

742 

WWUS20 KWNS 041552

SEL7

SPC WW 041552

IAZ000-ILZ000-042200-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 557

NWS STORM PREDICTION CENTER NORMAN OK

1055 AM CDT SAT AUG 4 2012



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       EAST CENTRAL AND SOUTHEAST IOWA

       NORTHWEST ILLINOIS



EFFECTIVE THIS SATURDAY MORNING AND AFTERNOON FROM 1055 AM UNTIL

500 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 65

STATUTE MILES NORTH AND SOUTH OF A LINE FROM 20 MILES NORTH

NORTHWEST OF KNOXVILLE IOWA TO 45 MILES EAST OF MOLINE ILLINOIS.

FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED WATCH

OUTLINE UPDATE (WOUS64 KWNS WOU7).



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...A LINE OF STORMS ACROSS CENTRAL IA ALONG THE SURFACE

COLD FRONT WILL LIKELY INTENSIFY THIS AFTERNOON WHILE SPREADING EWD

ACROSS THE MS RIVER.  STRONG SURFACE HEATING ACROSS SE IA/NW IL WILL

CONTRIBUTE TO MODERATE-STRONG INSTABILITY TO SUPPORT THE

INTENSIFYING STORMS...WHILE DEEP-LAYER VERTICAL SHEAR WILL BE

SUFFICIENT FOR BOTH ORGANIZED LINE SEGMENTS/BOWS AND PERHAPS SOME

EMBEDDED SUPERCELL STRUCTURES.  LARGE HAIL WILL BE POSSIBLE WITH THE

STRONGEST STORMS...THOUGH THE PRIMARY THREAT SHOULD BE DAMAGING

WINDS THROUGH THE AFTERNOON.



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 550. MEAN STORM

MOTION VECTOR 27035.





...THOMPSON

