SWODY1
SPC AC 171220
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0720 AM CDT SUN AUG 17 2008
VALID 171300Z - 181200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATER THIS AFTERNOON INTO
TONIGHT FOR THE ORE CASCADES AREA...
...WA/ORE CASCADES AREA THIS AFTERNOON INTO TONIGHT...
A PLUME OF LOW-MID LEVEL MOISTURE HAS SPREAD NWD OVER ORE/WA...NEAR
AND IMMEDIATELY W OF THE CASCADES...AND THIS MOISTURE HAS SUPPORTED
A FEW THUNDERSTORMS OVERNIGHT FROM NW ORE INTO WRN WA. THIS INITIAL
CONVECTION WILL LIKELY WEAKEN LATER THIS MORNING...WITH ADDITIONAL
THUNDERSTORM DEVELOPMENT EXPECTED OVER THE ORE CASCADES LATER THIS
AFTERNOON/EVENING IN CONJUNCTION WITH DAYTIME HEATING AND THE
APPROACH OF EMBEDDED WEAK SPEED MAXIMA IN ADVANCE OF AN ERN PAC MID
LEVEL TROUGH. OBSERVATIONS OF STEEP MID LEVEL LAPSE RATES ATOP
BOUNDARY LAYER DEWPOINTS RANGING FROM THE MID 50S TO LOCALLY LOWER
60S WILL CONTRIBUTE TO MLCAPE VALUES OF 1000-1500 J/KG BY LATER THIS
AFTERNOON/EVENING. THE COMBINATION OF MODERATE INSTABILITY AND
GRADUALLY INCREASING VERTICAL SHEAR THIS AFTERNOON INTO TONIGHT
APPEARS SUFFICIENT FOR SUPERCELLS CAPABLE OF PRODUCING LARGE HAIL
AND DAMAGING WINDS.
...UPPER GREAT LAKES THIS AFTERNOON INTO EARLY TONIGHT...
A MID LEVEL SHORTWAVE TROUGH OVER SRN SASKATCHEWAN THIS MORNING WILL
MOVE ESEWD TO NRN WI/UPPER MI BY TONIGHT...IN PHASE WITH A LARGER
SCALE TROUGH OVER ONTARIO. A SURFACE COLD FRONT WILL ACCOMPANY THE
ONTARIO WAVE. THE PRE-FRONTAL WARM SECTOR WILL BE CHARACTERIZED BY
RESIDUAL BOUNDARY LAYER DEWPOINTS OF 58-62 F AND SURFACE
TEMPERATURES WILL WARM TO 85-90 F THIS AFTERNOON. THESE
NEAR-SURFACE CONDITIONS...COMBINED WITH A PLUME OF 7 C/KM MID LEVEL
LAPSE RATES...WILL RESULT IN MLCAPE VALUES OF 500-1250 J/KG IN
ADVANCE OF THE COLD FRONT.
WIDELY SCATTERED THUNDERSTORMS MAY FORM THIS AFTERNOON ACROSS WI/MI
AS DAYTIME HEATING REMOVES CONVECTIVE INHIBITION IN THE WARM SECTOR.
THE MORE FOCUSED AREA FOR THUNDERSTORM DEVELOPMENT WILL BE IN
PROXIMITY TO THE COLD FRONT ACROSS ONTARIO AND THE MN ARROWHEAD THIS
EVENING. DEEP LAYER VERTICAL SHEAR WILL BE RELATIVELY WEAK IN THE
WARM SECTOR AND A LITTLE STRONGER NEAR THE FRONT. MARGINALLY SEVERE
HAIL AND GUSTY WINDS WILL BE POSSIBLE WITH THE STRONGEST
STORMS...BUT THE OVERALL SEVERE THREAT WILL BE LIMITED BY MODEST
INSTABILITY/VERTICAL SHEAR AND THE ARRIVAL OF THE COLD FRONT
PRIMARILY AFTER DARK.
...SRN HIGH PLAINS THIS AFTERNOON/EVENING...
A CLOSED MID LEVEL LOW WILL DRIFT SEWD OVER CO THROUGH TONIGHT AS AN
UPSTREAM RIDGE OVER THE GREAT BASIN/NRN ROCKIES CONTINUES TO WEAKEN
GRADUALLY. SEVERAL DAYS OF RELATIVELY WIDESPREAD CONVECTION HAS
REDUCED LAPSE RATES IN THE AREA FROM NRN AZ ACROSS NM TO ERN CO.
THE NET RESULT WILL BE RATHER WEAK INSTABILITY IN THE ZONE OF
STRONGER VERTICAL SHEAR OVER NM /S OF THE MID LEVEL LOW/. ISOLATED
LARGE HAIL/DAMAGING GUSTS WITH A STORM OR TWO CANNOT BE RULED OUT
WHERE POCKETS OF SURFACE HEATING LOCALLY ENHANCE INSTABILITY FROM NM
INTO SE CO THIS AFTERNOON...BUT THE OVERALL SEVERE STORM THREAT
APPEARS RATHER MARGINAL.
...FL KEYS AND EXTREME S FL EARLY MONDAY...
TROPICAL STORM FAY IS FORECAST TO CROSS CUBA AND ENTER THE FL
STRAITS EARLY MONDAY MORNING. THE STORM IS NOT EXPECTED TO BE
PARTICULARLY STRONG BY THIS TIME AS A RESULT OF RECENT LAND
INTERACTIONS /SEE LATEST NHC ADVISORIES FOR DETAILS/.
STILL...LOW-MID LEVEL WINDS/VERTICAL SHEAR MAY INCREASE ENOUGH TO
SUPPORT A LOW PROBABILITY OF OUTER RAIN BAND SUPERCELLS ACROSS THE
KEYS/S FL NEAR THE END OF THE PERIOD.
..THOMPSON/LEVIT.. 08/17/2008
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