

000 

WWUS20 KWNS 240008

SEL6

SPC WW 240008

IAZ000-ILZ000-INZ000-MOZ000-LMZ000-240500-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 646

NWS STORM PREDICTION CENTER NORMAN OK

710 PM CDT THU AUG 23 2007



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       SOUTHEAST IOWA

       NORTHERN ILLINOIS

       EXTREME NORTHWEST INDIANA

       EXTREME NORTHEAST MISSOURI

       LAKE MICHIGAN



EFFECTIVE THIS THURSDAY NIGHT FROM 710 PM UNTIL MIDNIGHT CDT.



HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70

MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.



THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 45

STATUTE MILES NORTH AND SOUTH OF A LINE FROM 65 MILES WEST

NORTHWEST OF BURLINGTON IOWA TO 20 MILES SOUTH OF VALPARAISO

INDIANA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU6).



REMEMBER...A SEVERE THUNDERSTORM WATCH MEANS CONDITIONS ARE

FAVORABLE FOR SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH

AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR

THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS

AND POSSIBLE WARNINGS. SEVERE THUNDERSTORMS CAN AND OCCASIONALLY

DO PRODUCE TORNADOES.



OTHER WATCH INFORMATION...THIS SEVERE THUNDERSTORM WATCH REPLACES

SEVERE THUNDERSTORM WATCH NUMBER 640. WATCH NUMBER 640 WILL NOT

BE IN EFFECT AFTER 710 PM CDT. CONTINUE...WW 641...WW 642...WW

643...WW 644...WW 645...



DISCUSSION...THUNDERSTORM CLUSTERS WILL LIKELY PERSIST FOR THE NEXT

FEW HOURS ALONG THE COMPOSITE OUTFLOW BOUNDARY FROM NRN IL INTO SE

IA.  A FEED OF MODERATE INSTABILITY WILL MAINTAIN STRONG UPDRAFTS

CAPABLE OF PRODUCING LARGE HAIL...WHILE THE PRIMARY SEVERE THREAT

WILL BE DAMAGING OUTFLOW WINDS WITH STORM MERGERS AND BOWING

SEGMENTS.



AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT

TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500. MEAN STORM

MOTION VECTOR 27030.





...THOMPSON

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