WATCHES: Severe Thunderstorm Watch Number 646

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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