DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0216 AM CST SUN NOV 20 2011
VALID 221200Z - 231200Z
...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS CENTRAL/WRN GULF COAST
TO SRN APPALACHIANS AND OH VALLEY...
...SYNOPSIS...
STG SHORTWAVE TROUGH AND EMBEDDED MID-UPPER LOW -- NOW EVIDENT IN
MOISTURE CHANNEL IMAGERY OFFSHORE CA -- IS FCST TO MOVE EWD ACROSS
GREAT BASIN...SRN ROCKIES AND SRN HIGH PLAINS THROUGH DAY-2. BY
START OF PERIOD...THIS SYSTEM SHOULD ASSUME POSITIVE TILT AT 500
MB...ROUGHLY FROM ERN KS SWWD TO W-CENTRAL TX. ALTHOUGH OPERATIONAL
MODELS AND SREF MEMBERS START TO DIVERGE WITH AMPLITUDE AND SPEED OF
THIS PERTURBATION LATE DAY-2 INTO EARLY DAY-3...DIFFERENCES NOW
APPEAR MESOSCALE IN NATURE THROUGH THIS PERIOD. MID-UPPER TROUGH
SHOULD EXTEND FROM CENTRAL/SRN IL TO CENTRAL/ERN TX BY 23/00Z...AND
FROM OH SWWD OVER AL BY 23/12Z.
AT SFC...FRONTAL-WAVE CYCLONE SHOULD MOVE FROM SERN MO ENEWD ACROSS
LOWER GREAT LAKES/NRN APPALACHIANS REGION THROUGH PERIOD. COLD
FRONT SHOULD EXTEND FROM LOW SWWD ACROSS SERN OK AND CENTRAL/SW TX
BY 22/12Z...THEN FROM SRN INDIANA TO WRN/MID TN AND CENTRAL/SRN LA
BY 23/00Z. FRONT SHOULD REACH CENTRAL/SRN APPALACHIANS AND WRN FL
PANHANDLE BY END OF PERIOD.
...CENTRAL/WRN GULF COAST TO SRN APPALACHIANS AND OH VALLEY...
LATE DAY-2 INTO EARLY DAY-3...EXPECT BAND OF TSTMS TO BUILD SWWD
ALONG SFC COLD FRONT...WITH ATTENDANT RISK FOR OCNL DAMAGING GUSTS
AND ISOLATED HAIL FROM OZARKS TO CENTRAL/ERN TX. THROUGH LATE
AFTERNOON...SVR RISK SHOULD SHIFT EWD...WHILE EXPANDING NWD OVER OH
VALLEY IN STEP WITH EXPANDING WARM-SECTOR REGIME OF AT LEAST MRGL
BUOYANCY. CONVECTION ALSO SHOULD BACKBUILD ALONG/AHEAD OF FRONT
TOWARD COASTAL SE TX AND LA. DAMAGING TSTM GUSTS SHOULD REMAIN
MOST PREVALENT SVR MODE DURING AFTERNOON/EVENING...THOUGH TORNADOES
AND ISOLATED HAIL ALSO ARE POSSIBLE. MAIN UNCERTAINTIES RESULT FROM
DIFFERENCES IN PROGS OF SYSTEM SPEED...AND INVOLVE BOTH TIMING OF
SVR THREAT AREA-WIDE...AND NWD EXTENT OF SVR THREAT INTO OH VALLEY
REGION. SVR THREAT THEREFORE IS MORE CONDITIONAL WITH NWD AND EWD
EXTENT WHERE AVAILABILITY OF SFC-BASED EFFECTIVE LIFTED PARCELS IS
PROGRESSIVELY MORE QUESTIONABLE.
WARM-SECTOR AIR MASS WILL DESTABILIZE DIURNALLY FROM COMBINATION OF
BOUNDARY-LAYER THETAE ADVECTION AND POCKETS OF SFC DIABATIC HEATING.
EXPECT SFC DEW POINTS RANGING FROM LOWER 60S NEAR OH RIVER TO UPPER
60S/LOW 70S OVER COASTAL AREAS OF LA/MS/AL. FCST SOUNDINGS SUGGEST
THIS WILL OFFSET MODEST MIDLEVEL LAPSE RATES ENOUGH FOR MLCAPE
1000-1500 J/KG FROM GULF COAST TO TN VALLEY REGION...DIMINISHING TO
BELOW 500 J/KG OVER OH RIVER. DOMINANT CONVECTIVE MODE SHOULD
REMAIN QUASI-LINEAR...AS WINDS ALOFT PRECEDING POSITIVELY-TILTED
TROUGH CROSS LOW-LEVEL CONVERGENCE ZONE AT SMALL ANGLE.
HOWEVER...WITH LACK OF SIGNIFICANT ELEVATED MIXED-LAYER AIR MASS
AHEAD OF FRONT...MLCINH SHOULD BE RATHER SMALL. THIS MAY PERMIT
DISCRETE OR CLUSTERED DEVELOPMENT AHEAD OF MAIN LINE...WITH ENOUGH
DEEP-LAYER SHEAR TO SUPPORT CONDITIONAL SUPERCELL POTENTIAL AHEAD OF
MAIN BAND...IN ADDITION TO ANY EMBEDDED SUPERCELLS/BOWS/MESOVORTICES
IN QLCS.
CONVECTIVE REGIME WILL SHIFT EWD ACROSS LOWER DELTA...MID-SOUTH...OH
VALLEY AND TN VALLEY REGIONS THROUGH EVENING...IN STEP WITH OR
SLIGHTLY AHEAD OF SFC COLD FRONT. DURING LATE EVENING AND OVERNIGHT
HOURS...FAVORABLE BUOYANCY WILL BE OVERTAKEN FROM N-S AS MAIN TSTM
BAND ENCOUNTERS MORE STABLE BOUNDARY-LAYER TRAJECTORIES EMANATING
FROM RIDGING OVER ATLANTIC COAST REGION. SVR POTENTIAL ACCORDINGLY
SHOULD DIMINISH OVER NRN PORTIONS OF OUTLOOK AREA...AND BECOME MORE
CONDITIONAL/MRGL EWD ACROSS ERN PORTIONS AL/TN INTO WRN GA.
..EDWARDS.. 11/20/2011





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