SWODY2
SPC AC 211728
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1228 PM CDT MON SEP 21 2009
VALID 221200Z - 231200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
A LARGE CLOSED LOW...NOW IN THE PROCESS OF DEVELOPING ACROSS THE
CENTRAL HIGH PLAINS...IS EXPECTED TO REMAIN CUT-OFF FROM THE
STRONGER WESTERLIES...WHICH APPEAR LIKELY TO REMAIN NORTH OF THE
CANADIAN/U.S. BORDER AREA THROUGH THIS FORECAST PERIOD. MODELS ARE
SUGGESTIVE THAT A SLIGHT EASTWARD/NORTHEASTWARD SHIFT TO THE
CIRCULATION CENTER IS POSSIBLE DURING THE DAY TUESDAY...IN RESPONSE
TO A SIGNIFICANT IMPULSE PIVOTING AROUND ITS PERIPHERY. BUT...A
SLIGHT RETROGRESSION TOWARD THE CENTRAL ROCKIES IS POSSIBLE TUESDAY
NIGHT INTO EARLY WEDNESDAY...AS UPPER RIDGING BEGINS TO BUILD
NORTHEASTWARD THROUGH THE NORTHERN INTERMOUNTAIN REGION/
ROCKIES...DOWNSTREAM OF A POLAR IMPULSE APPROACHING THE BRITISH
COLUMBIA/PACIFIC NORTHWEST COAST. AT THE SAME TIME...GUIDANCE
INDICATES THAT SUBTROPICAL RIDGING WILL REMAIN STRONG ACROSS THE
GULF OF MEXICO AND SOUTHEASTERN STATES INTO THE WESTERN ATLANTIC.
...SOUTHERN PLAINS/WESTERN GULF COAST INTO LOWER MS VALLEY...
AN INCREASINGLY SHALLOW LEADING EDGE OF LOW-LEVEL COOLING ASSOCIATED
WITH THE EVOLVING CLOSED LOW IS EXPECTED TO CONTINUE TO ADVANCE
SOUTHEASTWARD TOWARD THE LOWER MISSISSIPPI VALLEY/WESTERN GULF COAST
REGION...WELL AHEAD OF THE STRONGER COOLING ALOFT...AND THE STRONGER
BELT OF CYCLONIC MID-LEVEL FLOW. WHILE THE PRE-FRONTAL LOW-LEVEL
ENVIRONMENT WILL LIKELY BE VERY MOIST...WITH SURFACE DEW POINTS IN
THE LOWER 70S...AND SUPPORTIVE OF MODERATE LEVELS OF CAPE...IT
APPEARS THAT LOWER/MID TROPOSPHERIC LAPSE RATES OF ANY SIGNIFICANCE
WILL BE GENERALLY CONFINED TO THE LOWER RIO GRANDE VALLEY/DEEP SOUTH
TEXAS. CLOUD COVER/PRECIPITATION ASSOCIATED WITH WEAKENING
CONVECTION COULD BE FAIRLY EXTENSIVE FROM SOUTHEAST TEXAS INTO THE
OZARKS EARLY IN THE PERIOD...LIMITING PRE-FRONTAL SURFACE HEATING.
AND...WEAKENING PRE-FRONTAL LOW-LEVEL CONVERGENCE MAY BECOME THE
MOST PROMINENT FORCING FOR WARM SECTOR CONVECTIVE DEVELOPMENT...IN
THE PRESENCE OF WEAK LOW-LEVEL FLOW FIELDS/SHEAR.
THESE UNCERTAINTIES ALL CONTINUE TO INDICATE UNCERTAIN/LOW SEVERE
POTENTIAL FOR TUESDAY/TUESDAY NIGHT. THERMODYNAMIC PROFILES
PROBABLY WILL BE FAVORABLE FOR AT LEAST ONE OR TWO LATE AFTERNOON
STRONG/SEVERE STORMS NEAR THE LOWER RIO GRANDE VALLEY...BUT IT IS
STILL UNCLEAR WHETHER OR NOT ANY SUCH ACTIVITY WILL REMAIN CONFINED
TO THE HIGHER TERRAIN WEST OF THE RIVER.
..KERR.. 09/21/2009
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