

000 

WWUS20 KWNS 231731

SEL0

SPC WW 231731

ILZ000-INZ000-WIZ000-LMZ000-240100-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 640

NWS STORM PREDICTION CENTER NORMAN OK

1230 PM CDT THU AUG 23 2007



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       NORTHERN AND CENTRAL ILLINOIS

       NORTHWEST INDIANA

       EXTREME SOUTHERN WISCONSIN

       LAKE MICHIGAN



EFFECTIVE THIS THURSDAY AFTERNOON AND EVENING FROM 1230 PM UNTIL

800 PM CDT.



HAIL TO 2 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 90

STATUTE MILES NORTH AND SOUTH OF A LINE FROM 10 MILES WEST

SOUTHWEST OF MOLINE ILLINOIS TO 30 MILES EAST NORTHEAST OF MEIGS

FIELD ILLINOIS.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU0).



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.



DISCUSSION...CONVECTIVE SYSTEM MOVING EWD ACROSS ERN IA IS EXPECTED

TO PROGRESS ENEWD AND INTENSIFY THIS AFTERNOON.  MCS IS IN ADVANCE

OF A SHORT WAVE TROUGH MOVING INTO EXTREME WRN IA...AND IS EMBEDDED

WITHIN A LARGELY UNIDIRECTIONAL WSWLY WIND FIELD WITH SPEEDS OF

40-50 KT ABOVE THE PBL.  VISIBLE IMAGERY SHOWS MAINLY CLEAR SKIES

OVER IL AND EXTREME SRN WI AHEAD OF THE MCS WHICH WILL ALLOW

CONTINUED STRONG HEATING OF VERY MOIST AIR MASS.  DAMAGING WIND

GUSTS AND OCCASIONAL HAIL WILL BE POSSIBLE WITH STRONGEST CELLS AS

THEY MOVE ENEWD 35-40 KT.



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

TO 2 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

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

MOTION VECTOR 24035.





...WEISS

