

757 

WWUS20 KWNS 131922

SEL1

SPC WW 131922

SDZ000-WYZ000-140300-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 491

NWS STORM PREDICTION CENTER NORMAN OK

120 PM MDT TUE AUG 13 2013



THE NWS STORM PREDICTION CENTER HAS ISSUED A



* SEVERE THUNDERSTORM WATCH FOR PORTIONS OF

  WESTERN SOUTH DAKOTA

  NORTHEASTERN WYOMING



* EFFECTIVE THIS TUESDAY AFTERNOON AND EVENING FROM 120 PM UNTIL

  900 PM MDT.



* PRIMARY THREATS INCLUDE...

  SEVERAL LARGE HAIL EVENTS WITH A FEW VERY LARGE HAIL EVENTS TO 2

    INCHES IN DIAMETER POSSIBLE

  SEVERAL DAMAGING WIND GUSTS TO 70 MPH POSSIBLE



THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 70

STATUTE MILES EAST AND WEST OF A LINE FROM 25 MILES NORTH OF

BUFFALO SOUTH DAKOTA TO 75 MILES SOUTH SOUTHEAST OF RAPID CITY

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

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU1).



PRECAUTIONARY/PREPAREDNESS ACTIONS...



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...ISOLATED SEVERE STORM DEVELOPMENT IS ALREADY UNDERWAY

TO THE WEST OF A CONVECTIVELY GENERATED OR ENHANCED MID-LEVEL

VORTICITY CENTER NOW MIGRATING SOUTHEASTWARD INTO THE WESTERN

DAKOTAS.  ADDITIONAL STORMS ARE BEGINNING TO FORM NEAR THE BLACK

HILLS...AND FURTHER DESTABILIZATION WITHIN THE LOW-LEVEL UPSLOPE

FLOW REGIME ACROSS WESTERN SOUTH DAKOTA APPEARS LIKELY WITH

ADDITIONAL HEATING THIS AFTERNOON.  DESPITE WEAK TO MODEST

SHEAR...LARGE-SCALE FORCING FOR UPWARD VERTICAL MOTION MAY

CONTRIBUTE TO A GROWING...INCREASINGLY ORGANIZED SOUTHEASTWARD

PROPAGATING MESOSCALE CONVECTIVE SYSTEM ACROSS MUCH OF THE REMAINDER

OF WESTERN SOUTH DAKOTA THROUGH EARLY EVENING.



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 500. MEAN STORM

MOTION VECTOR 32020.





...KERR

