SWODY1
SPC AC 251956
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0256 PM CDT SUN SEP 25 2011
VALID 252000Z - 261200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE ARKLATEX
AND MID MS VALLEY...
SEVERAL CHANGES HAVE BEEN MADE TO THE OUTLOOK AT 20Z. THE FIRST
CHANGE IS TO ADD A 5 PERCENT HAIL PROBABILITY ACROSS GA AND SC WHERE
ISOLATED LARGE HAIL HAS BEEN REPORTED BY A FEW OF THE STRONGER
CELLS. THE LOW-END SEVERE PROBABILITIES HAVE BEEN EXPANDED NWD INTO
PARTS OF WCNTRL NC WHERE THUNDERSTORMS ARE DEVELOPING ON THE NRN END
OF MODERATE INSTABILITY. THE SECOND CHANGE IS TO TRIM ON THE WRN
EDGE OF THE THUNDER-LINE ACROSS SCNTRL MO WHERE A COLD FRONT AND LOW
CLOUD COVER IS KEEPING THE AIRMASS STABLE. THE THIRD CHANGE IS TO
REMOVE PARTS OF ERN PA FROM THE THUNDER AREA WHERE THE MODELS DO NOT
DEVELOP STORMS. NO OTHER CHANGES HAVE BEEN MADE TO THE OUTLOOK.
..BROYLES.. 09/25/2011
.PREV DISCUSSION... /ISSUED 1130 AM CDT SUN SEP 25 2011/
...LOWER MS VALLEY AREA THIS AFTERNOON/EVENING...
A SERIES OF SPEED MAXIMA WILL CONTINUE TO ROTATE AROUND THE SRN/ERN
PERIPHERY OF THE CLOSED LOW OVER THE MID MS VALLEY. AT THE
SURFACE...A LOW IN EXTREME SE OK WILL DEVELOP ENEWD ACROSS AR THIS
AFTERNOON ALONG A SURFACE BAROCLINIC ZONE...REACHING THE LOWER OH
VALLEY THIS EVENING. THE LOW WILL THEN DEEPEN/OCCLUDE OVERNIGHT
WHILE MOVING NWD TOWARD SRN LAKE MI. AT THE SAME TIME...A TRAILING
COLD FRONT WILL CONTINUE SEWD ACROSS N/NE TX AND AR INTO NW MS AND
NRN LA...PROVIDING A FOCUS FOR SEVERE THUNDERSTORM DEVELOPMENT THIS
AFTERNOON/EVENING.
LOW-LEVEL MOISTURE IS RETURNING QUICKLY NEWD FROM E TX/LA...WITH
MID-UPPER 60S BOUNDARY LAYER DEWPOINTS NOW AS FAR NE AS SRN/CENTRAL
AR. THE MOISTURE WILL CONTINUE TO SPREAD NEWD THROUGH THIS
EVENING...WITH 60 DEWPOINTS POTENTIALLY REACHING AS FAR N AS WRN KY.
MEANWHILE...STEEP MIDLEVEL LAPSE RATES HAVE SPREAD EWD TO THE MS
RIVER FROM THE SRN PLAINS...ABOVE THE MOISTENING BOUNDARY LAYER.
THESE PROCESSES...IN COMBINATION WITH DAYTIME HEATING IN CLOUD
BREAKS...WILL RESULT IN MLCAPE VALUES OF 2000-2500 J/KG INTO AR.
LOW-MIDLEVEL ASCENT ALONG THE FRONT AND LOW-LEVEL DESTABILIZATION
WILL ALLOW THUNDERSTORM DEVELOPMENT BY MID AFTERNOON ACROSS
CENTRAL/NE AR...WITH CONVECTION EXPECTED TO DEVELOP BOTH NEWD FROM
THERE...AND BACKBUILD ALONG THE FRONT TOWARD NRN LA THIS EVENING.
DEEP-LAYER VERTICAL SHEAR AND INSTABILITY WILL BE FAVORABLE FOR
BROKEN BAND OF STORMS ALONG THE FRONT...WITH THE POTENTIAL FOR A FEW
EMBEDDED SUPERCELLS. LOW-LEVEL SHEAR AND MOISTURE WILL BE
SUFFICIENT FOR A TORNADO OR TWO WITH THE MORE DISCRETE STORMS ALONG
THE FRONT IN AR THIS AFTERNOON/EVENING. DAMAGING WINDS AND LARGE
HAIL WILL BE THE MAIN THREATS WITH THE BACK-BUILDING STORMS LATER
THIS EVENING...WHILE DAMAGING WINDS SHOULD BE THE PRIMARY CONCERN
FARTHER TO THE NE TOWARD THE LOWER OH VALLEY WHERE LOW-LEVEL
MOISTURE/INSTABILITY WILL BE MORE LIMITED. OVERNIGHT...VERTICAL
SHEAR WILL BE FAVORABLE FOR ROTATING STORMS ACROSS INDIANA/WRN OH TO
THE E OF THE DEEPENING LOW...BUT WEAK INSTABILITY WILL TEMPER THESE
THREATS.
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