWRN GULF OF MEXICO RETREATING NWD
TONIGHT AS A FRONTAL WAVE DEVELOPS AHEAD OF UPSTREAM SYSTEM.
...UPPER OH VALLEY/CNTRL APPALACHIANS TODAY...
LOW-TOPPED CONVECTION WITH EMBEDDED TSTMS WILL PERSIST AHEAD OF THE
COLD FRONT...FORCED BY DCVA IN ADVANCE OF VORTICITY MAXIMUM
MENTIONED IN THE SYNOPSIS. HERE...FLOW FIELD WILL REMAIN QUITE
STRONG /I.E. 70-80 KT AT 500 MB AND 60-65 KT AT 700 MB/ WITH MORE
THAN SUFFICIENT VERTICAL SHEAR FOR ORGANIZED STORM MODES. VISIBLE
SATELLITE IMAGERY SHOWS A CORRIDOR OF CLEARING AHEAD OF THE COLD
FRONT ACROSS PARTS OF WRN/CNTRL OH INTO CNTRL KY WHERE LOCALLY
STRONGER DIABATIC HEATING AND BOUNDARY LAYER DEWPOINTS IN THE LOW
50S MAY YIELD SBCAPE VALUES OF A COUPLE HUNDRED J/KG. GIVEN THE
STRONG FORCING AND HIGH MOMENTUM FLOW FIELD...IT IS POSSIBLE THAT
ONE OR MULTIPLE BANDS OF LOW-TOPPED STORMS MAY BE ABLE TO ORGANIZE
THIS AFTERNOON WITH THE PRIMARY HAZARD BEING DAMAGING WIND GUSTS.
ELSEWHERE...WIDESPREAD CLOUDS WILL LIMIT AIR MASS DESTABILIZATION
AND THE POTENTIAL FOR A MORE ROBUST SEVERE WEATHER THREAT.
...CNTRL/ERN GULF STATES TODAY...
A MOIST LOW-LEVEL AIR MASS CHARACTERIZED BY BOUNDARY LAYER DEWPOINTS
IN THE 60S WILL BE MAINTAINED AHEAD OF DECELERATING COLD FRONT.
HOWEVER...SIMILAR TO TUESDAY...LAPSE RATES AND WIDESPREAD CLOUDS
WILL LIMIT AIR MASS DESTABILIZATION WITH MLCAPE VALUES REMAINING AOB
500-1000 J/KG. WEAK LOW-LEVEL WAA AND MASS CONFLUENCE WILL REMAIN
THE PRIMARY PROCESSES SUSTAINING BANDS OF SHOWERS AND TSTMS AS LEAD
MIDLEVEL SYSTEM CONTINUES TO SHIFT EWD/NEWD AWAY FROM THE REGION.
AND GIVEN THAT 45-55 KT OF DEEP SWLY SHEAR WILL BE MAINTAINED ACROSS
THE WARM SECTOR...LOW WIND/TORNADO PROBABILITIES REMAIN WARRANTED.
HOWEVER...RATHER STRONG LOW-LEVEL SHEAR CURRENTLY OBSERVED BY
REGIONAL VADS WILL DIMINISH TODAY AS LLJ MIGRATES AWAY FROM THE
AREA. THIS IN CONJUNCTION WITH THE WEAK INSTABILITY WILL LIMIT THE
POTENTIAL FOR A MORE ROBUST SEVERE WEATHER THREAT.
...SERN TX/SRN LA EARLY THURSDAY...
COLD FRONT CURRENTLY STALLING OVER THE NWRN GULF OF MEXICO /PER
SATELLITE IMAGERY/ WILL BEGIN TO ACCELERATE NWD OVERNIGHT IN
RESPONSE TO THE APPROACH OF THE NRN MEXICO SYSTEM AND RESULTANT
CYCLOGENESIS OFF THE TX COAST. 12Z DETERMINISTIC AND ENSEMBLE MODEL
GUIDANCE INDICATES A HIGH PROBABILITY OF SURFACE-BASED INSTABILITY
REMAINING LARGELY OFFSHORE THROUGH 22/12Z WITH DEVELOPING STORMS
LIKELY ROOTED WITHIN A WEAKLY UNSTABLE ENVIRONMENT ABOVE A SHALLOW
NEAR-SURFACE LAYER. WHILE ISOLATED SEVERE WEATHER WILL BE POSSIBLE
TOWARD 22/12Z...IT APPEARS THAT THE GREATER POTENTIAL WILL BE
DELAYED UNTIL THE DAY 2 PERIOD.
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