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SPC AC 181240
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0740 AM CDT TUE MAR 18 2008
VALID 181300Z - 191200Z
...THERE IS A MDT RISK OF SVR TSTMS OVER ERN TX...SRN AR...AND
NRN/WRN LA...
...THERE IS A SLGT RISK OF SVR TSTMS FROM THE SRN PLAINS ACROSS THE
LOWER MS VALLEY...
...SYNOPSIS...
SPLIT FLOW PATTERN IS EVIDENT IN SATELLITE IMAGERY WITH A
PROGRESSIVE SHORT WAVE TROUGH IN THE NRN BRANCH OF THE WESTERLIES
DIGGING SEWD ACROSS THE NRN PLAINS...WHILE AN UPPER LOW OVER NRN
MEXICO HAS BEEN MOVING SLOWLY EWD. THESE TWO SYSTEMS ARE FORECAST
TO PHASE TODAY WITH THE SRN STREAM UPPER LOW ACCELERATING ENEWD
ACROSS THE SRN PLAINS TONIGHT.
AT THE SURFACE...A SW-NE ORIENTED FRONT FROM THE MIDDLE RIO GRANDE
VALLEY ACROSS N CENTRAL TX AND SRN MO IS FORECAST TO MOVE SEWD...AS
A SURFACE LOW NE OF JCT GRADUALLY DEVELOPS NEWD ACROSS NERN TX INTO
AR. THE SRN PART OF THE FRONT IS EXPECTED TO MOVE MORE RAPIDLY EWD
ACROSS CENTRAL AND SERN TX THIS AFTERNOON AND EVENING...CONTINUING
ACROSS LA AND MS TONIGHT.
...CENTRAL/ERN TX INTO LA/SRN AR...
BAND OF THUNDERSTORMS FROM NE OF DRT INTO N CENTRAL TX IS SLOWLY
SPREADING EWD AS CELLS MOVE NEWD. MOST OF THE CONVECTION HAS BEEN
ELEVATED TO THE NORTHWEST OF THE FRONT...HOWEVER...SEVERAL
SURFACE-BASED SUPERCELLS ON THE LEADING EDGE OF CONVECTIVE BAND WERE
ABLE TO PERSIST FOR SEVERAL HOURS EARLIER THIS MORNING BEFORE
WEAKENING AROUND 11Z.
EAST OF THE FRONT OVER PARTS OF CENTRAL/ERN TX INTO LA AND SRN AR
TEMPERATURES HAVE REMAINED IN THE LOW 70S AND DEW POINTS ARE IN THE
MID/UPPER 60S. MODEL GUIDANCE IS CONSISTENT SHOWING INCREASING
CONVECTION DEVELOPING ALONG/AHEAD OF THE FRONT OVER ERN TX DURING
THE MORNING AND EARLY AFTERNOON. THE AIR MASS IS EXPECTED TO BECOME
MODERATELY UNSTABLE WITH MLCAPE OF 1000-2000 J/KG OVER PARTS OF ERN
TX INTO WRN LA...ESPECIALLY IN AREAS WHERE BREAKS IN CLOUDS WILL
ENHANCE DIABATIC HEATING. REGIONAL 12Z SOUNDINGS...WIND PROFILER
AND VWP DATA INDICATE VERY STRONG LOW LEVEL WINDS THAT VEER WITH
HEIGHT IN THE LOWEST 3 KM...WITH SLIGHT BACKING IN THE MIDDLE LEVELS
IN ASSOCIATION WITH THE 60-70 KT SSWLY WINDS AHEAD OF THE UPPER LOW.
LARGE CLOCKWISE TURNING HODOGRAPHS WITH EFFECTIVE SHEAR OF 50-60 KT
AND 0-1 KM SRH OF 200-400 MS/S2 WILL BE SUPPORTIVE OF SUPERCELLS
FOR ANY DISCRETE STORMS THAT DEVELOP THIS MORNING AND AFTERNOON.
STRONGER CELLS WILL BE CAPABLE OF PRODUCING LARGE HAIL...DAMAGING
WIND GUSTS...AND TORNADOES...WITH POTENTIAL FOR A FEW STRONG
TORNADOES.
THE PRIMARY SEVERE CONVECTIVE ACTIVITY IS EXPECTED TO DEVELOP INTO A
LINEAR SYSTEM WITH EMBEDDED BOW ECHOES/ISOLATED SUPERCELLS ACROSS
ERN TX DURING THE AFTERNOON...SPREADING EWD INTO LA AND THE LOWER MS
VALLEY TONIGHT. INSTABILITY IS FORECAST TO DIMINISH WITH EASTWARD
EXTENT OVER THE LOWER MS VALLEY LATER THIS EVENING WITH DAMAGING
WIND GUSTS BECOMING THE PRIMARY SEVERE THREAT AFTER
00-03Z...ALTHOUGH ISOLATED TORNADOES WILL STILL BE POSSIBLE GIVEN
THE CONTINUED VERY FAVORABLE LOW LEVEL SHEAR.
...MID MS AND TN VALLEY THROUGH SRN PARTS OF THE OH VALLEY...
DESPITE AN AXIS OF LOWER 60S DEWPOINTS ADVECTING NEWD ALONG THE
SW-NE BAROCLINIC ZONE...INSTABILITY WILL REMAIN WEAK IN THIS REGION
OWING TO EXPECTED WIDESPREAD CLOUDS AND MODEST MID LEVEL LAPSE
RATES. HOWEVER...VERTICAL WIND PROFILES WILL SUPPORT ORGANIZED STORM
STRUCTURES...INCLUDING BOW ECHOES AND ISOLATED SUPERCELLS...ALONG
THE SW-NE ORIENTED BOUNDARY WITHIN ZONE OF FORCING ASSOCIATED WITH
NEWD MIGRATING LOW LEVEL JET AND SURFACE LOW. ISOLATED DAMAGING WIND
AND TORNADOES WILL BE POSSIBLE. THE THREAT APPEARS SOMEWHAT MORE
MARGINAL TOWARD THE TN AND OH VALLEY WHERE INSTABILITY WILL BE MORE
LIMITED.
..WEISS/JEWELL.. 03/18/2008
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