SWODY2
SPC AC 260558
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1258 AM CDT SAT SEP 26 2009
VALID 271200Z - 281200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE WRN AND CENTRAL
GREAT LAKES...
...SYNOPSIS...
MODELS ARE IN AGREEMENT INDICATING THE INTENSIFICATION/
AMPLIFICATION OF A SHORTWAVE TROUGH...CURRENTLY APPROACHING BRITISH
COLUMBIA...AS THIS SYSTEM DIGS SEWD FROM THE CENTRAL CANADIAN
PROVINCES THROUGH THE UPPER MS VALLEY TO GREAT LAKES/OH VALLEY.
MEANWHILE...THE UPPER LOW...NOW LOCATED OVER THE CENTRAL U.S. IS
EXPECTED TO REMAIN PROGRESSIVE...BUT SHOULD DE-AMPLIFY AS IT TRACKS
ENEWD TO THE MIDDLE AND NRN ATLANTIC COASTS.
A COLD FRONT ACCOMPANYING THE AMPLIFYING SYSTEM INTO THE GREAT LAKES
REGION SHOULD EXTEND SWWD FROM NRN MN TO THE CENTRAL PLAINS AT 12Z
SUNDAY. THIS BOUNDARY IS EXPECTED TO TRACK QUICKLY ESEWD THROUGH
MUCH OF THE GREAT LAKES AND OH VALLEY BY 12Z MONDAY. IN THE
EAST...A SURFACE LOW IS EXPECTED TO TRACK ENEWD FROM THE MID
ATLANTIC COAST...REACHING EAST OF MAINE BY THE END OF THE FORECAST
PERIOD.
...GREAT LAKES REGION...
STRONG HEIGHT FALLS /100-200 METERS AT 500 MB/ ARE EXPECTED TO
SPREAD ACROSS THE WRN/CENTRAL GREAT LAKES REGION FROM SUNDAY
AFTERNOON TO 12Z MONDAY. ALTHOUGH STRONG FORCING FOR ASCENT AND
STRENGTHENING WIND FIELDS ARE EXPECTED WITH THIS AMPLIFYING
MID/UPPER LEVEL TROUGH...MODELS SUGGEST THE DEGREE OF
DESTABILIZATION SHOULD TEND TO REMAIN SOMEWHAT LIMITED.
RESIDUAL SURFACE DEWPOINTS IN THE LOWER-MID 50S ARE EXPECTED TO
INCREASE INTO THE UPPER 50S TO NEAR 60 F ACROSS CENTRAL/ERN WI INTO
ERN UPPER MI BY MID-LATE SUNDAY AFTERNOON. GIVEN THE FAST MOVEMENT
OF THE DIGGING TROUGH/COLD FRONT...A RATHER NARROW CORRIDOR OF
HIGHER MOISTURE VALUES IS EXPECTED AHEAD OF THE COLD FRONT IN THE
WRN GREAT LAKES. PLUME OF STEEP MID LEVEL LAPSE RATES SPREADING EWD
AHEAD OF THE UPPER TROUGH AND ATOP THE MOISTURE RETURN/SURFACE
HEATING WILL RESULT IN A NARROW AXIS OF MLCAPE 500-1000 J/KG.
DESPITE THE WEAKER INSTABILITY...A STRONG ZONE OF DEEP FORCING WITH
THE VIGOROUS SHORTWAVE TROUGH IS EXPECTED TO SUPPORT A SQUALL LINE
DEVELOPING OVER CENTRAL/ERN WI LATER SUNDAY AFTERNOON AND SPREADING
EWD INTO LOWER MI SUNDAY NIGHT. STRONG WIND GUSTS WILL BE THE
PRIMARY THREAT WITH THIS LOW-TOPPED SQUALL LINE...THOUGH STEEP LAPSE
RATES AND STRONG DEEP LAYER SHEAR SUGGEST A THREAT FOR HAIL. A
TORNADO OR TWO MAY OCCUR WITH ANY INITIAL STORMS ALONG THE FRONT
OVER ERN/SERN WI WHERE INSTABILITY WILL BE GREATER AND LOW LEVEL
DIRECTIONAL SHEAR STRONGER. A DAMAGING WIND THREAT COULD PERSIST
EWD INTO WRN LOWER MI SUNDAY EVENING AS THE UPPER SYSTEM INTENSIFIES
FURTHER AND AT LEAST MARGINAL INSTABILITY DEVELOPS INTO THIS AREA.
...SRN NEW ENGLAND...
ALTHOUGH THE NAM/GFS INDICATE THE SURFACE LOW TRACKING ACROSS LONG
ISLAND SUNDAY AFTERNOON AND THROUGH SRN NEW ENGLAND SUNDAY
EVENING...CLOUDINESS/PRECIPITATION PRECEDING THE SURFACE LOW SHOULD
TEND TO MAINTAIN A MORE STABLE AIR MASS ACROSS SRN NEW ENGLAND.
THIS WOULD TEND TO LIMIT THE INLAND PROGRESSION OF A COASTAL/WARM
FRONT AND THE LIKELIHOOD FOR THE SURFACE LOW TO REACH TOO FAR
INLAND. ALTHOUGH INSTABILITY WILL BE QUITE WEAK ACROSS THIS REGION
SUNDAY AFTERNOON...STRONG LOW LEVEL SHEAR VALUES ALONG THE TRACK OF
THE SURFACE LOW COULD SUPPORT A BRIEF TORNADO. HOWEVER...THE MODEL
DIFFERENCES IN THE TRACK OF THE SURFACE LOW PRECLUDES THE
INTRODUCTION OF LOW SEVERE PROBABILITIES.
..PETERS.. 09/26/2009
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