WATCHES: Tornado Watch Number 689

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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