DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 171244
SWODY1
SPC AC 171241

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0641 AM CST MON DEC 17 2012

VALID 171300Z - 181200Z

...THERE IS A SLGT RISK OF SVR TSTMS SERN AL/FL PANHANDLE TO SRN/ERN
SC...

...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS NRN MID TN TO OH
VALLEY REGION...

...SYNOPSIS...
PROGRESSIVE UPPER-AIR PATTERN FCST THROUGH REMAINDER OF PERIOD.
SERIES OF SHORTWAVE PERTURBATIONS WILL CONTRIBUTE TO EWD SHIFT AND
VERY GRADUAL DEAMPLIFICATION OF SYNOPTIC TROUGHING NOW OVER CENTRAL
CONUS. ONE SHORTWAVE TROUGH -- NOW EXTENDING FROM WRN OZARKS REGION
SSWWD ACROSS CENTRAL/S-CENTRAL TX -- SHOULD SHIFT EWD TO MIDDLE/ERN
TN...AL...AND NWRN GULF BY 00Z...BEFORE WEAKENING AND MOVING
OFFSHORE CAROLINAS DURING 06-12Z TIME FRAME. CLOSELY FOLLOWING THAT
ON THE N END IS SEPARATE PERTURBATION NOW EVIDENT IN MOISTURE
CHANNEL IMAGERY OVER PORTIONS KS/NEB. THIS FEATURE SHOULD MOVE TO
NEAR MO/IL BORDER BY 00Z...THEN CENTRAL APPALACHIANS BY END OF
PERIOD.

AT SFC...EXTENSIVE CONVECTION AND PRECIP FROM PRIOR DAY REINFORCED
PRE-EXISTING/RESIDUAL BAROCLINIC ZONE VIA OUTFLOW...SHIFTING IT SWD
ACROSS NRN GULF. BOUNDARY WAS ANALYZED AT 11Z FROM EXTREME SERN GA
WSWWD PAST TLH TO ABOUT 40 S VPS...TO INFLECTION POINT ABOUT 40 S
MOB...THEN SWWD ACROSS PENINSULAR PLAQUEMINES PARISH LA...TO NEAR
COASTAL TERREBONNE/ST MARY PARISHES. INLAND RETREAT AND CHARACTER
OF THIS BOUNDARY ACROSS SERN CONUS WILL BE MAJOR FACTOR IN SERN
CONUS SVR POTENTIAL TODAY--MORE DETAILS BELOW.

FARTHER NW...FRONTAL-WAVE LOW SHOULD MOVE NEWD GENERALLY UP OH RIVER
TODAY...REACHING SRN OH/SWRN PA REGION BY 00Z THEN WRN NY AROUND
12Z. ACCOMPANYING WEAK COLD FRONT SHOULD REACH NRN/WRN AL AND
EXTREME SERN PORTIONS MS/LA BY 00Z...THEN COASTAL PLAIN OR COAST OF
CAROLINAS AND SERN GA NEAR END OF PERIOD.

...SERN CONUS...
COMPLEX AND SOMEWHAT CONDITIONAL SCENARIO APPEARS HERE...WITH
GREATEST SVR THREAT PROBABLY MANIFEST AS DAMAGING GUSTS AND PERHAPS
A TORNADO OR TWO THIS AFTERNOON INTO EVENING. WHILE MID-UPPER FLOW
AND DEEP SHEAR WILL BE STG OVER ENTIRE REGION...MRGL INSTABILITY AND
WEAK LAPSE RATES--EVEN IN WARM SECTOR--SHOULD KEEP PRIMARY SVR
THREAT INVOF BOUNDARY...WHICH ITSELF PRESENTS SOME UNCERTAINTIES
ATTM.

