SPC AC 171944
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0144 PM CST TUE FEB 17 2009
VALID 172000Z - 181200Z
...THERE IS A SLGT RISK OF SVR TSTMS TONIGHT ACROSS PARTS OF THE LWR
MS VALLEY....
...SOUTHEAST TEXAS THROUGH THE LWR MS/OH AND TENNESSEE VALLEYS...
SURFACE CYCLOGENESIS IS WELL UNDERWAY TO THE LEE OF THE CENTRAL
ROCKIES. AS THE SURFACE LOW CENTER CONTINUES TO DEEPEN AND MIGRATE
EAST OF THE CENTRAL HIGH PLAINS...TOWARD THE MIDDLE MISSISSIPPI
VALLEY THROUGH THE REMAINDER OF THE PERIOD...A SOUTHERLY RETURN FLOW
OF MOISTURE WILL STRENGTHEN OFF THE WESTERN OF MEXICO. UPPER
60S/LOWER 70S SURFACE DEW POINTS ARE CURRENTLY PRESENT ACROSS THE
WESTERN GULF OF MEXICO INTO THE TEXAS COASTAL PLAIN. AND...AT LEAST
UPPER 50S/LOWER 60S SURFACE DEW POINTS ARE EXPECTED TO ADVECT
NORTHEASTWARD ALONG AN AXIS THROUGH THE LOWER MISSISSIPPI VALLEY BY
12Z WEDNESDAY.
MOISTURE RETURN HAS BEEN QUICKER THAN THE RETREAT OF A COLD SURFACE
RIDGE...NOW ENCOMPASSING MOST AREAS FROM THE MISSISSIPPI VALLEY
EASTWARD. THIS HAS RESULTED IN INCREASING SHALLOW CONVECTION AND
PRECIPITATION WITHIN A ZONE OF BROAD ISENTROPIC ASCENT ALONG AND
EAST OF THE LOWER MISSISSIPPI VALLEY. HOWEVER...WARMING ALOFT
BENEATH SHORT WAVE RIDGING IS SUPPRESSING THE FORMATION OF
THUNDERSTORMS. THIS PROBABLY WILL REMAIN THE CASE THROUGH THE MID
EVENING HOURS...PERHAPS MUCH OF THE NIGHT...PARTICULARLY ALONG THE
AXIS OF SURFACE-BASED...OR NEAR SURFACE-BASED...DESTABILIZATION
DEVELOPING NORTHEASTWARD OUT OF SOUTHEAST TEXAS.
MODELS ARE STILL NOT PARTICULARLY CLEAR ON THE TIMING/LOCATION OF
THE INITIATION OF STORMS TONIGHT. HOWEVER...THE JUXTAPOSITIONING OF
STRONGER LOW-LEVEL MOISTENING AND STEEPENING ISENTROPIC ASCENT SEEMS
MOST PROBABLE BENEATH THE DIFLUENT REGIME BETWEEN THE POLAR AND
SUBTROPICAL JET AXES...EAST OF THE CREST OF THE UPPER RIDGE...
ACROSS EASTERN ARKANSAS INTO WESTERN TENNESSEE/NORTHERN MISSISSIPPI
DURING THE 03-06Z TIME FRAME. THIS ACTIVITY SHOULD DEVELOP EASTWARD
TOWARD THE UPPER TENNESSEE VALLEY OVERNIGHT. THE ONSET OF AT LEAST
WEAK COOLING IN THE CAPPING LAYER TOWARD THE 09-12Z TIME FRAME MAY
SUPPORT ADDITIONAL STORM DEVELOPMENT NEAR OR JUST WEST OF THE
MISSISSIPPI/ARKANSAS BORDER AREA. IF THIS OCCURS...
DESTABILIZATION IN THE PRESENCE OF STRONG SHEAR WILL BE MORE
SUPPORTIVE OF A SEVERE THREAT...WITH A FEW SUPERCELLS CAPABLE OF
PRODUCING AT LEAST LARGE HAIL POSSIBLE.
...CENTRAL CALIFORNIA...
A RESIDUAL CYCLONIC MID-LEVEL CIRCULATION CENTER IS IN THE PROCESS
OF MIGRATING INLAND....NORTH OF SAN FRANCISCO BAY. BUT...
OROGRAPHICALLY BACKED...SOUTHEASTERLY SURFACE WINDS BENEATH 20-30 KT
SOUTHERLY 850 FLOW ARE MAINTAINING SIZABLE CLOCKWISE CURVED
LOW-LEVEL HODOGRAPHS ACROSS THE INTERIOR VALLEY. THIS MAY PERSIST
THROUGH A LARGE PORTION OF THE AFTERNOON. GIVEN THE COLD MID-LEVEL
ENVIRONMENT ACROSS THIS REGION...THERE APPEARS TO BE ENOUGH
DEVELOPING BREAKS IN THE OVERCAST TO ALLOW FOR POCKETS OF SURFACE
HEATING AND DESTABILIZATION SUPPORTIVE OF LOW TOPPED STORM
DEVELOPMENT. IF THIS OCCURS...THE RISK FOR ROTATING UPDRAFTS
CAPABLE OF PRODUCING SOME HAIL OR A BRIEF TORNADO IS NOT COMPLETELY
NEGLIGIBLE.
..KERR.. 02/17/2009
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z
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