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889 

WWUS20 KWNS 262224

SEL9

SPC WW 262224

NDZ000-SDZ000-270600-



URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 689

NWS STORM PREDICTION CENTER NORMAN OK

525 PM CDT TUE JUL 26 2011



THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF



       SOUTH-CENTRAL NORTH DAKOTA

       CENTRAL SOUTH DAKOTA



EFFECTIVE THIS TUESDAY AFTERNOON AND WEDNESDAY MORNING FROM 525

PM UNTIL 100 AM 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 75 STATUTE

MILES EAST AND WEST OF A LINE FROM 20 MILES WEST OF CHAMBERLAIN

SOUTH DAKOTA TO 85 MILES NORTH NORTHEAST OF MOBRIDGE SOUTH

DAKOTA.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE ASSOCIATED

WATCH OUTLINE UPDATE (WOUS64 KWNS WOU9).



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 686...WW 687...WW 688...



DISCUSSION...INCIPIENT SURFACE-BASED TSTM DEVELOPMENT IS UNDERWAY

FROM PIR-MBG...N OF SURFACE LOW AND ATTENDANT WARM FRONT.

MODIFICATION OF 20Z ABR SOUNDING FOR CURRENT SURFACE CONDITIONS NEAR

WARM FRONT SUGGESTS THAT CAP HAS BECOME QUITE WEAK WITH MLCAPE

VALUES AROUND 2000-3000 J/KG.  MOREOVER...STRENGTHENING MIDLEVEL

WIND FIELD ASSOCIATED WITH DE-AMPLIFYING SHORT WAVE TROUGH OVER THE

NRN HIGH PLAINS COUPLED WITH BACKED LOW-LEVEL FLOW N OF WARM FRONT

IS RESULTING IN A RATHER STRONGLY SHEARED ENVIRONMENT.  AS

SUCH...SETUP APPEARS FAVORABLE FOR SUPERCELL EVOLUTION WITH A RISK

FOR TORNADOES...LARGE HAIL AND LOCALLY DAMAGING WINDS.



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 25020.





...MEAD

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