

000 

WWUS20 KWNS 101727

SEL9

SPC WW 101727

IAZ000-NEZ000-SDZ000-102200-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 579

NWS STORM PREDICTION CENTER NORMAN OK

1230 PM CDT FRI AUG 10 2007



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       EXTREME NORTHWEST IOWA

       NORTHEAST NEBRASKA

       EXTREME SOUTHEAST SOUTH DAKOTA



EFFECTIVE THIS FRIDAY AFTERNOON FROM 1230 PM 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 35

STATUTE MILES NORTH AND SOUTH OF A LINE FROM 50 MILES WEST

SOUTHWEST OF YANKTON SOUTH DAKOTA TO 30 MILES EAST NORTHEAST OF

SIOUX CITY IOWA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU9).



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 CLUSTER OF SEVERE STORMS ALONG THE MO RIVER MAY

PERSIST INTO THE AFTERNOON WHILE DEVELOPING ESEWD ALONG THE

INSTABILITY GRADIENT.  MODERATE-STRONG INSTABILITY AND 30-40 KT

EFFECTIVE BULK SHEAR SHOULD SUPPORT STORMS WITH SOME SUPERCELL

STRUCTURES.  THOUGH THE INITIAL CONVECTION WAS LIKELY

ELEVATED...LOW-LEVEL DESTABILIZATION MAY ALLOW THE STORMS TO ROOT IN

THE BOUNDARY LAYER DURING THE AFTERNOON...WHICH WOULD INCREASE THE

THREAT FOR DAMAGING WINDS AND LARGE HAIL.



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 28025.





...THOMPSON

