Severe Thunderstorm Watch 558

Issued

Aug 4, 2012, 1:46 PM CDT (18:46 UTC)

Expired

Aug 4, 2012, 5:47 PM CDT

In force

4 hr

Issued by

the Storm Prediction Center

Largest hail expected

2 in

Strongest gust expected

60 kt

The Chicago radar (KLOT), 81 km from the middle of the area, began a volume at 18:46 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

20%

One or more EF2 to EF5 tornadoes

5%

Ten or more severe wind reports

70%

One or more gusts of 65 knots or more

40%

Ten or more severe hail reports

20%

One or more hailstones of 2 inches or more

10%

Six or more combined severe hail and wind reports

90%

• Mesoscale Discussion 1698Aug 4, 10:26 AM CDT
• Mesoscale Discussion 1699Aug 4, 1:09 PM CDT
• Mesoscale Discussion 1701Aug 4, 3:57 PM CDT

119 
WWUS20 KWNS 041851
SEL8
SPC WW 041851
ILZ000-INZ000-WIZ000-LMZ000-050100-

URGENT - IMMEDIATE BROADCAST REQUESTED
SEVERE THUNDERSTORM WATCH NUMBER 558
NWS STORM PREDICTION CENTER NORMAN OK
150 PM CDT SAT AUG 4 2012

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

       NORTHEAST ILLINOIS
       EXTREME NORTHWEST INDIANA
       SOUTHEAST WISCONSIN
       LAKE MICHIGAN

EFFECTIVE THIS SATURDAY AFTERNOON AND EVENING FROM 150 PM UNTIL
800 PM CDT.

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 70
STATUTE MILES EAST AND WEST OF A LINE FROM 35 MILES NORTHWEST OF
MILWAUKEE WISCONSIN TO 60 MILES SOUTH SOUTHEAST OF MARSEILLES
ILLINOIS.  FOR A COMPLETE DEPICTION OF THE WATCH SEE THE
ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU8).

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.

OTHER WATCH INFORMATION...CONTINUE...WW 557...

DISCUSSION...WELL-DEVELOPED MCS WITH EMBEDDED BOWING STRUCTURES WILL
CONTINUE TO MOVE EWD/ENEWD THROUGH THE AFTERNOON ACROSS N/NE IL AND
SE WI.  STRONG INSTABILITY IS PRESENT IN ADVANCE OF THE MCS AND
CONVECTIVE INHIBITION HAS WEAKENED WITH DAYTIME HEATING...SUCH THAT
NEW STORM DEVELOPMENT AHEAD OF THE MCS AND ADDITIONAL CELL MERGERS
WILL AUGMENT MCS MOTION.  THE ENVIRONMENT WILL REMAIN FAVORABLE FOR
ORGANIZED BOWING SEGMENTS AND PERHAPS SOME EMBEDDED SUPERCELL
STRUCTURES...ESPECIALLY ACROSS NRN IL WHERE THE STRONGEST
INSTABILITY RESIDES.  RELATIVELY WIDESPREAD DAMAGING WINDS CAN BE
EXPECTED WITH THIS MCS THROUGH THE AFTERNOON...AS WELL AS ISOLATED
LARGE HAIL.  AN ISOLATED TORNADO CANNOT BE RULED OUT WITH MERGING
STORMS AND CIRCULATIONS WITHIN THE MCS.

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 600. MEAN STORM
MOTION VECTOR 26045.


...THOMPSON

Severe Thunderstorm Watches in Illinois, 2012Severe Thunderstorm Watches in Wisconsin, 2012Severe Thunderstorm Watches in Indiana, 2012Every severe thunderstorm watch in 2012

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.