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WWUS20 KWNS 041851

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URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 558

NWS STORM PREDICTION CENTER NORMAN OK

150 PM CDT SAT AUG 4 2012



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       NORTHEAST ILLINOIS

       EXTREME NORTHWEST INDIANA

       SOUTHEAST WISCONSIN

       LAKE MICHIGAN



EFFECTIVE THIS SATURDAY AFTERNOON AND EVENING FROM 150 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 70

STATUTE MILES EAST AND WEST OF A LINE FROM 35 MILES NORTHWEST OF

MILWAUKEE WISCONSIN TO 60 MILES SOUTH SOUTHEAST OF MARSEILLES

ILLINOIS.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU8).



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...CONTINUE...WW 557...



DISCUSSION...WELL-DEVELOPED MCS WITH EMBEDDED BOWING STRUCTURES WILL

CONTINUE TO MOVE EWD/ENEWD THROUGH THE AFTERNOON ACROSS N/NE IL AND

SE WI.  STRONG INSTABILITY IS PRESENT IN ADVANCE OF THE MCS AND

CONVECTIVE INHIBITION HAS WEAKENED WITH DAYTIME HEATING...SUCH THAT

NEW STORM DEVELOPMENT AHEAD OF THE MCS AND ADDITIONAL CELL MERGERS

WILL AUGMENT MCS MOTION.  THE ENVIRONMENT WILL REMAIN FAVORABLE FOR

ORGANIZED BOWING SEGMENTS AND PERHAPS SOME EMBEDDED SUPERCELL

STRUCTURES...ESPECIALLY ACROSS NRN IL WHERE THE STRONGEST

INSTABILITY RESIDES.  RELATIVELY WIDESPREAD DAMAGING WINDS CAN BE

EXPECTED WITH THIS MCS THROUGH THE AFTERNOON...AS WELL AS ISOLATED

LARGE HAIL.  AN ISOLATED TORNADO CANNOT BE RULED OUT WITH MERGING

STORMS AND CIRCULATIONS WITHIN THE MCS.



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 600. MEAN STORM

MOTION VECTOR 26045.





...THOMPSON

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