SWODY1
SPC AC 081951
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0251 PM CDT WED OCT 08 2008
VALID 082000Z - 091200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTN ACROSS PARTS OF
SE AL...SWRN GA...AND THE FLORIDA PANHANDLE....
...SYNOPSIS..
WITHIN THE STRONGER NORTHERN BRANCH OF SPLIT POLAR WESTERLIES...A
VIGOROUS SHORT WAVE TROUGH IS NOW PIVOTING EASTWARD NEAR/JUST NORTH
OF THE CENTRAL CANADIAN/U.S. BORDER AREA...TOWARD ONTARIO AND THE
UPPER GREAT LAKES REGION. THIS IS NEAR THE EXIT REGION OF A STRONG
ZONAL JET STREAK WHICH IS PROGGED TO CONTINUE NOSING EASTWARD
THROUGH THE LOWER GREAT LAKES REGION BY 12Z THURSDAY...AS THE
UPSTREAM PATTERN BEGINS TO AMPLIFY ACROSS THE EASTERN PACIFIC INTO
THE WESTERN UNITED STATES.
MEANWHILE...AS A DOWNSTREAM NORTHERN BRANCH IMPULSE BEGINS TO LIFT
NORTHEAST OF THE GREAT LAKES REGION...THE SOUTHERN BRANCH CLOSED
LOW...NOW EAST OF THE LOWER MISSISSIPPI VALLEY...WILL CONTINUE TO
BECOME CUT-OFF FROM THE STRONGER FLOW. THE EASTWARD MOTION OF THIS
SLOW MOVING SYSTEM WILL LIKELY SLOW FURTHER SOMETIME LATER TONIGHT
INTO THURSDAY...AS RIDGING BEGINS TO BUILD WITHIN THE NORTHERN
STREAM TO ITS NORTH...AHEAD OF THE DIGGING TROUGH ACROSS THE PACIFIC
COAST STATES AND INTERMOUNTAIN WEST.
...SOUTHEAST...
IN RESPONSE TO THE EVOLVING LARGE-SCALE UPPER PATTERN...A BELT OF
30-40 KT SOUTHERLY 850 MB FLOW FROM THE EASTERN GULF COAST THROUGH
THE LOWER GREAT LAKES...AHEAD OF A WEAKENING SURFACE FRONT...IS
PROGGED TO WEAKEN SUBSTANTIALLY THROUGH EARLY EVENING. AS THIS
OCCURS...LOW-LEVEL HODOGRAPHS ACROSS THE FLORIDA PANHANDLE THROUGH
SOUTHEAST ALABAMA AND SOUTHERN GEORGIA WILL GRADUALLY DECREASE IN
SIZE THROUGH THE REMAINDER OF THE AFTERNOON. AND...POTENTIAL FOR
MODERATE TO STRONG LOW-LEVEL MESOCYCLONES WITH A RISK FOR TORNADOES
IS EXPECTED TO DIMINISH CORRESPONDINGLY.
BUT...THE RISK FOR TORNADOES MAY NOT BECOME COMPLETELY NEGLIGIBLE AS
DISCRETE VIGOROUS CONVECTIVE DEVELOPMENT PERSISTS WITHIN A ZONE OF
BROADER SCALE ISENTROPIC ASCENT ONLY SLOWLY SPREADING EASTWARD
ACROSS SOUTHERN GEORGIA/NORTHERN FLORIDA LATE THIS AFTERNOON AND
EVENING. SOUTH OF A DEVELOPING SURFACE FRONT ACROSS THE CAROLINA
PIEDMONT INTO CENTRAL GEORGIA...IN THE WAKE OF A WEAKENING/
RETREATING SURFACE RIDGE...THE BOUNDARY LAYER MOISTURE VALUES AND
RELATIVE HUMIDITIES HAVE BECOME QUITE HIGH ACROSS THIS REGION.
AND...THE DECREASE IN LOW-LEVEL SHEAR MAY BE OFFSET SOMEWHAT BY
INCREASINGLY FAVORABLE AMBIENT VERTICAL VORTICITY NEAR THE
APPROACHING LOW/MID-LEVEL CIRCULATION.
OTHERWISE...INCREASING CONVECTIVE INSTABILITY ASSOCIATED WITH
MID-LEVEL COOLING/DRYING NEAR THE LOW MAY PROMOTE THE RISK FOR SMALL
TO MARGINALLY SEVERE HAIL IN ONGOING STORMS NEAR A LINGERING
LOW-LEVEL MOIST AXIS CURVING ACROSS PARTS OF WESTERN/CENTRAL ALABAMA
INTO NORTHEAST MISSISSIPPI. THIS POTENTIAL SHOULD DIMINISH THIS
EVENING AS UPPER SYSTEM CONTINUES SLOWLY EASTWARD... AND LOW-LEVELS
COOL IN RESPONSE TO LOSS OF DAYTIME HEATING AND/OR WEAKENING SURFACE
FRONTAL PASSAGE.
...PACIFIC NORTHWEST...
AS THE UPPER TROUGH DIGS INTO THE WESTERN STATES...LOW-LEVEL
MOISTENING AND MID-LEVEL COOLING ARE EXPECTED TO CONTRIBUTE TO
INCREASING INSTABILITY TO THE WEST OF THE CASCADES TONIGHT.
SCATTERED STORMS ASSOCIATED WITH A BAND OF EMBEDDED STRONGER FORCING
FOR UPWARD VERTICAL MOTION MAY APPROACH COASTAL AREAS TOWARD THE END
OF THE PERIOD.
...UPPER GREAT LAKES...
MID-LEVEL COOLING ON THE SOUTHERN FRINGE OF THE APPROACHING VIGOROUS
SHORT WAVE IMPULSE WILL CONTRIBUTE TO WEAK DESTABILIZATION AND AT
LEAST LOW PROBABILITIES FOR THUNDERSTORMS LATE THIS AFTERNOON AND
TONIGHT...FROM PARTS OF NORTHERN MINNESOTA INTO THE VICINITY OF LAKE
SUPERIOR.
..KERR.. 10/08/2008
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