DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 091958
SWODY1
SPC AC 091956

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0156 PM CST WED JAN 09 2013

VALID 092000Z - 101200Z

...NO SVR TSTM AREAS FORECAST...

THE ONLY CHANGE TO THE OUTLOOK AT 20Z IS TO REMOVE PART OF THE 5
PERCENT WIND DAMAGE AND 2 PERCENT TORNADO PROBABILITIES ALONG THE
MIDDLE TX COAST. THIS AREA IS BEHIND CONVECTION THAT IS MOVING INTO
FAR SE TX. THE SEVERE THREAT SHOULD CONTINUE THROUGH THE LATE
AFTERNOON ACROSS FAR SE TX AND FAR SW LA WHERE INSTABILITY IS VERY
WEAK BUT DEEP LAYER SHEAR COULD BE STRONG ENOUGH FOR A MARGINAL
SEVERE THREAT.

..BROYLES.. 01/09/2013

.PREV DISCUSSION... /ISSUED 1008 AM CST WED JAN 09 2013/

...SYNOPSIS...

A POLAR BRANCH SHORT-WAVE TROUGH OVER THE FAR NERN PACIFIC WILL
ADVANCE INTO THE PACIFIC NW AND CA THIS PERIOD...AMPLIFYING WITH
TIME OWING TO THE EQUATORWARD PROGRESSION OF MID TO UPPER-LEVEL JET
STREAKS INTO THE BASE OF THE PARENT TROUGH. THESE DEVELOPMENTS WILL
LEAD TO THE NWD ACCELERATION OF A SUBTROPICAL BRANCH UPPER LOW FROM
NRN MEXICO INTO W-CNTRL TX. THIS LATTER SYSTEM WILL BE ATTENDED BY
A 50-70 KT MIDLEVEL JET STREAK WHICH WILL OVERSPREAD THE ERN HALF OF
TX NWD THROUGH THE RED RIVER VALLEY.

IN THE LOW LEVELS...A SURFACE LOW ANALYZED ALONG THE TX COAST NEAR
PSX AS OF 16Z WILL DEVELOP NWWD INTO THE TX HILL COUNTRY BY
10/00Z...AND INTO NWRN TX BY THE END OF THE D1 PERIOD. MEANWHILE...
A WARM FRONT --DELINEATING CP AND MODIFIED CP AIR MASSES-- CURRENTLY
FROM THE LOW NEWD THROUGH E-CNTRL TX AND CNTRL LA WILL LIFT SLOWLY
NWD. A SECONDARY WARM FRONT --WHICH DEMARCATES THE MODIFIED CP AIR
FROM THAT OF TRUE MARITIME ORIGIN-- IS EXPECTED TO REMAIN ALONG THE
SHELF WATERS OF THE NRN GOM. FINALLY...A COLD FRONT WILL ADVANCE
EWD ALONG THE TX COAST TOWARD THE LOWER SABINE RIVER VALLEY.

...MIDDLE/UPPER TX COAST INTO SWRN LA...

AS OF MID-MORNING...A PRE-FRONTAL CONVECTIVE BAND EXISTED FROM NEAR
CLL SWD INTO THE NWRN GOM APPROXIMATELY 50-80 NM E OF BRO. THIS
ACTIVITY DEFINES THE SRN PORTION OF THE WARM CONVEYOR BELT OF THE
SUBTROPICAL BRANCH SYNOPTIC SYSTEM...WITH A BROAD ZONE OF ISENTROPIC
ASCENT AND DCVA AHEAD OF THE PARENT UPPER LOW LIKELY BEING THE
PRIMARY PROCESSES FORCING THIS ACTIVITY. LATEST CONVECTION-ALLOWING
AND MESOSCALE MODEL GUIDANCE ARE IN RELATIVELY GOOD AGREEMENT IN
SUGGESTING A GRADUAL EWD PROGRESSION OF THIS CONVECTIVE BAND INTO
THE LOWER SABINE RIVER VALLEY BY TONIGHT AS THE PARENT UPPER LOW
BEGINS TO LIFT NWD TOWARD THE RED RIVER VALLEY.

AS MENTIONED ABOVE...IT APPEARS THAT THE SRN-MOST WARM FRONT WHICH
DELIMITS THE NRN EXTENT OF A HIGH THETA-E...MT AIR MASS WILL REMAIN
OFFSHORE THROUGH THE PERIOD. THIS COUPLED WITH WIDESPREAD CLOUDS
AND PRECIPITATION WILL LIMIT AIR MASS DESTABILIZATION ALONG THE
COASTAL PLAINS OF TX INTO SWRN LA. THIS IS CONFIRMED BY
MODEL-DERIVED FORECAST SOUNDINGS WHICH SHOW THE MAINTENANCE OF A
SHALLOW...NEAR-SURFACE STABLE LAYER AND MLCAPE VALUES GENERALLY LESS
THAN 500 J/KG. THUS...DESPITE MODESTLY STRONG LOW-LEVEL AND
DEEP-LAYER SHEAR WHICH WILL PERSIST ACROSS THE REGION...THE OVERALL
SETUP DOES NOT APPEAR TO WARRANT ANY MORE THAN LOW TORNADO AND
DAMAGING WIND PROBABILITIES.

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