During the morning hours of July 21, a cold front extended from northcentral Wisconsin southwestward through northwestern Iowa.  Ahead of the cold front, over the upper midwest, a very humid, unstable airmass had set up.  By late morning, a small cluster of thunderstorms had developed over northwestern Illinois.  These storms tracked into northcental Illinois, reaching their peak intensity over Ogle County, producing wind gusts up to 58 mph and causing isolated incidents of damage to trees.  This storm cluster then weakend as it moved eastward.By early afternoon, more thunderstorms began to develop over northeastern Illinois.  A small area of strong thunderstorms initially developed over Kane and western Will Counties and rapidly developed into a line of severe storms extending over the Chicago metro area.  The line of storms rapidly moved through Du Page, Cook and Will Counties and into northwestern Indiana.  These storms left a path of wind damage over southern portions of the Chicago metro area and into rural sections of northwestern Indiana.  Strong wind gusts were also measured by coastal observing stations of Lake Michigan.  Numerous incidents of wind damage were observed, with a considerable number of trees knocked down or large limbs torn from trees.  Power poles were knocked down over many locations.  While the primary threat was strong winds, there were also isolated reports of penny to nickel sized hail as well.

Thunderstorm Wind in OGLE, ILLINOIS
From 21-JUL-04 09:00:00 to 21-JUL-04 09:00:00 (CST)
Location: KINGS
Magnitude: 50 EG
Reported by: GENERAL PUBLIC

Large limbs torn off of trees.

Thunderstorm Wind in OGLE, ILLINOIS
From 21-JUL-04 08:50:00 to 21-JUL-04 08:50:00 (CST)
Location: CHANA
Magnitude: 50 EG
Reported by: GENERAL PUBLIC

Trees knocked down by strong wind gusts.

Thunderstorm Wind in DU PAGE, ILLINOIS
From 21-JUL-04 15:20:00 to 21-JUL-04 15:30:00 (CST)
Location: NAPERVILLE
Magnitude: 60 EG
Reported by: AMATEUR RADIO

A 24 inch diameter tree was knocked down and a 12 inch branch was torn from a tree.

Hail in KENDALL, ILLINOIS
From 21-JUL-04 15:25:00 to 21-JUL-04 15:30:00 (CST)
Location: YORKVILLE
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 16:00:00 to 21-JUL-04 16:00:00 (CST)
Location: BURBANK
Magnitude: 60 EG
Reported by: TRAINED SPOTTER

Large tree limbs, trees, power line and power poles were knocked down.

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 16:01:00 to 21-JUL-04 16:01:00 (CST)
Location: BRIDGEVIEW
Magnitude: 60 EG
Reported by: AMATEUR RADIO

A 7 inch diameter tree was snapped in half at 87th St. and Roberts Rd.

Thunderstorm Wind in WILL, ILLINOIS
From 21-JUL-04 15:18:00 to 21-JUL-04 15:18:00 (CST)
Location: PLAINFIELD
Magnitude: 50 EG
Reported by: EMERGENCY MANAGER

A tree fell on to a power line.

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 16:00:00 to 21-JUL-04 16:00:00 (CST)
Location: CHICAGO LAWN
Magnitude: 60 EG
Reported by: TRAINED SPOTTER

Large tree limbs were down near 64th St. and Maple Ave.

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 16:00:00 to 21-JUL-04 16:00:00 (CST)
Location: ALSIP
Magnitude: 60 EG
Reported by: LAW ENFORCEMENT

Large limbs, several trees and flag poles were knocked down.

Hail in WILL, ILLINOIS
From 21-JUL-04 15:26:00 to 21-JUL-04 15:31:00 (CST)
Location: ROMEOVILLE
Magnitude: 0.88 
Reported by: EMERGENCY MANAGER

The hail was observed Interstate 55 and Budler Rd.

Thunderstorm Wind in OGLE, ILLINOIS
From 21-JUL-04 09:00:00 to 21-JUL-04 09:00:00 (CST)
Location: ROCHELLE
Magnitude: 50 EG
Reported by: LAW ENFORCEMENT

Large limbs torn off of trees.

Thunderstorm Wind in DU PAGE, ILLINOIS
From 21-JUL-04 15:33:00 to 21-JUL-04 15:33:00 (CST)
Location: WOODRIDGE
Magnitude: 50 EG
Reported by: EMERGENCY MANAGER

Tree limbs were down at the intersection of Rt. 53 and 75th St.

Thunderstorm Wind in WILL, ILLINOIS
From 21-JUL-04 15:15:00 to 21-JUL-04 15:20:00 (CST)
Location: BOLINGBROOK
Magnitude: 50 EG
Reported by: TRAINED SPOTTER

Tree limbs were reported down at the intersection of Boughton Rd. and Weber Rd and trees were knocked down at the intersection of Royce Rd. and Concord Rd.

Thunderstorm Wind in DU PAGE, ILLINOIS
From 21-JUL-04 15:42:00 to 21-JUL-04 15:42:00 (CST)
Location: WILLOWBROOK
Magnitude: 60 EG
Reported by: AMATEUR RADIO

Large tree limbs were reported down near Interstate 55 and Rt. 83.

Hail in KANE, ILLINOIS
From 21-JUL-04 15:00:00 to 21-JUL-04 15:05:00 (CST)
Location: MONTGOMERY
Magnitude: 0.88 
Reported by: EMERGENCY MANAGER

The hail was reported at the intersection of Rt. 30 and Rt. 31.

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 15:35:00 to 21-JUL-04 15:35:00 (CST)
Location: LEMONT
Magnitude: 55 EG
Reported by: AMATEUR RADIO

Trees were knocked down on Main St. between Cortland Rd. and Rt. 83.

Thunderstorm Wind in COOK, ILLINOIS
From 21-JUL-04 15:58:00 to 21-JUL-04 15:58:00 (CST)
Location: CHICAGO HGTS
Magnitude: 50 EG
Reported by: EMERGENCY MANAGER

Large tree limbs were reported down.

Hail in COOK, ILLINOIS
From 21-JUL-04 15:55:00 to 21-JUL-04 16:00:00 (CST)
Location: CHICAGO
Magnitude: 0.75 
Reported by: TRAINED SPOTTER

Large hail was reported near the intersection of 64th St. and Maple Ave.