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WWUS20 KWNS 122027

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SPC WW 122027

IAZ000-MNZ000-NDZ000-SDZ000-130300-



URGENT - IMMEDIATE BROADCAST REQUESTED

TORNADO WATCH NUMBER 603

NWS STORM PREDICTION CENTER NORMAN OK

325 PM CDT THU AUG 12 2010



THE NWS STORM PREDICTION CENTER HAS ISSUED A

TORNADO WATCH FOR PORTIONS OF



       EXTREME NORTHWEST IOWA

       WEST CENTRAL AND SOUTHWEST MINNESOTA

       EXTREME SOUTHEAST NORTH DAKOTA

       EASTERN SOUTH DAKOTA



EFFECTIVE THIS THURSDAY AFTERNOON AND EVENING FROM 325 PM UNTIL

1000 PM CDT.



TORNADOES...HAIL TO 3 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 60 MILES NORTHWEST OF

ALEXANDRIA MINNESOTA TO 55 MILES SOUTHWEST OF SIOUX FALLS SOUTH

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

WATCH OUTLINE UPDATE (WOUS64 KWNS WOU3).



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 601...WW 602...



DISCUSSION...PRIMARY CONCERN WILL BE TIMING OF SURFACE-BASED

THUNDERSTORM DEVELOPMENT THIS AFTERNOON.  MODIFIED SOUNDINGS AND

OBJECTIVE ANALYSES SUGGEST THAT THE CAP IS WEAKENING ALONG AN E-W

BOUNDARY FROM NE SD INT W CENTRAL MN...BUT IT IS NOT CLEAR IS NEW

STORMS WILL EVOLVE FROM WITHIN THE BAND OF MIDLEVEL CONVECTION OVER

ERN SD...OR DEVELOP NEAR OR W OF THE MIDLEVEL BAND.  STILL...THE

BACKED LOW-LEVEL FLOW AND BOUNDARY LAYER DEWPOINTS IN THE 70S ARE

CONTRIBUTING TO A FAVORABLE ENVIRONMENT FOR SUPERCELLS WITH

TORNADOES AND VERY LARGE HAIL...ESPECIALLY NEAR THE OUTFLOW/WARM

FRONT.  OTHERWISE...THERE ARE SOME QUESTIONS ABOUT THE NRN EXTENT OF

THE TORNADO THREAT GIVEN SOME TENDENCY FOR THE BOUNDARY TO MOVE

SLOWLY NWD IN NE SD...WHICH MAY BE OFFSET BY RAIN REINFORCING THE

BOUNDARY IN MN.  FARTHER S...INSTABILITY IS STRONG...BUT LOW-LEVEL

SHEAR GRADUALLY WEAKENS CLOSER TO THE SD/NEB BORDER.



AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL

SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE

WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO

550. MEAN STORM MOTION VECTOR 25020.





...THOMPSON

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