SPC AC 222334
DAY 1 CONVECTIVE OUTLOOK RESENT 2
NWS STORM PREDICTION CENTER NORMAN OK
0634 PM CDT TUE SEP 22 2009
VALID 222000Z - 231200Z
...NO SVR TSTM AREAS FORECAST...
...20Z UPDATE...
...DEEP SOUTH TEXAS...
CONVECTIVE DEVELOPMENT WITHIN A SURFACE TROUGH EXTENDING FROM SOUTH
OF THE RIO GRANDE RIVER INTO DEEP SOUTH TEXAS IS SLOWLY BEING
UNDERCUT BY A SHALLOW SURFACE COLD FRONT ADVANCING SOUTHWARD THROUGH
DEEP SOUTH TEXAS. THIS FRONT APPEARS LIKELY TO PROGRESS SOUTH OF
THE RIVER BY 22-23Z. UNTIL IT DOES...THE FRONT/TROUGH INTERSECTION
MAY BECOME THE FOCUS FOR INCREASINGLY VIGOROUS THUNDERSTORM STORM
ACTIVITY IN THE PRESENCE OF FAIRLY STEEP LAPSE RATES AND LARGE CAPE
ON THE ORDER OF 3000 J/KG. WHERE SURFACE TEMPERATURES HAVE HEATED
INTO THE MID/UPPER 90S...NEAR/WEST THROUGH SOUTHWEST OF THE
BROWNSVILLE AREA...SIZABLE BOUNDARY LAYER TEMPERATURE/DEW POINT
SPREADS MAY BE FAVORABLE FOR LOCALIZED DOWNBURSTS WITH POTENTIALLY
DAMAGING SURFACE GUSTS. OTHERWISE... SOME HAIL IS POSSIBLE...BUT
THIS THREAT APPEARS LIMITED BY WEAK VERTICAL SHEAR.
...CENTRAL STATES...
THE SHALLOW...CONVECTIVELY GENERATED OR ENHANCED LEADING EDGE OF
STRONGER SURFACE COOLING APPEARS TO HAVE ADVANCED OUT AHEAD OF THE
MAIN COLD FRONT ACROSS THE LOWER MISSISSIPPI VALLEY. AND...WEAK OR
UNFOCUSED LOW-LEVEL FORCING FOR CONVECTIVE DEVELOPMENT...COUPLED
WITH WEAK LOWER/MID TROPOSPHERIC FLOW FIELDS AND SHEAR...ARE
CONTRIBUTING TO LOW SEVERE POTENTIAL. POCKETS OF MODERATELY STRONG
POTENTIAL INSTABILITY ARE DEVELOPING WITH HEATING OF A VERY MOIST
AIR MASS CHARACTERIZED BY 70+F SURFACE DEW POINTS AND PRECIPITABLE
WATER AROUND 2 INCHES...FROM THE CENTRAL GULF COAST NORTHWARD INTO
PARTS OF THE TENNESSEE/LOWER OHIO VALLEYS. THIS COULD BE FAVORABLE
FOR LOCALIZED DOWNBURSTS WITH SURFACE GUSTS POSSIBLY APPROACHING
SEVERE LIMITS. BUT...HEAVY RAINFALL APPEARS THE PRIMARY POTENTIAL
HAZARD.
..KERR.. 09/22/2009
.PREV DISCUSSION... /ISSUED 1127 AM CDT TUE SEP 22 2009/
...SYNOPSIS...
THE CLOSED LOW OVER THE CENTRAL HIGH PLAINS IS EVOLVING INTO A
CUTOFF/EQUIVALENT BAROTROPIC SYSTEM THAT WILL REMAIN
QUASI-STATIONARY THROUGH TONIGHT. AS A RESULT...LITTLE SURFACE
REFLECTION OF THE MID LEVEL LOW IS EXPECTED IN THE WARM SECTOR...AND
WARM SECTOR FLOW/VERTICAL SHEAR WILL LIKEWISE REMAIN RELATIVELY WEAK
THROUGH THE PERIOD. ADDITIONALLY...OVERNIGHT PRE-FRONTAL CONVECTION
AND ASSOCIATED OUTFLOW HAVE PROGRESSED EWD INTO ERN IL...SEWD INTO
CENTRAL MS/LA... AND SWD INTO S TX. THE RICHER MOISTURE RESIDES E
OF THIS PRE-FRONTAL BOUNDARY...WITH THE NOTABLY STRONGER INSTABILITY
CONFINED TO MS/LA AND S TX WHERE CLOUDS ARE LESS PREVALENT AND
BOUNDARY LAYER DEWPOINTS ARE IN THE MID 70S. CONTINUED DAYTIME
HEATING WILL BOOST MLCAPE VALUES TO 2000-3000 J/KG THIS AFTERNOON
ALONG AND E/S OF THE OUTFLOW BOUNDARY ACROSS MS/LA AND S TX.
SOME BOUNDARY LAYER RECOVERY WILL OCCUR TODAY ACROSS ERN AR/NE LA/NW
MS WHERE CLOUD BREAKS WILL ALLOW DAYTIME HEATING TO ERODE THE
WEAKENING COLD POOL. HOWEVER...IT IS NOT CLEAR THAT THIS AREA OF
DESTABILIZATION WILL EXTEND FAR ENOUGH WEST TO BE IN PHASE WITH THE
SYNOPTIC COLD FRONT AND THE ERN FRINGE OF THE SOMEWHAT STRONGER MID
LEVEL FLOW. THE OUTFLOW BOUNDARY ACROSS S TX WILL ALSO CONTINUE TO
MOVE SWD AWAY FROM THE STRONGER MID LEVEL FLOW. THUS...RELATIVELY
WEAK VERTICAL SHEAR IN THE WARM SECTOR SUGGESTS THAT ORGANIZED/
PERSISTENT SEVERE STORMS ARE UNLIKELY...AND THAT ANY SEVERE STORM
THREAT WILL BE LIMITED TO ISOLATED LARGE HAIL AND/OR STRONG OUTFLOW
GUSTS WITH PRIMARILY MULTICELL STORMS ALONG THE REMNANT BOUNDARIES
THIS AFTERNOON/EVENING.
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