SWODY1
SPC AC 010551
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1251 AM CDT THU OCT 01 2009
VALID 011200Z - 021200Z
...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS SERN/N-CENTRAL TX AND
SWRN LA TO AR/MO OZARKS...
...SYNOPSIS...
UPPER-AIR PATTERN WILL REMAIN HIGHLY AMPLIFIED ACROSS CONUS THROUGH
PERIOD. CYCLONE INITIALLY CENTERED OVER SRN QUE IS FCST TO MOVE
SLOWLY IN CYCLONICALLY CURVING LOOP -- MAINLY N OF ST LAWRENCE
RIVER. MEANWHILE STG SHORTWAVE TROUGH -- NOW ANALYZED OVER
CENTRAL/NRN ROCKIES STATES -- IS FCST TO EVOLVE INTO CLOSED LOW BY
EARLY THIS PERIOD OVER NEB...MOVING EWD TO IA BY 2/12Z.
ASSOCIATED SFC CYCLONE -- NOW ELONGATED ALONG ARCHING COLD FRONT
OVER NWRN KS AND S-CENTRAL NEB -- IS FCST TO MOVE EWD INVOF KS/NEB
BORDER EARLY IN PERIOD...THEN MOVE/REDEVELOP UNDER CYCLONE
ALOFT...OVER NRN IA/SRN MN. AS THIS OCCURS...COLD FRONT WILL SWEEP
SEWD FROM CENTRAL PLAINS...REACHING ERN MO...N-CENTRAL AR...SERN
OK...AND W-CENTRAL TX BY ABOUT 2/00Z. FRONT THEN SHOULD PROCEED
EWD/SEWD TO S-CENTRAL TX...W-CENTRAL/SWRN LA...CENTRAL MS...AND WRN
KY BY END OF PERIOD...AS PARENT CYCLONE OCCLUDES. OLDER FRONTAL
ZONE -- TRAILING FROM NWRN ATLANTIC AND DEMARCATING NRN EDGE OF
MOIST/RETURN-FLOW AIR -- IS EXPECTED TO LIFT SLOWLY ENEWD ACROSS
ARKLATEX AND WRN GULF COASTAL REGION THROUGH PERIOD...BUT WILL BE
OVERTAKEN BY COLD FRONT FROM N-S ACROSS LOWER MS VALLEY REGION.
...PORTIONS SERN/N-CENTRAL TX AND SWRN LA TO AR/MO OZARKS...
EARLY-PERIOD TSTMS IN CLUSTERS AND BANDS MAY BE ONGOING OVER
PORTIONS MO/IA...ROOTED IN LAYER OF ELEVATED BUOYANCY AND STG
ISENTROPIC ASCENT TO LFC. MAIN CONCERN WITH ANY SUCH ACTIVITY WILL
BE MRGL SVR HAIL...CARRYING OVER THREAT FROM PREVIOUS PERIOD. AS
LLJ WEAKENS THROUGH MID-LATE MORNING...SO SHOULD THIS
CONVECTION...CROSSING MID/UPPER MS VALLEY.
MAIN SVR CONCERN IS WITH DIURNAL CONVECTIVE INITIATION INVOF SFC
COLD FRONT...WHERE ROBUST LINE OF TSTMS PRODUCING DAMAGING GUSTS AND
MOSTLY LOW-END SVR HAIL IS EXPECTED TO FORM BY MID-LATE
AFTERNOON....THEN MOVE EWD ACROSS CATEGORICAL OUTLOOK AREA. SOME
INITIATION-TIMING UNCERTAINTIES STILL EXIST...COMPELLING LOOSER W-E
GRADIENT OF PROBABILITIES THAN ACTUAL SVR POTENTIAL IS LIKELY TO
EVOLVE ACROSS SWRN MO...ERN OK AND N TX. ONCE TSTMS DO
FORM...EXPECT RAPID UPSCALE GROWTH NEWD AND SWWD ALONG/AHEAD OF COLD
FRONT. CONVECTIVE MODE MAY TRANSITION THROUGH BRIEF MIX OF
MULTICELLS AND SUPERCELLS TO LINEAR...RESULTING IN LONG SQUALL LINE
AFTER DARK FROM SERN OZARKS REGION AT LEAST TO UPPER TX COASTAL
PLAIN. THIS LINE SHOULD SHIFT EWD ACROSS PORTIONS LOWER MS VALLEY
THEREAFTER...THREAT DIMINISHING WITH EWD EXTENT AFTER DARK AS
1. FOREGOING BOUNDARY LAYER COOLS DIABATICALLY...REDUCING AVAILABLE
SFC-BASED BUOYANCY...AND
2. CONVECTIVE BAND OUTRUNS HIGHEST THETAE BOUNDARY LAYER AIR MASS
FROM N-S AND INGESTS MORE STABLE AND INCOMPLETELY MODIFIED
CONTINENTAL TRAJECTORIES.
STRONGEST VERTICAL SHEAR PROFILES ARE FCST TO OCCUR ACROSS NRN
PORTION OF OUTLOOK AREA...ESPECIALLY OVER OZARKS IN ZONE WHERE
BUOYANT ZONE WILL BE NARROWING AND DIMINISHING IN MAGNITUDE...AS
COLD FRONT OVERTAKES RETURN-FLOW BOUNDARY. SFC DEW POINTS MID 60S F
OVER SRN MO...AND UPPER 60S/LOWER 70S ACROSS MUCH OF ERN OK...AR AND
NRN/ERN TX -- WILL COMBINE WITH DIABATIC HEATING TO SUPPORT MLCAPES
RANGING FROM LESS THAN 1000 J/KG NEAR MO RIVER TO AROUND 3000 J/KG
OVER PORTIONS N-CENTRAL/NE TX.
SWLY SFC WINDS ARE EXPECTED TO LIMIT SFC CONVERGENCE AND LOW LEVEL
SHEAR OVER MUCH OF AREA...WITH SIGNIFICANT COMPONENT OF MEAN FLOW
VECTOR BEING PARALLEL TO FRONTAL ZONE. FCST HODOGRAPHS ARE ENLARGED
ACROSS OZARKS REGION DUE TO COMBINATION OF CONSIDERABLE BOUNDARY
LAYER SPEED SHEAR AND SLGT VEERING WITH HEIGHT...HOWEVER CONCERNS
OVER CONVECTIVE MODE AND EXTENT OF FAVORABLE INSTABILITY PRECLUDE
MORE THAN MRGL TORNADO PROBABILITIES ATTM.
..EDWARDS/GARNER.. 10/01/2009
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