A regional myth states that
Lake Michigan protects Chicago from tornadoes; this is not true, and while the relatively cooler air coming off the lake can influence some tornadoes, others have been seen forming near the lakefront and some have moved over the lake itself. Another myth states that urban development and
Chicago's tall buildings protect the city from tornadoes; buildings have a negligible effect on the formation and path of a tornado.
Ted Fujita was the lead figure for tornado science and research, and was a professor at the
University of Chicago. His research included comparing the
intensity of tornadoes to the damage patterns of the
atomic bombings of Hiroshima and Nagasaki, furthering the theory of the
downburst pattern of damaging winds, and developing the
Fujita scale for ranking tornadoes based on the damage they cause. Following the Plainfield tornado in 1990, Fujita said that a tornado in the
Chicago Loop, the most developed part of the city, couldn't be ruled out: A study by National Weather Service Chicago, Illinois found that a violent (F4–F5) tornado strikes the Chicago metropolitan area on average once every 9.8 years, with F3 and F2 tornadoes much more common at once every 4.5 and 1.3 years on average respectively. A majority of both deaths and injuries caused by tornadoes in the area were the result of F4 tornadoes, with a large amount of total fatalities in the region originating on just three days – March 28, 1920, April 21, 1967, and August 28, 1990. The study concludes by stating that "[t]he Chicago area is overdue for a major tornado", while also mentioning that, as rural areas are developed into populated places, more people are at risk of the impacts of a tornado. In a second study analyzing local storm reports between 2001 and 2020, the National Weather Service determined the time of year when severe weather happened based on the amount of days on which a severe weather report occurs in the Chicago region. The study found that there are on average three days with reports of severe weather between the months of May and August, with tornado days specifically occurring most commonly in May and June. Days with damaging wind events were found to be most common in June and July. Severe weather typically occurs between 1 and 9p.m. local time, with tornadoes being especially common between 4 and 8p.m. local time. The study also found that only about 5% of tornadoes in the region are rated F3 or higher, with only 1.6% being rated F4 to F5. An apparent increase in tornado reports between 1960 and 2020 was attributed to increased access to electronic means to report severe weather, a higher population in the Chicago and
Rockford metropolitan areas, and the rise of
trained volunteer storm spotters. An apparent decrease in the amount of days with significant tornadoes, those rated F2 or above, from about two every year to only one, was attributed to improved building codes. A 2014 study by Ashley et al. attempted to quantify the increasing risk of a tornado based on the development of suburban and exurban areas in the Chicago area using synthetic tornado paths, influenced by historical tornadoes including the
2011 Joplin tornado. A hypothetical "worst case" scenario was determined to impact as many as 200,000 people with direct tornadic intensity, whereas the overall population at risk of a tornado was increasing significantly with time, especially expanding high-density suburban development in the
Chicago metropolitan area, in what was dubbed an "expanding bull’s-eye effect". The highly dense risk exposure that exists in Chicago's central business district could pose a "catastrophic disaster potential" due to critical infrastructure being overwhelmed. == Preparations ==