DAY2SVR: Nws Storm Prediction Center Norman Ok

ACUS02 KWNS 170559
SWODY2
SPC AC 170557

DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1257 AM CDT MON SEP 17 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS SRN NY/SWRN NEW
ENGLAND TO NEAR GA/FL/AL BORDER JUNCTION...

...SYNOPSIS...
IN MID-UPPER LEVELS...STG NRN-STREAM AMPLIFICATION HAS
BEGUN...ASSOCIATED INITIALLY WITH INTENSIFYING SHORTWAVE TROUGH
MOVING SSEWD INTO DAKOTAS. SUBSTANTIAL HEIGHT FALLS WILL CONTINUE
INTO DAY-2 OVER MUCH OF MS VALLEY...GREAT LAKES AND TN
VALLEY/CENTRAL GULF COAST STATES. NET RESULT SHOULD BE
HIGH-AMPLITUDE/SYNOPTIC-SCALE TROUGH BY START OF PERIOD...FROM
HUDSON BAY SWD DOWN MS VALLEY TO SRN LA. THIS WILL INCLUDE
NRN-STREAM SHORTWAVE...REACHING WI/IA/NRN MO BY THAT TIME...AND SRN
STREAM PERTURBATION NOW MANIFEST AS CLOSED CYCLONE OVER S-CENTRAL
TX. LATTER FEATURE IS EXPECTED TO PIVOT ACROSS SE TX...NERN GULF
AND COASTAL LA DAY-1...REACHING JAN/MSY/BVE REGION AS
DEAMPLIFYING/OPEN SHORTWAVE BY 18/12Z. THAT FEATURE WILL WEAKEN
GREATLY DAY-2...ACCELERATING NEWD ACROSS SRN PIEDMONT AND DELMARVA
BY END OF PERIOD...AS LARGE-SCALE TROUGH CONTINUES TO AMPLIFY.
MEANWHILE...EMBEDDED NRN-STREAM SHORTWAVE SHOULD PIVOT EWD OVER
IL/INDIANA/OH/LOWER MI...THEN WEAKEN AND EJECT NEWD OVER LOWER GREAT
LAKES AND SERN ONT DURING 19/06Z-19/12Z TIME FRAME.

AT SFC...COLD FRONT NOW ANALYZED FROM LS TO SWRN NEB IS FCST TO
STRENGTHEN AND SWEEP SEWD/SWD OVER CENTRAL/SRN PLAINS AND MUCH OF MS
VALLEY. BY START OF PERIOD..FRONT SHOULD REACH ERN PORTIONS
OH/KY/TN...AL...AND SWRN LA. WEAK FRONTAL-WAVE LOW OR MESOLOW MAY
DEVELOP ALONG THAT BOUNDARY EARLY IN PERIOD OVER CENTRAL
APPALACHIANS...IN ADVANCE OF NRN-STREAM SHORTWAVE TROUGH...MOVING
NEWD ACROSS WRN/CENTRAL PA AND WRN/NRN NY DURING DAYLIGHT HOURS. BY
19/00Z...FRONTAL POSITION IS EXPECTED TO BE NEAR ADIRONDACKS...SWD
THEN SWWD ACROSS ERN PA...CENTRAL VA...WRN NC...CENTRAL/WRN GA AND
CENTRAL PANHANDLE OF FL. BY END OF PERIOD...FRONT SHOULD REACH ERN
NEW ENGLAND...OUTER BANKS...COASTAL GA AREA AND NERN/CENTRAL GULF.

...NY/WRN NEW ENGLAND TO NERN GULF COAST...
NARROW LINE OF CONVECTION SHOULD DEVELOP EARLY IN PERIOD AND CROSS
MID-ATLC AND CENTRAL PIEDMONT REGIONS...WITH DAMAGING WIND BEING
PRIMARY THREAT. A FEW TORNADOES ALSO ARE POSSIBLE.

AT START OF PERIOD...MORE THAN ONE BAND OR CLUSTER OF TSTMS MAY BE
ONGOING...MOST LIKELY OVER SRN APPALACHIANS REGION AND OVER ERN
OH/WRN PA AREA. AS EACH MOVES INTO PROGRESSIVELY HIGHER-THETAE AIR
MASS...SVR POTENTIAL WILL INCREASE...WHILE EVOLUTION OF MORE UNIFIED
SQUALL LINE IS EXPECTED. FLOW ALOFT WILL BE MOSTLY PARALLEL TO
CONVECTIVE FORCING...INDICATING RAPID DEVELOPMENT OF LINEAR MODE
THAT WOULD BE MAINTAINED THROUGH PERIOD. OVER MID-ATLC REGION...SFC
LOW SHOULD LIFT NEWD FAST ENOUGH TO OUTPACE NWD EXTENT OF FAVORABLE
DESTABILIZATION. BEFORE THAT HAPPENS...HOWEVER...RELATED
ISALLOBARIC FORCING AND BACKING OF LOW-LEVEL FLOW TO ITS E SHOULD
AFFECT NRN FRINGE OF AT LEAST MRGLLY UNSTABLE AIR DURING
DAY...BENEATH 50-65 KT LLJ. THIS WOULD ENLARGE LOW-LEVEL HODOGRAPHS
FAVORABLY FOR SUPERCELL/TORNADO POTENTIAL WITH ANY
DISCRETE/SUSTAINED CELLS THAT CAN FORM AHEAD OF MAIN LINE...AND QLCS
TORNADO THREAT WHERE SQUALL LINE ENCOUNTERS UNSTABLE AND SFC-BASED
EFFECTIVE INFLOW PARCELS.

BUOYANCY WILL DIMINISH WITH NWD EXTENT AMIDST WEAK MIDLEVEL LAPSE
RATES. FCST SOUNDINGS SUGGEST MLCAPE RANGING FROM AROUND 1000 J/KG
OVER ATLC COASTAL DELMARVA TO LESS THAN 100 J/KG OVER ERN NY.
STILL...NEAR-NEUTRAL TO VERY SLIGHTLY UNSTABLE THERMODYNAMIC
PROFILES MAY BE SUFFICIENT TO SUPPORT CONDITIONAL WIND POTENTIAL
OVER PARTS OF ERN NY AND WRN NEW ENGLAND...GIVEN STRENGTH OF
AMBIENT/DEEP-LAYER FLOW.

HODOGRAPHS SHOULD DIMINISH IN SIZE WITH SWWD EXTENT AS LLJ GETS
WEAKER AND SFC WINDS ARE RELATIVELY VEERED. HOWEVER...GREATER
BUOYANCY IS EXPECTED OVER VA/CAROLINAS...GIVEN RICHER LOW-LEVEL
MOISTURE AND MORE TIME FOR CLOUD-MUTED SFC DIABATIC HEATING IN
ADVANCE OF MAIN TSTM BAND. MEANWHILE...SERN RIM OF FAVORABLE
DEEP-LAYER FLOW/SHEAR WILL OVERSPREAD WRN FRINGES OF WARM
SECTOR...CONTRIBUTING TO SVR POTENTIAL. HOWEVER...WITH SWWD
EXTENT...THREAT WILL BECOME MORE TRANSIENT AND CONDITIONAL...AS
STRONGEST DEEP-LAYER ASCENT AND ISALLOBARIC FORCING PULL AWAY FROM
THOSE PORTIONS MS/AL/FL PANHANDLE ALONG/AHEAD OF FRONT.

..EDWARDS.. 09/17/2012

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