

000 

WWUS20 KWNS 212046

SEL1

SPC WW 212046

NEZ000-SDZ000-220300-



URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 631

NWS STORM PREDICTION CENTER NORMAN OK

345 PM CDT TUE AUG 21 2007



THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF



       NORTHEAST NEBRASKA

       SOUTHEAST SOUTH DAKOTA



EFFECTIVE THIS TUESDAY AFTERNOON AND EVENING FROM 345 PM UNTIL

1000 PM CDT.



TORNADOES...HAIL TO 2.5 INCHES IN DIAMETER...THUNDERSTORM WIND

GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE

AREAS.



THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 70 STATUTE

MILES EAST AND WEST OF A LINE FROM 45 MILES NORTH NORTHWEST OF

YANKTON SOUTH DAKOTA TO 40 MILES EAST SOUTHEAST OF BURWELL

NEBRASKA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU1).



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 629...WW 630...



DISCUSSION...CUMULUS IS INCREASING E OF THE SURFACE LOW ALONG THE

SD/NEB BORDER...IN THE CORRIDOR FROM ONL TO 9V9.  THE CAP IS

WEAKENING THIS AREA AND THUNDERSTORM INITIATION APPEARS LIKELY IN

THE NEXT COUPLE OF HOURS...WITH STRONG INSTABILITY AND DEEP LAYER

VERTICAL SHEAR FAVORABLE FOR SUPERCELLS.  THE INITIAL STORMS MAY

POSE A RISK FOR A COUPLE OF TORNADOES GIVEN THE STRONG INSTABILITY

AND MODEST LOW-LEVEL SHEAR/LOWER 70S DEWPOINTS NEAR THE REMNANT

BOUNDARY ACROSS EXTREME NE NEB.  OTHERWISE...LARGE HAIL AND DAMAGING

WINDS WILL BE THE PRIMARY SEVERE THREATS THROUGH THE LATE EVENING.



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

SURFACE AND ALOFT TO 2.5 INCHES. EXTREME TURBULENCE AND SURFACE

WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO

600. MEAN STORM MOTION VECTOR 29025.





...THOMPSON

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