SWODY1
SPC AC 210547
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1147 PM CST TUE NOV 20 2007
VALID 211200Z - 221200Z
..THERE IS A SLGT RISK OF SVR TSTMS FROM PARTS OF THE ARKLATEX INTO
THE MID SOUTH...
..SYNOPSIS...
DYNAMIC LARGE-SCALE PATTERN WILL EXIST THIS PERIOD WITH THE PRIMARY
FEATURE OF INTEREST BEING EWD PROGRESSION OF MID-LEVEL TROUGH FROM
NRN AND CNTRL HIGH PLAINS INTO THE GREAT LAKES AND MID MS VALLEY.
EMBEDDED SHORT WAVE TROUGH AND ASSOCIATED 70-80 KT MID-LEVEL JET
STREAK CURRENTLY DIGGING SEWD THROUGH THE FOUR CORNERS REGION WILL
TRANSLATE EWD THROUGH THE CNTRL PLAINS...MID MS VALLEY AND
EVENTUALLY INTO THE GREAT LAKES BY THURSDAY MORNING.
IN THE LOW LEVELS...WEAK SURFACE LOW INITIALLY OVER ERN OK IS
FORECAST TO DEEPEN WHILE DEVELOPING RAPIDLY NEWD THROUGH THE MID MS
VALLEY TODAY INTO ERN OH/WRN PA BY THE END OF THE PERIOD.
TRAILING...STRONG COLD FRONT WILL PUSH EWD/SEWD...LIKELY EXTENDING
FROM LOW PRESSURE NEAR THE CONFLUENCE OF THE OH AND MS RIVERS SWWD
INTO ERN TX BY LATE AFTERNOON. THIS BOUNDARY WILL BE THE PRIMARY
FOCUS FOR STRONG TO POTENTIALLY SEVERE TSTM DEVELOPMENT THIS
AFTERNOON INTO EVENING.
..ARKLATEX INTO MID SOUTH/MID MS VALLEY...
21/00Z OBSERVED SOUNDINGS OVER THE LOWER MS VALLEY INDICATE THAT A
RELATIVELY MOIST BOUNDARY LAYER IS ALREADY PRESENT WITH DEWPOINTS IN
THE LOW/MID 60S AND LOWEST MEAN MIXING RATIOS OF 11-13 G/KG. SWLY
25-35 KT LLJ WILL SUPPORT THE NEWD TRANSPORT OF THIS AIR MASS INTO
THE MID MS AND LOWER OH VALLEYS LATER TODAY. WHEN COUPLED WITH
MODEST MID-LEVEL LAPSE RATES OF 6.5-7.0 C/KM...ENVIRONMENT IS
EXPECTED TO BECOME MODERATELY UNSTABLE BY AFTERNOON WITH MLCAPES OF
500-1000 J/KG OVER NRN PORTIONS OF SLIGHT RISK AREA...TO AS HIGH AS
1500-2000 J/KG ACROSS SERN TX/ERN LA.
LATEST SHORT TERM MODEL GUIDANCE SUGGESTS THAT PRE-FRONTAL CLUSTERS
OF TSTMS MAY BE ONGOING AT THE ONSET OF THE PERIOD OVER PORTIONS OF
AR INTO WRN PARTS OF TN AND MS IN ASSOCIATION WITH WEAK SHORT WAVE
TROUGH CURRENTLY MOVING THROUGH THE ARKLATEX. THESE STORMS ARE
EXPECTED TO INCREASE IN AREAL COVERAGE THROUGH THE DAY EWD ACROSS MS
INTO WRN AL AND MIDDLE TN...ALONG ERN EDGE OF DEVELOPING INSTABILITY
AXIS. THE MORE INTENSE STORMS WILL EXHIBIT ORGANIZED/ROTATING
UPDRAFTS WITH THE PRIMARY SEVERE THREATS BEING DAMAGING WINDS AND
PERHAPS A TORNADO.
MORE VIGOROUS TSTM DEVELOPMENT IS EXPECTED TO OCCUR THIS AFTERNOON
ALONG COLD FRONT FROM SERN MO/NERN AR SWWD INTO THE ARKLATEX AS
STRONGER HEIGHT FALLS ATTENDANT TO SHORT WAVE TROUGH AND MID-LEVEL
JET STREAK OVERSPREAD DESTABILIZING AIR MASS. WHILE THE STRONGER
MID AND HIGH-LEVEL WIND FIELDS WILL LAG SURFACE WARM SECTOR TO THE
W/NW...FORECAST SOUNDINGS INDICATE 35-40 KT OF DEEP...SWLY SHEAR
OVER THE MID SOUTH...DECREASING TO 25-30 KT ACROSS THE ARKLATEX.
THIS COMBINATION OF MODERATE INSTABILITY AND VERTICAL SHEAR SUGGEST
THE POTENTIAL FOR EMBEDDED BOWING AND SUPERCELL STRUCTURES WITH A
THREAT OF DAMAGING WINDS...LARGE HAIL AND PERHAPS A TORNADO OR TWO.
THIS SEVERE THREAT WILL LIKELY CONTINUE ACROSS WRN TN AND NRN/CNTRL
PARTS OF MS BEFORE WEAKENING LATER TONIGHT AS BOUNDARY LAYER SLOWLY
STABILIZES.
.MEAD/GUYER.. 11/21/2007
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