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 the western Chicago suburbs and  rapidly developed into a line of severe storms extending over the rest of the metro area.  The line of storms rapidly moved eastward into northwestern Indiana.  These storms left a path of wind damage over southern and southeastern  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 up to 1 inch diameter hail as well.

Hail in JASPER, INDIANA
From 21-JUL-04 17:42:00 to 21-JUL-04 17:47:00 (CST)
Location: REMINGTON
Magnitude: 0.75 
Reported by: GENERAL PUBLIC

Thunderstorm Wind in JASPER, INDIANA
From 21-JUL-04 17:50:00 to 21-JUL-04 17:50:00 (CST)
Location: WHEATFIELD
Magnitude: 50 EG
Reported by: LAW ENFORCEMENT

Numerous trees were reported to be knocked down.

Thunderstorm Wind in NEWTON, INDIANA
From 21-JUL-04 17:15:00 to 21-JUL-04 17:15:00 (CST)
Location: LAKE VLG
Magnitude: 50 EG
Reported by: LAW ENFORCEMENT

Trees were down on State Road 10, near Lake Village.

Hail in JASPER, INDIANA
From 21-JUL-04 17:20:00 to 21-JUL-04 17:25:00 (CST)
Location: RENSSELAER
Magnitude: 1.00 
Reported by: EMERGENCY MANAGER

Thunderstorm Wind in LAKE, INDIANA
From 21-JUL-04 17:25:00 to 21-JUL-04 17:25:00 (CST)
Location: MERRILLVILLE
Magnitude: 50 EG
Reported by: TRAINED SPOTTER

Trees were damaged and a traffic light was broken on Rt. 30.

Thunderstorm Wind in NEWTON, INDIANA
From 21-JUL-04 17:25:00 to 21-JUL-04 17:25:00 (CST)
Location: MOROCCO
Magnitude: 50 EG
Reported by: LAW ENFORCEMENT

Trees were knocked down on 114th St.

Flash Flood in LAKE, INDIANA
From 21-JUL-04 16:25:00 to 21-JUL-04 19:25:00 (CST)
Location: HIGHLAND
Reported by: EMERGENCY MANAGER

Street flooding up to the curbs with portions of 45th St. impassible.

Thunderstorm Wind in LAKE, INDIANA
From 21-JUL-04 16:27:00 to 21-JUL-04 16:27:00 (CST)
Location: GARY
Magnitude: 65 EG
Reported by: GENERAL PUBLIC

Trees, limbs and power lines were reported down.

Thunderstorm Wind in PORTER, INDIANA
From 21-JUL-04 16:35:00 to 21-JUL-04 16:35:00 (CST)
Location: BEVERLY SHRS
Magnitude: 60 EG
Reported by: PARK/FOREST SERVICE

A 20 foot tree was knocked down and blocked a road at the Indiana Dunes National Seashore.

Thunderstorm Wind in PORTER, INDIANA
From 21-JUL-04 16:35:00 to 21-JUL-04 16:35:00 (CST)
Location: PORTER
Magnitude: 50 EG
Reported by: LAW ENFORCEMENT

Tree limbs and power lines were knocked down.