Severe Thunderstorm Watch 821

Issued

Sep 2, 2011, 3:48 AM CDT (08:48 UTC)

Expired

Sep 2, 2011, 9:28 AM CDT

In force

6 hr

Issued by

the Storm Prediction Center

Largest hail expected

1.5 in

Strongest gust expected

60 kt

The Minneapolis radar (KMPX), 162 km from the middle of the area, began a volume at 08:49 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

Two or more tornadoes

10%

One or more EF2 to EF5 tornadoes

2%

Ten or more severe wind reports

40%

One or more gusts of 65 knots or more

10%

Ten or more severe hail reports

20%

One or more hailstones of 2 inches or more

5%

Six or more combined severe hail and wind reports

70%

• Mesoscale Discussion 2093Sep 2, 3:27 AM CDT
• Mesoscale Discussion 2094Sep 2, 7:29 AM CDT

867 
WWUS20 KWNS 020851
SEL1
SPC WW 020851
IAZ000-MNZ000-WIZ000-021600-

URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 821
NWS STORM PREDICTION CENTER NORMAN OK
350 AM CDT FRI SEP 2 2011

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

       NORTHERN IOWA
       SOUTH CENTRAL AND SOUTHEASTERN MINNESOTA
       SOUTHWEST WISCONSIN

EFFECTIVE THIS FRIDAY MORNING FROM 350 AM UNTIL 1100 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 45
STATUTE MILES EITHER SIDE OF A LINE FROM 10 MILES SOUTH SOUTHWEST
OF SPENCER IOWA TO 45 MILES EAST SOUTHEAST OF LA CROSSE
WISCONSIN.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU1).

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...CLUSTER OF TSTMS THAT HAS FORMED/STRENGTHENED OVER NW
IA IN THE PAST HOUR LIKELY IS ROOTED AROUND 800 MB...ABOVE SHALLOW
POST-COLD FRONTAL INVERSION.  NEVERTHELESS...GIVEN FACT THAT CLUSTER
IS EMBEDDED IN 35 KT MEAN...DEEP...UNIDIRECTIONAL WSW FLOW...AND
WILL HAVE ACCESS TO INCREASINGLY RICH MOISTURE WITH EWD EXTENT /PW
AOA 1.50 IN/...SETUP COULD SUPPORT SMALL BOWING SEGMENTS WITH HIGH
WIND.  THE STORMS SHOULD CONTINUE TO MOVE/DEVELOP E/ENE ACROSS NRN
IA AND PERHAPS SRN MN/SW WI ALONG ZONE OF INCREASING
FRONTOGENESIS/ASCENT ON S SIDE OF ND UPR IMPULSE.

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 500. MEAN STORM
MOTION VECTOR 26030.


...CORFIDI

Severe Thunderstorm Watches in Iowa, 2011Severe Thunderstorm Watches in Minnesota, 2011Severe Thunderstorm Watches in Wisconsin, 2011Every severe thunderstorm watch in 2011

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.