DAY2SVR: Nws Storm Prediction Center Norman Ok

ACUS02 KWNS 211729
SWODY2
SPC AC 211727

DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1227 PM CDT FRI SEP 21 2012

VALID 221200Z - 231200Z

...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NORTHEAST
AND MID-ATLANTIC REGIONS...

...SYNOPSIS...
MODELS CONTINUE TO SHOW A LARGE UPPER TROUGH OVER ERN CANADA AND THE
ERN HALF OF THE U.S. SHIFTING SLOWLY EWD ON SAT. THIS WILL OCCUR AS
A COMPACT SHORTWAVE TROUGH...CURRENTLY MOVING SWD THROUGH SRN
MANITOBA PER MOISTURE CHANNEL IMAGERY...TRACKS THROUGH THE MIDWEST/
OH VALLEY REGION...REACHING NY/WRN NEW ENGLAND SAT NIGHT. A COLD
FRONT ACCOMPANYING A LEAD SHORTWAVE IMPULSE AND THE PARENT LONGWAVE
TROUGH SHOULD EXTEND FROM SRN ONTARIO SSWWD THROUGH THE UPPER OH
VALLEY TO MIDDLE TN AND THE MID SOUTH AT 12Z SAT. THIS BOUNDARY
WILL PROGRESS EWD ACROSS THE APPALACHIAN CREST THROUGH MIDDAY...AND
THEN OFF THE ATLANTIC COAST BY THE END OF THE PERIOD. THIS FRONT
AND A PRE-FRONTAL TROUGH SHOULD BE THE PRIMARY FOCI FOR SAT
AFTERNOON AND EVENING CONVECTIVE ACTIVITY.

MEANWHILE FARTHER W...LARGE-SCALE RIDGING WILL PREVAIL...THOUGH A
SMALL-SCALE LOW/TROUGH IS FORECAST TO DRIFT ESEWD ACROSS THE PAC NW
REGION SUPPORTING SOME INCREASE IN CONVECTIVE ACTIVITY/ISOLATED
TSTMS.

...PARTS OF THE NORTHEAST AND MID-ATLANTIC REGION...
ALTHOUGH LOW LEVEL MOISTURE/WARM AIR ADVECTION IS EXPECTED IN
ADVANCE OF THE APPROACHING COLD FRONT...THE FORECAST OF WEAK
MIDLEVEL LAPSE RATES CONTINUES TO SUGGEST INSTABILITY WILL BE MODEST
AT BEST WITH MLCAPE OF 500-1000 J/KG. AT THIS TIME...THE GREATEST
INSTABILITY IS FORECAST FROM NERN MD THROUGH ERN PA/NJ TO SERN NY.
EFFECTIVE BULK SHEAR WILL INCREASE THROUGH THE DAY AS DEEP LAYER
WIND FIELDS STRENGTHEN WITH THE EWD SHIFT OF THE ERN U.S. LONGWAVE
TROUGH AND THE APPROACH OF THE MIDWEST SHORTWAVE TROUGH.

CONVECTION WITH EMBEDDED TSTMS ARE EXPECTED TO DEVELOP ALONG A
PRE-FRONTAL TROUGH AND HIGHER TERRAIN OF THE CENTRAL APPALACHIANS
INTO CENTRAL NY SAT AFTERNOON...WITH ACTIVITY BEING MAINTAINED AS
THE COLD FRONT OVERTAKES THE TROUGH AND ADVANCES INTO ERN NY...ERN
PA TO THE BALTIMORE/WASHINGTON DC METRO AREAS LATE SAT AFTERNOON/
EVENING. THE JUXTAPOSITION OF EVEN MODEST INSTABILITY AND EFFECTIVE
BULK SHEAR OF 35-40 KT SUGGEST THE POTENTIAL FOR STORM ORGANIZATION
INCLUDING SOME SUPERCELLS...WITH DAMAGING WINDS REMAINING THE
PRIMARY THREAT.

FARTHER S...GREATER VEERING OF LOW LEVEL WINDS INTO VA SUGGESTS
CONVERGENCE WILL BE WEAKER...AND THUS LIMIT THE DEVELOPMENT OF MORE
ROBUST/SUSTAINED UPDRAFTS. FOR THIS REASON...SEVERE PROBABILITIES
HAVE BEEN REDUCED INTO VA. MEANWHILE...FAR N ACROSS UPSTATE
NY...WEAKER INSTABILITY IS EXPECTED TO LIMIT THE DEVELOPMENT OF
ORGANIZED SEVERE WEATHER WITH NWD EXTENT...AND THUS THE SLIGHT RISK
HAS BEEN REMOVED FROM THAT REGION TOO.

..PETERS.. 09/21/2012

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