SWODY1
SPC AC 251255
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0755 AM CDT SAT OCT 25 2008
VALID 251300Z - 261200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
THE CLOSED MID LEVEL LOW OVER LAKE MI THIS MORNING IS EVOLVING INTO
AN OPEN WAVE IN RESPONSE TO UPSTREAM HEIGHT FALLS/MEAN FLOW INCREASE
WITH A STRONG MID-UPPER JET MOVING SEWD FROM THE SRN PRAIRIE
PROVINCES TOWARD THE NRN PLAINS. THE OPEN WAVE WILL EJECT EWD OVER
THE OH VALLEY TODAY...AND NEWD OVER THE SAINT LAWRENCE VALLEY AND
INTO QUEBEC OVERNIGHT. THE MID LEVEL TROUGH WILL BE ACCOMPANIED BY
A SURFACE COLD FRONT THAT WILL PROGRESS E OF THE APPALACHIANS BY
MIDDAY...REACHING THE MID ATLANTIC TO ERN NY BY LATE EVENING...AND
THE REMAINDER OF NEW ENGLAND OVERNIGHT. THE PRIMARY CONVECTIVE
THREATS IN THE CONUS WILL BE FOCUSED ALONG THE FRONT FROM THE MID
ATLANTIC INTO SRN NEW ENGLAND...AND ACROSS NRN MN/UPPER GREAT LAKES
IN ADVANCE OF THE DIGGING JET STREAK OVER THE NRN PLAINS.
...MID ATLANTIC/ERN NY/SRN NEW ENGLAND THROUGH EARLY TONIGHT...
INCREASING LOW-LEVEL MOISTURE WILL SPREAD NWD FROM THE WRN ATLANTIC
IN ADVANCE OF THE COLD FRONT...WITH BOUNDARY LAYER DEWPOINTS LIKELY
REACHING 58-60 F AS FAR N AS ERN NY BY LATE AFTERNOON.
HOWEVER...WIDESPREAD CLOUDS/RAIN WILL GREATLY LIMIT LOW-MID LEVEL
LAPSE RATES AND ANY POTENTIAL FOR SURFACE HEATING. THIS WILL LEAVE
THE PRE-FRONTAL ENVIRONMENT CHARACTERIZED BY MOIST PROFILES AND WEAK
INSTABILITY ROOTED ABOVE THE BOUNDARY LAYER...AND RATHER MEAGER
POTENTIAL FOR SURFACE-BASED INSTABILITY OR STRONG/DEEP DOWNDRAFTS.
STILL...LOW-LEVEL WIND FIELDS AND VERTICAL SHEAR WILL BE QUITE
STRONG IN ADVANCE OF THE COLD FRONT /60-70 KT SLY FLOW IN THE LOWEST
2 KM AGL/. A NARROW BAND OF FORCED CONVECTION COULD DEVELOP ALONG
THE FRONT DURING THE AFTERNOON AND PERSIST INTO EARLY
TONIGHT...WHERE DOWNDRAFTS WITHIN THE HEAVIER PRECIPITATION CORES
COULD TRANSPORT SOME OF THIS HIGH MOMENTUM FLOW TO THE GROUND.
...NRN MN TO THE UPPER GREAT LAKES LATE TODAY INTO TONIGHT...
AN INTENSIFYING MID-UPPER JET WILL TRANSLATE FROM THE SRN PRAIRIE
PROVINCES THIS MORNING TO THE UPPER MS VALLEY TONIGHT. COLD
TEMPERATURES ALOFT AND STEEP LOW-MID LEVEL LAPSE RATES ON THE
CYCLONIC SIDE OF THE JET /AS OBSERVED IN MANITOBA THIS MORNING/ WILL
COMBINE WITH DAYTIME HEATING TO SUPPORT WEAK SURFACE-BASED
INSTABILITY LATE THIS AFTERNOON FROM ERN ND INTO NRN MN.
ISOLATED-WIDELY SCATTERED LOW-TOPPED THUNDERSTORMS ARE EXPECTED THIS
AFTERNOON ACROSS THIS AREA. STEEP LOW-LEVEL LAPSE RATES AND 40-50
KT FLOW WITHIN A FEW THOUSAND FEET OF THE GROUND COULD SUPPORT
MARGINALLY SEVERE CONVECTIVE GUSTS WITH THE STRONGER SHOWERS/TSTMS
THIS AFTERNOON.
THE LEFT EXIT REGION OF THE MID-UPPER JET AND STEEP LAPSE RATES WILL
OVERSPREAD THE UPPER GREAT LAKES TONIGHT. A FEW THUNDERSTORMS WILL
CONTINUE OVERNIGHT...WITH BUOYANCY ENHANCED SOME BY THE SENSIBLE AND
LATENT HEAT FLUXES FROM THE RELATIVELY WARM LAKE MI.
..THOMPSON/GRAMS.. 10/25/2008
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