TORNADO WATCH #678

[NWIASpotter]

URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 678
NWS STORM PREDICTION CENTER NORMAN OK
315 PM CDT SUN AUG 1 2004

THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF

EXTREME NORTHWEST IOWA
SOUTHWEST MINNESOTA
EASTERN SOUTH DAKOTA

EFFECTIVE THIS SUNDAY AFTERNOON AND EVENING FROM 315 PM UNTIL 1000
PM CDT.

TORNADOES...HAIL TO 3 INCHES IN DIAMETER...THUNDERSTORM WIND GUSTS
TO 80 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE AREAS.

THE TORNADO WATCH AREA IS ALONG AND 75 STATUTE MILES EAST AND WEST
OF A LINE FROM 45 MILES WEST NORTHWEST OF ALEXANDRIA MINNESOTA TO
10 MILES SOUTH OF WORTHINGTON MINNESOTA.

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.

DISCUSSION...AIRMASS OVER EASTERN SD IS BECOMING VERY UNSTABLE WITH
DEWPOINTS NEAR 70F AND MLCAPE VALUES AROUND 3000 J/KG. COMBINATION
OF DAYTIME HEATING AND APPROACHING UPPER TROUGH WILL ERODE
CAP...LEADING TO ISOLATED SEVERE THUNDERSTORM DEVELOPMENT ACROSS WW
AREA IN THE NEXT 1-3 HOURS. STORMS WILL LIKELY BECOME SUPERCELLULAR
WITH A RISK OF VERY LARGE HAIL AND ISOLATED TORNADOES.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
SURFACE AND ALOFT TO 3 INCHES. EXTREME TURBULENCE AND SURFACE WIND
GUSTS TO 70 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO 500.
MEAN STORM MOTION VECTOR 30030.


...HART