WATCHES: Severe Thunderstorm Watch Number 500

WWUS20 KWNS 212247
SEL0
SPC WW 212247
MTZ000-NDZ000-SDZ000-WYZ000-220600-

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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