SWODY1
SPC AC 261956
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0256 PM CDT SAT SEP 26 2009
VALID 262000Z - 271200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE LOWER OH VALLEY...
...LOWER OH VALLEY/MIDDLE MS VALLEY...
AN INCREASINGLY OPEN/PROGRESSIVE UPPER LOW WILL CONTINUE TO
TRANSITION EASTWARD OVER THE MIDDLE MS/LOWER OH RIVER VALLEYS
THROUGH TONIGHT. WHILE TOTAL CAPE IS RELATIVELY MODEST /AROUND 500 J
PER KG MLCAPE/...STEEP LAPSE RATE ENVIRONMENT/STRONG LOW LEVEL
BUOYANCY AND AMPLE VERTICAL SHEAR/AMBIENT VERTICAL VORTICITY
ATTENDANT TO THE UPPER LOW WILL CONTINUE TO SUPPORT SEVERE HAIL/WIND
AND PERHAPS A BRIEF TORNADO OR TWO INTO EARLY EVENING. REFERENCE
MESOSCALE DISCUSSION 2033 AND SEVERE TSTM WATCH 744 FOR ADDITIONAL
DETAILS.
...AL/GA...
WHILE HEAVY RAINFALL WILL LIKELY REMAIN THE PRIMARY RISK...VERY
MOIST AIRMASS AND A GRADUALLY STRENGTHENING DEEP LAYER WIND FIELD
MAY SUPPORT SOME SEMI-ORGANIZED/SUSTAINED BOWING SEGMENTS/PERHAPS
BRIEF SUPERCELLS IN VICINITY OF EASTWARD ADVANCING CONVECTIVE
LINE/EFFECTIVE COLD FRONT AND/OR NW-SE EFFECTIVE WARM FRONT ACROSS
GA. AS SUCH...DAMAGING WINDS/PERHAPS A TORNADO REMAIN POSSIBLE ON AN
ISOLATED BASIS.
...COASTAL NC...
WHILE A COOL/STABLE AIRMASS PERSISTS THIS AFTERNOON ACROSS THE
CAROLINAS...WILL MAINTAIN LOW TORNADO/WIND PROBABILITIES FOR COASTAL
NC FOR POSSIBLE LATTER PERIOD INLAND MOVEMENT OF WARM/MOIST SECTOR
ATTENDANT TO POTENTIALLY DEEPENING SURFACE LOW.
..GUYER.. 09/26/2009
.PREV DISCUSSION... /ISSUED 1111 AM CDT SAT SEP 26 2009/
LARGE SCALE PATTERN TRANSITION TO MORE FALL LIKE UNDERWAY AS COLD
UPPER LOW OVER THE PLAINS LAST SEVERAL DAYS NOW ON THE MOVE WITH
VIGOROUS UPSTREAM SYSTEM ONSHORE SWRN CANADA DRIVING INTO NRN PLAINS
BY SUN AM.
A VERY MOIST/TROPICAL AIR MASS HAS ADVECTED NEWD ACROSS SERN STATES
INTO SRN APPALACHIANS AHEAD OF UPPER LOW CIRCULATIONS AND ITS ZONE
OF LARGE SCALE ASCENT NOW MOVING EWD ACROSS MID MS/TN VALLEYS.
...LOWER OH VALLEY...
COLD LOW STILL WELL DEFINED AND LOCATED OVER NWRN MO THIS AM. BY
THIS EVENING CIRCULATION CENTER WILL HAVE MOVED INTO SRN IL PRECEDED
BY A DRY SLOT ASSOCIATED WITH THE BAND OF STRONG MID/UPPER WINDS
ROTATING EWD ACROSS LOWER OH/TN VALLEYS THIS AFTERNOON.
LOW LEVEL SHEAR IS EXPECTED TO REMAIN MARGINAL FOR ORGANIZED SEVERE
WITH LITTLE IN THE WAY OF SURFACE DEVELOPMENT EXPECTED. THE
STEEPENING LAPSE RATES WITH APPROACH OF COLD UPPER LOW...AND 50-60KT
OF DEEP LAYER SHEAR ALONG WITH SURFACE HEATING IN THE DRY SLOT
EXPECTED TO SUPPORT SURFACE BASED THUNDERSTORMS BY MID AFTERNOON
VICINITY LOWER OH VALLEY. WITH MLCAPES AT BEST NO GREATER THAN 1000
J/KG...SEVERE THREAT SHOULD BE PRIMARILY LARGE HAIL AND LOCALIZED
STRONG/DAMAGING WINDS. THERE IS THE POSSIBILITY OF ROTATING STORMS
GIVEN THE DEEP LAYER SHEAR WHICH WOULD ENHANCE SEVERE POTENTIAL
INCLUDING BRIEF TORNADOES. ACTIVITY SHOULD QUICKLY DROP BELOW
SEVERE THIS EVENING WITH LOSS OF HEATING AND THE EXPECTED LIMITED
LOW LEVEL SHEAR.
...AL/GA...
REF MCD 2031
WHILE AIRMASS VERY MOIST AND PWAT IN EXCESS OF 2 INCHES WEAK LOW
LEVEL SHEAR AND NEARLY SATURATED MOIST PROFILES WILL LIMIT SEVERE
POTENTIAL THIS AREA. SEE MCD 2031 FOR DETAILED DISCUSSION ON THIS
MARGINAL THREAT AREA.
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