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WWUS20 KWNS 212247

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URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 500

NWS STORM PREDICTION CENTER NORMAN OK

445 PM MDT SAT JUL 21 2012



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       EASTERN MONTANA

       WESTERN NORTH DAKOTA

       WESTERN SOUTH DAKOTA

       NORTHEAST WYOMING



EFFECTIVE THIS SATURDAY AFTERNOON FROM 445 PM UNTIL MIDNIGHT MDT.



HAIL TO 2 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS TO 70

MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.



THE SEVERE THUNDERSTORM WATCH AREA IS APPROXIMATELY ALONG AND 75

STATUTE MILES EAST AND WEST OF A LINE FROM 65 MILES NORTH

NORTHWEST OF WILLISTON NORTH DAKOTA TO 75 MILES SOUTH OF RAPID

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

ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU0).



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...STRONG TO SEVERE TSTMS SHOULD INCREASE INTO THE EVENING

HOURS ALONG LEE/THERMAL TROUGH SITUATED FROM ERN MT TO NERN WY. THIS

DEVELOPMENT IS IN RESPONSE TO DIURNAL HEATING CYCLE...TERRAIN

INFLUENCES...AND POSSIBLY SUBTLE/WEAK LARGE SCALE ASCENT ASSOCIATED

WITH A LOW AMPLITUDE SHORT WAVE TROUGH IN CANADA THAT WILL SLIGHTLY

SUPPRESS UPPER RIDGE AXIS OVER THE REGION. RESULTING STORMS WILL TAP

INTO A CORRIDOR OF STRONG SURFACE-BASED INSTABILITY ALIGNED WITH THE

LEE TROUGH WHERE CAPE VALUES OF 1000-2000 J PER KG ARE ESTIMATED

BASED ON LATEST OBSERVATIONS. COMBINATION OF EFFECTIVE SHEAR

MAGNITUDES ON THE ORDER OF 30-50KT AND DEEPLY MIXED BOUNDARY LAYER

AIR MASS SHOULD CONTRIBUTE TO BOTH LARGE HAIL PRODUCTION AND

DAMAGING WINDS. OVERALL STORM COVERAGE MAY NOT BE GREAT BUT ENSEMBLE

PROBABILITIES SUGGEST TWO AREAS OF POTENTIALLY GREATER SEVERE STORM

THREAT...ONE NEAR THE BORDER OF MT/ND INTO WRN ND THROUGH THE

EVENING...THE OTHER AROUND THE BLACK HILLS REGION OF SD.



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 30020.





...CARBIN

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