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895 

WWUS20 KWNS 012343

SEL0

SPC WW 012343

SDZ000-020700-



URGENT - IMMEDIATE BROADCAST REQUESTED

SEVERE THUNDERSTORM WATCH NUMBER 640

NWS STORM PREDICTION CENTER NORMAN OK

640 PM CDT WED SEP 1 2010



THE NWS STORM PREDICTION CENTER HAS ISSUED A

SEVERE THUNDERSTORM WATCH FOR PORTIONS OF



       EASTERN SOUTH DAKOTA



EFFECTIVE THIS WEDNESDAY NIGHT AND THURSDAY MORNING FROM 640 PM

UNTIL 200 AM CDT.



HAIL TO 1.5 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 60

STATUTE MILES EAST AND WEST OF A LINE FROM 35 MILES EAST

NORTHEAST OF ABERDEEN SOUTH DAKOTA TO 55 MILES SOUTH OF MITCHELL

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...THUNDERSTORMS ARE BEGINNING TO INCREASE IN ADVANCE OF A

COLD FRONT AND DRY LINE MOVING EWD ACROSS CENTRAL SD.  INCREASING

SOUTHERLY LOW LEVEL WINDS WILL RESULT IN CONTINUED NWD RETURN OF LOW

LEVEL MOISTURE WITH THE ENVIRONMENT DESTABILIZING THIS EVENING.

VERTICAL WIND PROFILES EXHIBIT STRONG VEERING AND INCREASING FLOW

WITH HEIGHT...PROVIDING FAVORABLE SHEAR TO ENHANCE STORM

ORGANIZATION AND INTENSITY.  DYNAMIC FORCING FOR LARGE SCALE ASCENT

WILL INCREASE TONIGHT IN ADVANCE OF A NEGATIVELY TILTED SHORT WAVE

TROUGH MOVING EWD TOWARD THE NRN PLAINS...WITH THREAT FOR SEVERE

STORMS POSSIBLY CONTINUING INTO THE OVERNIGHT HOURS.



AVIATION...A FEW SEVERE THUNDERSTORMS WITH HAIL SURFACE AND ALOFT

TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE WIND GUSTS TO 60

KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 450. MEAN STORM

MOTION VECTOR 25035.





...WEISS

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