DAY3SVR: Nws Storm Prediction Center Norman Ok

ACUS03 KWNS 160730
SWODY3
SPC AC 160729

DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0229 AM CDT SUN SEP 16 2012

VALID 181200Z - 191200Z

...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS NY TO SRN
APPALACHIANS/PIEDMONT REGIONS...

...SYNOPSIS...
SUBSTANTIAL MID-UPPER PATTERN CHANGE IS UNDERWAY AND WILL OCCUR IN
EARNEST DAY-2. SHORTWAVE TROUGH NOW OVER FAR W TX AND NRN CHIHUAHUA
WILL PHASE WITH NRN-STREAM TROUGH...AMIDST BROAD REGIME OF HEIGHT
FALLS. BY START OF PERIOD...FULL-LATITUDE/SYNOPTIC-SCALE TROUGH
SHOULD EXTEND FROM HUDSON BAY ACROSS LS AND MS VALLEY. THIS FEATURE
WILL SHIFT EWD TO LH...OH...TN VALLEY REGION AND N-CENTRAL GULF BY
19/12Z. STRONGEST EMBEDDED SHORTWAVE TROUGH IS FCST TO PIVOT
CYCLONICALLY FROM SERN IA/NRN MO REGION ACROSS SRN IL TO ERN OH/WRN
PA.

PRECEDING THAT PERTURBATION...STG SFC COLD FRONT SHOULD BEGIN PERIOD
OVER PORTIONS OH...MID-ERN TN...AL AND NWRN GULF...REACHING WRN
NY...WRN PA...WV...WRN GA AND CENTRAL/WRN FL PANHANDLE BY 19/00Z.
BY END OF PERIOD...FRONT SHOULD EXTEND FROM ERN QUE ACROSS COASTAL
MID-ATLC OR SRN NEW ENGLAND...SSWWD TO NRN FL. UNCERTAINTIES IN
FRONTAL SPEED GROW WITH TIME AFTER MIDDLE OF PERIOD...IN STEP WITH
GREATER SPREAD IN PROGS REGARDING EJECTION PHASE SPEED OF MAIN
EMBEDDED SHORTWAVE.

...NY TO SRN APPALACHIANS/PIEDMONT REGIONS...
CORRIDOR OF CONVECTIVE DAMAGING-WIND POTENTIAL IS EXPECTED TO SHIFT
EWD ACROSS OUTLOOK AREA FROM MORNING THROUGH AFTN AND AT LEAST INTO
MID-LATE EVENING.

BUOYANCY IS EXPECTED TO INCREASE WITH EWD EXTENT IN PROXIMITY TO
HIGHER BOUNDARY-LAYER MOISTURE NEAR ATLC COAST. MEANWHILE...DEEP
SHEAR AND FRONTAL FORCING GENERALLY WILL BE GREATER WITH NWD
EXTENT...AND WILL SPREAD OVER MORE OF WARM SECTOR WITH TIME.
INSTABILITY WILL BE VERY LIMITED OVER CENTRAL/NRN APPALACHIANS
REGION EWD INTO MID-ATLC. FCST SOUNDINGS REASONABLY SUGGEST
INFLOW-LAYER MLCAPE FOR PRIMARY CONVECTIVE BAND REMAINING BELOW 1000
J/KG MUCH OF PERIOD...AND BELOW 500 J/KG EARLY IN PERIOD AND FROM
PA/SRN NY NWD. HOWEVER...COMBINATION OF WEAK CINH AND STG FRONTAL
FORCING SHOULD YIELD NARROW...PROBABLY LOW-TOPPED BAND OF CONVECTION
MOVING EWD ACROSS INTERIOR MID-ATLC. THOUGH TIMING OF ITS COASTAL
APCH WILL BE AFTER DARK...CONVECTION WILL BE MOVING INTO HIGHER
LOW-LEVEL THETAE...AND MAY BE SUSTAINED AT SVR LEVELS INTO SOME OF
OVERNIGHT HOURS.

ONE COMPLICATING FACTOR MAY BE PRESENCE OF BAND OR CLUSTERS OF
CONVECTION ONGOING EARLY IN PERIOD...RELATED TO INTENSE LOW-LEVEL
WAA/MOISTURE TRANSPORT BELT OVER PORTIONS WRN VA/NC...ERN TN AND/OR
NRN GA. WHILE MUCH OF THIS ACTIVITY LIKELY WOULD OCCUR ATOP
RESIDUAL REGIME OF COOL/DAMMING AIR...STG LOW-LEVEL THETAE ADVECTION
WILL ERODE THAT REGIME WITH TIME AHEAD OF TSTMS. CONVECTION WILL
SHIFT NEWD FROM THOSE AREAS INTO AT LEAST MRGL BOUNDARY-LAYER
INSTABILITY ACROSS CAROLINAS/VA...WHILE ADDITIONAL ACTIVITY MAY
DEVELOP IN FAVORABLE MOIST/DIABATICALLY HEATED AIR MASS OVER PARTS
OF SC/GA. TIMING/PLACEMENT OF SRN/WRN PORTION OF SVR THREAT
THEREFORE IS QUITE UNCERTAIN...BUT UNCONDITIONAL PROBABILITIES
SUPPORTING CATEGORICAL OUTLOOK ARE DRAWN OVER PORTIONS CAROLINAS/GA
IN DEFERENCE TO FCST OVERLAP OF FAVORABLE MOISTURE AND DEEP SPEED
SHEAR.

..EDWARDS.. 09/16/2012

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