SWODY1
SPC AC 181940
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0140 PM CST WED FEB 18 2009
VALID 182000Z - 191200Z
...THERE IS A MDT RISK OF SVR TSTMS LATE THIS AFTN ACROSS PARTS OF E
CNTRL MS...NRN AND CNTRL AL INTO NRN/WRN GA....
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS MUCH OF THE CNTRL/ERN
GULF STATES INTO PARTS OF THE OH VALLEY....
DRY LINE STRUCTURE...NEAR THE INTERFACE OF THE PRE-FRONTAL THERMAL
RIDGE AXIS AND THE WESTERN GRADIENT OF LOW-LEVEL MOISTURE RETURN OFF
THE GULF OF MEXICO...APPEARS LIKELY TO REMAIN THE PRIMARY FOCUS FOR
VIGOROUS CONVECTIVE DEVELOPMENT THROUGH THE REMAINDER OF THE
AFTERNOON. THIS CURRENTLY EXTENDS FROM THE LOWER MISSISSIPPI VALLEY
INTO THE LOWER OHIO VALLEY...BUT IS SHIFTING EAST
SOUTHEASTWARD...AND WILL BE OVERTAKEN BY THE COLD FRONT ACROSS
KENTUCKY...MUCH OF TENNESSEE AND NORTHERN MISSISSIPPI DURING THE
NEXT FEW HOURS.
...KENTUCKY/TENNESSEE INTO MID ATLANTIC STATES...
AS THE EXIT REGION OF A 90-100 KT 500 MB JET STREAK SPREADS
NORTHEAST OF REGION...INTO THE UPPER OHIO VALLEY...IT IS UNCLEAR IF
ACTIVITY WILL EVOLVE INTO A MORE CONSOLIDATED FRONTAL LINE...OR
REMAIN IN THE FORM OF BROKEN LINES OR DISCRETE SUPERCELLS. BUT... A
SEVERE THREAT IS EXPECTED TO PERSIST UNTIL LAPSE RATES STABILIZE AS
THE NOSE OF THE PRE-FRONTAL THERMAL RIDGE BECOMES SUPPRESSED
SOUTHWARD INTO THE NORTHERN GULF STATES LATER THIS AFTERNOON.
VIGOROUS CONVECTIVE ACTIVITY MAY ATTEMPT TO SPREAD EASTWARD AHEAD OF
THE AMPLIFYING UPPER TROUGH AXIS...ACROSS THE CENTRAL APPALACHIANS.
HOWEVER...THIS WILL BECOME FURTHER REMOVED FROM AN INCREASINGLY
PINCHED OFF LOW-LEVEL INSTABILITY AXIS TO THE WEST OF THE
APPALACHIANS...AND THE SEVERE THREAT IS EXPECTED TO BECOME
PROGRESSIVELY DIMINISHED WITH EASTWARD EXTENT.
...LWR MISSISSIPPI VALLEY INTO THE SOUTH ATLANTIC COAST STATES...
HIGH RESOLUTION MODELS SUGGEST POTENTIAL FOR AN EVOLVING CONVECTIVE
CLUSTER IS NOT PARTICULARLY HIGH. BUT...THE BEST CHANCE FOR MORE
ORGANIZED CONVECTIVE DEVELOPMENT...IF IT WERE TO OCCUR...APPEARS TO
BE IN ASSOCIATION WITH STRONGER MID-LEVEL FORCING NEAR THE BASE OF
THE AMPLIFYING UPPER TROUGH. THIS IS EXPECTED TO GRADUALLY
DEVELOP/STRENGTHEN ACROSS NORTHERN ALABAMA INTO NORTHERN/CENTRAL
GEORGIA LATE THIS AFTERNOON AND EVENING... BEFORE SPREADING
SOUTHEASTWARD TOWARD THE SOUTH ATLANTIC COAST TONIGHT. SCATTERED
STORM DEVELOPMENT IS ALREADY UNDERWAY ACROSS PARTS OF
NORTHERN/CENTRAL ALABAMA...WHERE DEEP LAYER SHEAR IS STRONG AND
LOW-LEVEL HODOGRAPHS STILL SIZABLE...SUPPORTING AT LEAST THE RISK
FOR A COUPLE OF SUPERCELLS WITH THE POTENTIAL FOR TORNADOES LATE
THIS AFTERNOON AND EARLY EVENING.
OTHERWISE...BREAKS IN CLOUD COVER ARE ALLOWING FOR INCREASED
INSOLATION WITHIN A MOISTENING BOUNDARY LAYER EAST OF THE LOWER
MISSISSIPPI VALLEY THROUGH MUCH OF CENTRAL/SOUTHERN GEORGIA. AS DEW
POINTS CLIMB...MIXED LAYER CAPE OF 500-1000 J/KG WILL BECOME MORE
PREVALENT LATE THIS AFTERNOON. THIS WILL SUPPORT INTENSIFICATION OF
CONVECTION ALONG THE PRE-FRONTAL MOISTURE GRADIENT...WITH PERHAPS
ADDITIONAL STORMS DEVELOPING TO THE EAST.
DUE TO LINGERING MID-LEVEL INHIBITION...AND THE WEAK TO MODEST
NATURE OF THE MID/UPPER FORCING...STORMS PROBABLY WILL REMAIN
DISCRETE AND SOMEWHAT WIDELY SCATTERED. IN THE PRESENCE OF STRONG
DEEP LAYER SHEAR...A FEW SUPERCELLS ARE LIKELY. DESPITE A TENDENCY
FOR SHRINKING LOW-LEVEL HODOGRAPHS DUE TO WEAKENING AND VEERING
LOW-LEVEL WIND FIELDS...ENVIRONMENT MAY STILL SUPPORT THE RISK FOR
ISOLATED TORNADOES...IN ADDITION TO LARGE HAIL/DAMAGING WINDS.
..KERR.. 02/18/2009
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