DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 251230
SWODY1
SPC AC 251228

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0628 AM CST TUE DEC 25 2012

VALID 251300Z - 261200Z

...THERE IS A MDT RISK OF SVR TSTMS FROM SERN TX THROUGH PORTIONS OF
CNTRL/SRN LA...CNTRL/SRN MS INTO THE SWRN QUARTER OF AL AND THE WRN
FL PNHDL...

...THERE IS A SLGT RISK OF SVR TSTMS FROM CNTRL TX EWD ACROSS THE
GULF COAST STATES TO WRN SC...

--SEVERE WEATHER OUTBREAK LIKELY TODAY AND TONIGHT FROM SERN TX
ACROSS THE GULF COAST STATES. STRONG...LONG-TRACKED TORNADOES AND
WIDESPREAD DAMAGING WINDS POSSIBLE--

...SYNOPSIS...

A 100+KT MIDLEVEL JET STREAK OVER SWRN TX WILL PROGRESS ESEWD TO
ALONG THE CNTRL GULF COAST BY LATE TONIGHT/EARLY WEDNESDAY...
CONTRIBUTING TO SUBSEQUENT AMPLIFICATION OF THE COMPANION SHORTWAVE
TROUGH AND EVENTUAL EVOLUTION OF A CLOSED CIRCULATION OVER THE LOWER
MS VALLEY. THESE DEVELOPMENTS WILL PROMOTE SIGNIFICANT FORCING FOR
ASCENT WITH A CORRIDOR OF 500-MB HEIGHT FALLS OF 100-150M/12HR
SPREADING FROM CNTRL TX INTO THE TN VALLEY BY 26/12Z.

IN THE LOW LEVELS...SURFACE CYCLONE OVER THE TX HILL COUNTRY WILL
DEEPEN TODAY WHILE DEVELOPING ENEWD ALONG A RETREATING WARM FRONT
INTO W-CNTRL MS BY 26/00Z AND MIDDLE TN BY THE END OF THE D1 PERIOD.
MEANWHILE...A STRENGTHENING COLD FRONT WILL SWEEP EWD THROUGH THE
SABINE RIVER VALLEY TODAY AND THE LOWER MS VALLEY OVERNIGHT.

...ERN TX EWD ACROSS THE GULF COAST STATES...

A NW-SE ORIENTED BAND OF ELEVATED TSTMS IS IN PROGRESS THIS MORNING
TO THE NORTH OF THE SURFACE WARM FRONT FROM N-CNTRL/NERN TX INTO
CNTRL LA...DRIVEN BY ISENTROPIC ASCENT AND WAA AT THE TERMINUS OF AN
INTENSIFYING LLJ. HERE...STEEPENING MIDLEVEL LAPSE RATES COINCIDE
WITH A MOISTENING...LOW-LEVEL AIR MASS...CONTRIBUTING TO MUCAPE
VALUES OF 1000-1500 J/KG PER 12Z FWD SOUNDING. GIVEN A KINEMATIC
ENVIRONMENT FEATURING 35-50 KT OF EFFECTIVE BULK SHEAR...THE SETUP
WILL REMAIN SUPPORTIVE OF SUPERCELLS CAPABLE OF LARGE HAIL.

TO THE S/SW OF THE ELEVATED TSTMS...AN OVERALL DEEPENING TO THE
SYNOPTIC SYSTEM WILL RESULT IN THE INTENSIFICATION AND BROADENING OF
A LLJ WHICH WILL AID IN THE NWD EXPANSION OF THE WARM SECTOR FROM
THE GULF COAST TODAY. WHILE DAYTIME HEATING WILL BE LIMITED BY
WIDESPREAD CLOUDS...STEEPENING MIDLEVEL LAPSE RATES AND BOUNDARY
LAYER DEWPOINTS RISING INTO THE MID 60S WILL YIELD A MODERATELY
UNSTABLE AIR MASS BY AFTERNOON TO THE S OF THE WARM FRONT WHERE
MLCAPE VALUES WILL APPROACH 1000-1500 J/KG.

LATEST CONVECTION-ALLOWING MODEL GUIDANCE SUGGESTS THAT
SURFACE-BASED OR NEAR SURFACE BASED STORMS /INCLUDING SUPERCELLS/
WILL DEVELOP AND INCREASE IN COVERAGE THIS MORNING INTO AFTERNOON
INVOF THE RETREATING WARM FRONT OVER PORTIONS OF SERN TX EWD INTO
CNTRL LA. BY MID TO LATE AFTERNOON...THESE DATA SUGGEST THAT STORMS
WILL BECOME ORIENTED IN A N-S ORIENTED BROKEN BAND /LIKELY COMPRISED
OF SUPERCELLS AND BOWING SEGMENTS/ FROM NEAR THE DEEPENING SURFACE
LOW ALONG THE MS/LA BORDER SWD ALONG THE SURGING COLD FRONT. A
SIMILAR CONVECTIVE CONFIGURATION IS LIKELY TO PERSIST ACROSS MS AND
AT LEAST THE WRN HALF OF AL THIS EVENING INTO TONIGHT.
THEREAFTER...INTENSE DCVA ASSOCIATED WITH NEGATIVELY-TILTED UPPER
SYSTEM AND LOW-LEVEL UPLIFT ALONG COLD FRONT MAY RESULT IN A
TRANSITION TO A NARROW BUT CONTINUOUS CONVECTIVE BAND WHICH WILL
ADVANCE EWD INTO GA...THE FL PNHDL AND WRN SC BY WEDNESDAY MORNING.

THE EWD PROGRESSION OF THE 100+KT MIDLEVEL JET STREAK ACROSS THE
WARM SECTOR AND CONCOMITANT INTENSIFICATION OF THE LLJ WILL RESULT
IN STRENGTHENING LOW-LEVEL AND DEEP-LAYER SHEAR...ESPECIALLY DURING
THE DEEPENING PHASE OF THE PARENT SYSTEM LATER TODAY INTO TONIGHT.
INDEED...FORECAST HODOGRAPHS AND PLAN VIEW KINEMATIC FIELDS INDICATE
EFFECTIVE BULK SHEAR VALUES OF 50-70 KT WITH EFFECTIVE SRH VALUES
INCREASING FROM 200-350 M2/S2 TODAY...TO IN EXCESS OF 400-500 M2/S2
TONIGHT.

EXPECT A MIXTURE OF SUPERCELLS AND BOWING LINE SEGMENTS/LEWPS TO
EVOLVE WITH AN ASSOCIATED RISK FOR TORNADOES...SOME OF WHICH COULD
BE POTENTIALLY LONG-TRACKED AND STRONG. DAMAGING WINDS WILL BECOME
INCREASINGLY WIDESPREAD LATER THIS AFTERNOON INTO THE OVERNIGHT
HOURS AS CONVECTION GROWS UPSCALE INTO A QLCS. FINALLY...HAIL
--SOME IN EXCESS OF 2 INCHES IN DIAMETER-- WILL BE POSSIBLE...
ESPECIALLY ACROSS PORTIONS OF SERN TX AND SWRN LA WHERE 12Z CRP/LCH
SOUNDINGS SHOWED THE PRESENCE OF A WELL-DEFINED EML WITH 700-500-MB
LAPSE RATES OF 8-8.5 C/KM.

..MEAD/LEITMAN.. 12/25/2012

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