COLD FRONTAL ZONE WILL BE CHARACTERIZED BY GRADUAL WIND SHIFT...WITH
VEERING WARM-SECTOR FLOW TO ITS E. AS SUCH...FRONTAL CONVERGENCE
WILL BE VERY LIMITED OVER THIS REGION...AND PRIMARY CONVECTIVE/SVR
CONCERN SHOULD BE AHEAD OF FRONT. PREFRONTAL BOUNDARY SHOULD MOVE
INLAND THIS MORNING AND BECOME MORE SHALLOW...SOME SEGMENTS OF IT
PERHAPS MIXING AWAY AMIDST SFC DIABATIC HEATING ON BOTH SIDES. THIS
PROCESS SHOULD ALLOW DESTABILIZING AIR MASS ACROSS PORTIONS SERN
AL...GA...FL PANHANDLE AND SC...THROUGH COMBINATION OF SFC HEATING
AND BOUNDARY-LAYER THETAE ADVECTION.

NWD EXTENT OF THIS DESTABILIZATION WILL BE AFFECTED BY ONGOING PLUME
OF MIXED CONVECTIVE/STRATIFORM PRECIP FROM SERN LA NEWD ACROSS
CENTRAL/SRN AL...FCST TO SPREAD/SHIFT NEWD OVER PORTIONS CENTRAL/NRN
GA/SC. SRN EDGE OF PRECIP PLUME MAY RE-ALIGN PRIMARY BAROCLINIC
ZONE WSW-ENE OR SW-NE WITH TIME...ESPECIALLY AS HIGHER THETAE
BOUNDARY LAYER DEVELOPS/SPREADS INLAND. AS SUCH...FAVORABLE
DESTABILIZATION FOR EFFECTIVELY SFC-BASED PARCELS APPEARS LESS
PROBABLE OVER NRN PORTIONS AL/GA...ERN TN AND WRN/UPSTATE SC. THIS
PART OF SVR OUTLOOK ACCORDINGLY HAS BEEN SEPARATED FROM KY LOBE.
BULK OF CONVECTION AND QPF SIGNAL APPARENT IN PROGS LIKELY WILL
REMAIN N OF THAT BOUNDARY OVER COOL LOW-LEVEL AIR MASS...REINFORCING
ITS EDGE OVER PORTIONS CENTRAL GA AND CENTRAL OR SRN SC. ALONG THAT
BOUNDARY...RELATIVE MAX IN LIFT AND VERTICAL SHEAR IS EVIDENT IN
MODIFIED FCST SOUNDINGS...WITH 40-50 KT EFFECTIVE SHEAR
MAGNITUDE...200-300 J/KG EFFECTIVE SRH...AND 30-40 KT 0-1 KM SHEAR
MAGNITUDES POSSIBLE. THIS COULD SUPPORT EITHER LEWP/BOW OR EMBEDDED
SUPERCELL EVOLUTIONS.

SVR POTENTIAL IS EXPECTED TO DIMINISH MARKEDLY BY EARLY-MID EVENING
AS BOUNDARY LAYER COOLS AND WINDS THEREIN VEER AHEAD OF COLD FRONT.

...NRN MID TN TO OH VALLEY REGION...
SVR POTENTIAL APPEARS MRGL BUT ADEQUATE TO MAINTAIN HAIL-DRIVEN
CATEGORICAL RISK FOR NOW. EXPECT VERTICAL JUXTAPOSITION OF DIABATIC
HEATING WITH PROGRESSIVELY STRONGER MID-UPPER DCVA/COOLING AHEAD OF
SHORTWAVES THROUGH AFTERNOON. IN CONJUNCTION WITH MRGL MOISTURE --
E.G. SFC DEW POINTS LOW-MID 50S F -- AIR MASS SHOULD DESTABILIZE
BEHIND ONGOING SHOWERS OVER THIS REGION...REDUCING CINH ENOUGH FOR
TSTMS TO FORM NEAR SFC LOW AND ACCOMPANYING FRONT. MIDLEVEL LAPSE
RATES SHOULD STEEPEN TO AROUND 7.5 DEG C/KM...SUPPORTING MLCAPE IN
300-800 J/KG RANGE. EFFECTIVE SHEAR MAGNITUDES 35-45 KT ARE
POSSIBLE...MOSTLY RELATED TO PRESENCE OF STG MID-UPPER WINDS GIVEN
WEAKNESS OF NEAR-SFC FLOW. RISK SHOULD DIMINISH RAPIDLY AFTER 00Z.

..EDWARDS/ROGERS.. 12/17/2012

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