SWODY3
SPC AC 240709
DAY 3 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0209 AM CDT THU SEP 24 2009
VALID 261200Z - 271200Z
...SYNOPSIS...
UPPER LOW INITIALLY OVER THE MID/LOWER MO VALLEY IS FORECAST TO
EVOLVE INTO AN OPEN WAVE WHILE ACCELERATING EWD THROUGH THE OH
VALLEY IN ADVANCE OF AMPLIFYING...UPSTREAM TROUGH OVER WRN CANADA.
AT THE SURFACE...LOW PRESSURE INITIALLY OVER THE OH VALLEY IS
EXPECTED TO DEEPEN SLIGHTLY WHILE DEVELOPING NEWD INTO SWRN ONTARIO
AND THE LOWER GREAT LAKES. TRAILING COLD FRONT WILL PUSH EWD
THROUGH THE OH/TN VALLEYS AND EVENTUALLY CNTRL/SRN
APPALACHIANS...WHILE TRAILING EXTENSION SETTLES MORE SLOWLY SWD
THROUGH THE LOWER MS VALLEY AND CNTRL GULF STATES. E OF THE
APPALACHIANS...WEDGE FRONT WILL RETREAT NWD TOWARD THE MID ATLANTIC
STATES.
...OH/TN VALLEYS INTO LOWER MS VALLEY AND CNTRL GULF STATES...
A VERY NARROW WARM SECTOR IS FORECAST SATURDAY OVER THE UPPER OH
VALLEY INTO CUMBERLAND PLATEAU REGION OWING TO EWD PROGRESSION OF
COLD FRONT FROM THE W AND COOL...STABLE LOW-LEVEL AIR MASS
ASSOCIATED WITH SURFACE RIDGE OVER THE LOWER LAKES. THUS...DESPITE
A SIGNIFICANT INCREASE IN VERTICAL SHEAR AND FORCING FOR
ASCENT...SEVERE WEATHER POTENTIAL IS EXPECTED TO BE LIMITED BY THE
MARGINAL INSTABILITY AND NARROW NATURE OF WARM SECTOR. BOTH
VERTICAL SHEAR AND FORCING FOR ASCENT WILL DECREASE WITH SWD
EXTENT...THOUGH WARM SECTOR WILL WIDEN...SUPPORTING SCATTERED TO
NUMEROUS STORMS ALONG AND AHEAD OF COLD FRONT. ISOLATED SEVERE
STORMS ARE QUITE POSSIBLE OVER PARTS OF THE LOWER MS VALLEY AND
CNTRL GULF STATES...THOUGH OVERALL THREAT MAY BE LIMITED BY THE
MARGINAL SHEAR.
ISOLATED SEVERE STORMS WILL REMAIN POSSIBLE SATURDAY NIGHT ALONG AND
S OF RETREATING WEDGE FRONT E OF THE APPALACHIANS. HERE
TOO...STRONGER FORCING FOR ASCENT AND VERTICAL SHEAR WILL REMAIN TO
THE N OF REGION WITHIN STABLE ENVIRONMENT.
PORTIONS OF REGION MAY NEED TO BE UPGRADED TO SLIGHT RISK ONCE
FINER-SCALE DETAILS BECOME MORE CLEAR.
...LOWER MO VALLEY INTO MIDWEST...
ISOLATED SEVERE STORMS WILL BE POSSIBLE SATURDAY AFTERNOON INTO
EVENING WITHIN POST-FRONTAL AIR MASS INVOF MIDLEVEL COLD POOL
ATTENDANT TO UPPER SYSTEM. HERE...STEEP LAPSE RATES AND RESIDUAL
BOUNDARY LAYER MOISTURE WILL CONTRIBUTE TO POCKETS OF AIR MASS
DESTABILIZATION THROUGH THE DIURNAL HEATING CYCLE. VERTICAL SHEAR
WILL BE SUPPORTIVE OF ORGANIZED/ROTATING STORMS...AND SHOULD
SUBSEQUENT MODEL GUIDANCE INDICATE THE DEVELOPMENT OF STRONGER
INSTABILITY...A SLIGHT RISK MAY BECOME NECESSARY FOR AREA.
..MEAD.. 09/24/2009
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