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000 

WWUS20 KWNS 212137

SEL8

SPC WW 212137

NEZ000-SDZ000-220500-



URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 438

NWS STORM PREDICTION CENTER NORMAN OK

340 PM MDT THU JUN 21 2007



THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF



       MUCH OF WESTERN NEBRASKA

       MUCH OF WESTERN SOUTH DAKOTA



EFFECTIVE THIS THURSDAY AFTERNOON AND EVENING FROM 340 PM UNTIL

1100 PM MDT.



TORNADOES...HAIL TO 4 INCHES IN DIAMETER...THUNDERSTORM WIND

GUSTS TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE

AREAS.



THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 95 STATUTE

MILES EAST AND WEST OF A LINE FROM 80 MILES NORTH NORTHEAST OF

RAPID CITY SOUTH DAKOTA TO 45 MILES SOUTH OF NORTH PLATTE

NEBRASKA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU8).



REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR

TORNADOES AND 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.



OTHER WATCH INFORMATION...CONTINUE...WW 434...WW 435...WW

436...WW 437...



DISCUSSION...VERY TO EXTREMELY UNSTABLE AIR MASS ACROSS WATCH AREA.

STORMS ARE RAPIDLY DEVELOPING ON WRN EDGE OF MOSTLY STILL CAPPED AIR

MASS.  WITH 30-40 KT OF SHEAR AND 3000PLUS J/KG CAPE...SUPERCELLS

WILL QUICKLY FORM WITH VERY LARGE HAIL AND TORNADOES LIKELY.  STORMS

WILL EVOLVE INTO CLUSTERS BY EVENING WITH INCREASING WIND DAMAGE

POTENTIAL AS THEY DEVELOP COLD POOLS AND SPREAD SEWD.



AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL

SURFACE AND ALOFT TO 4 INCHES. EXTREME TURBULENCE AND SURFACE

WIND GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO

550. MEAN STORM MOTION VECTOR 30020.





...HALES

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