Some facts about Chicago's Significant Tornadoes
There were 92 significant tornadoes in the 8 county Chicago metro area between 1855 and 2008.
The deadliest tornado occurred on April 21, 1967 during an outbreak of 5 significant tornadoes. A violent F4 tornado formed in Palos Hills in Cook County and traveled through Oak Lawn and the south side of Chicago. 33 people died and 500 people were injured by this 200 yard wide tornado that traveled 16 miles and caused over $50 million in damage.
The most recent significant tornadoes occurred on June 7, 2008 over Will and Cook Counties.
The only F5 tornado to ever strike the Chicago area was on August 28 1990. This tornado formed near Oswego and passed through Plainfield, Crest Hill, and Joliet. The tornado killed 29, injured 350, and caused $165 million in damage along a 16 mile path.
https://www.weather.gov/lot/sigchitorn
Conclusions
The Chicago metro area, including the city of Chicago, is prone to being struck by significant tornadoes, and sometimes violent tornadoes.
Tornadoes are most common in spring with a secondary peak in late summer through mid fall
Tornadoes are most frequent from early afternoon through the evening, with the peak around 5:00 to 6:00 PM.
For unknown reasons, the period from the 1950s through the 1970s was very active and violent in the Chicago area.
The Chicago area is overdue for a major tornado. An entire generation of Chicagoans has been born since the last violent tornado, and many more have transferred to the area from other parts of the country and other parts of the world, and have not experienced a major tornado here. This poses the danger of a vulnerable and unprepared population. Areas that were once open farm fields have been developed, putting more people in harms way.