Weather Bureau office issued a tornado watch at 1:50 p.m. CDT covering the northern half of Illinois plus southern Wisconsin, eastern Iowa, and western Indiana.

[citation needed] At 3:50 P.M., a violent multiple vortex tornado, later given an F4 rating, moved through Belvidere, Illinois, damaging the high school and overturning buses.

The second violent tornado of the day in Illinois may have developed as far southwest as Elgin but was first observed at about 5:00 p.m. CDT near Fox River Grove,[6][7] though its path is officially believed to have begun near Middlebury.

[6] According to official plots from Storm Data, the tornado lifted after hitting Acorn Acres, though non-tornadic damages to trees and buildings occurred as far as the intersection of Illinois Route 63 and Gilmer Road.

The F4 tornado that swept through Palos Hills, Oak Lawn, Hometown, Evergreen Park, and skipped through Chicago's Southside, killed 33 people.

[11] Near the Little Red Schoolhouse, in what is now the Forest Preserve District of Cook County, an observer first noted a funnel cloud to the south, moving east with hail up to .75 inches (19 mm) in diameter—but he was unable to report to the Weather Bureau as his telephone failed to give a dial tone.

It then grew in size to 450 feet (150 yd) wide and entered Palos Hills, destroying about five buildings—including two frame homes and a brick home—and snapping trees.

[11] Over the next six minutes, the tornado attained its maximum intensity as it tore a 16.2-mile (26.1 km) swath of damage through Oak Lawn, Hometown, and Evergreen Park at a 60-mile-per-hour (97 km/h) ground forward speed.

[11] In Oak Lawn, the tornado threw 25–40 vehicles from the intersection of Southwest Highway and W. 95th St. (US-12/20),[6] killing 16 people who were stuck in traffic during the rush hour.

[5] At the same intersection, the tornado destroyed the gym at Oak Lawn Community High School, including the locker room to which the students had been evacuated.

[11] It destroyed the brick pro shop at Beverly Country Club at 87th and Western Avenue, trapping several people who suffered only minor injuries.

It finally moved offshore as a waterspout at Rainbow Beach, producing a wind gust up to 100 miles per hour (160 km/h) at a water filtration plant on the lakefront shore.

The destroyed buildings included a high school, grocery store, tavern, market, motel, drive-in theater, restaurant, numerous apartments, and two gas stations.