SWODY2
SPC AC 110534
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1234 AM CDT SUN SEP 11 2011
VALID 121200Z - 131200Z
...THERE IS A SLGT RISK OF SVR TSTMS CENTRAL WI NWD ACROSS MUCH OF
LS REGION...
...SYNOPSIS...
IN MID-UPPER LEVELS...CYCLONE NOW QUASISTATIONARY OVER MID MS AND
LOWER OH RIVER VALLEYS IS FCST TO BECOME OPEN-WAVE TROUGH DAY-1. BY
12/12Z...TROUGH SHOULD EXTEND ALONG APPALACHIANS FROM PA-GA...THEN
OVER FL PANHANDLE AND JUST OFFSHORE MS RIVER MOUTH.
PRIMARY/ANCHORING 500-MB VORTICITY LOBE LIKEWISE WILL BE POSITIVELY
TILTED...WEAKENING SLOWLY WHILE MOVING EWD ACROSS CAROLINAS DURING
FIRST 12 HOURS OF PERIOD...THEN OFFSHORE. NRN-STREAM SHORTWAVE
TROUGH -- NOW PENETRATING MEAN RIDGE OVER NRN BC -- IS FCST TO TURN
SEWD ACROSS WRN CANADA DAY-1...REACHING SRN PORTIONS MB-SK BY
12/12Z. THIS PERTURBATION THEN SHOULD PIVOT ESEWD TO EWD ALONG
MN/ONT BORDER THEN OVER LS AND ADJOINING PORTIONS NRN ONT BY END OF
PERIOD. MEANWHILE...PERSISTENT CUT-OFF LOW -- EVIDENT IN MOISTURE
CHANNEL IMAGERY OVER CA CHANNEL ISLANDS -- SHOULD FILL VERY SLOWLY
WHILE DRIFTING ERRATICALLY NEAR PRESENT LOCATION.
AT SFC...COMPACT CYCLONE RELATED TO NRN-STREAM PERTURBATION SHOULD
BEGIN PERIOD NEAR MB-ND BORDER...MOVE EWD ACROSS NWRN ONT N OF MN
BORDER BY 13/00Z...NRN LS BY 13/06Z AND ACROSS NRN ONT THEREAFTER.
TRAILING COLD FRONT IS FCST TO CROSS NRN PLAINS AND EXTEND FROM LOW
TO SRN MN...SERN SD...AND CENTRAL/SWRN NEB BY 13/00Z. BY END OF
PERIOD...EXPECT COLD FRONT OVER NRN LOWER MI...NRN IL...NRN MO..AND
PORTIONS CENTRAL/SWRN KS.
...UPPER GREAT LAKES REGION...
SCATTERED TSTMS SHOULD DEVELOP DURING AFTERNOON AND EVENING INVOF
SFC COLD FRONT...ESPECIALLY OVER CATEGORICAL SLGT-RISK AREA WHERE
LOW-LEVEL CONVERGENCE WILL BE RELATIVELY MAXIMIZED IN CLOSEST
PROXIMITY TO SFC LOW. TSTM AND SVR PROBABILITIES WILL LESSEN WITH
SWWD EXTENT AS CAPPING STRENGTHENS AND FRONTAL ASCENT WEAKENS.
THOUGH PRE-FRONTAL SFC FLOW SHOULD BE SWLY...FCST HODOGRAPHS ARE
LARGE ENOUGH FROM CENTRAL WI NWD...AMIDST 40-50 KT EFFECTIVE SHEAR
MAGNITUDES...TO SUPPORT SOME SUPERCELL POTENTIAL. HOWEVER...PRIND
DOMINANT MODES FOR BULK OF EVENT SHOULD BE QUASI-LINEAR AND
CLUSTERED...WITH DAMAGING GUSTS AS MAIN THREAT. AT SFC...LEADING
EDGE OF ENHANCED LARGE-SCALE ASCENT/COOLING IN MIDLEVELS MAY
ACCOMPANY COLD FRONT OVER REGION. THIS WILL LEAD TO STEEPENING
LAPSE RATES ALOFT AS FRONTAL ASCENT IMPINGES ON DIABATICALLY HEATED
BOUNDARY LAYER CONTAINING UPPER 50S TO LOW 60S F SFC DEW POINTS.
ACCORDINGLY MODIFIED FCST SOUNDINGS YIELD 1000-2000 J/KG
MLCAPE...WITH SUFFICIENTLY WEAK CINH THAT FRONTAL LIFT SHOULD FORCE
TSTMS. SVR POTENTIAL SHOULD DIMINISH LATE EVENING INTO OVERNIGHT
HOURS AS ACTIVITY MOVES INTO PROGRESSIVELY MORE STABLE AIR MASS OVER
MI/LM.
..EDWARDS.. 09/11/2011
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