Tornadoes in Chicago
Tornadoes have struck Chicago multiple times. Chicago, the largest city in Illinois and the third largest in the United States, lies in an area susceptible to severe weather year-round, with the Romeoville National Weather Service office providing continuous severe weather outlooks for Chicago and surrounding regions. The city has been struck by multiple tornadoes, including the devastating [1967 1967 Oak Lawn tornado outbreak|Oak Lawn tornado outbreak|Oak Lawn tornado] which struck the South Side in 1967, and a multiple vortex tornado that struck the Loop in 1876. Studies of tornadoes in the region show that as Chicago's suburbs grow, they become more susceptible to tornadoes, with the strongest tornado in the region striking Plainfield in 1990. Chicago was also the home of the severe weather researcher Ted Fujita, a professor at the University of Chicago, who extensively contributed to the scientific model of the tornado. The first tornado on record in Illinois, which struck modern-day Des Plaines, led to correspondence between a Chicago newspaper and the Smithsonian Institution, the efforts of which across the country eventually gave rise to the creation of the National Weather Service.
Study
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 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 "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
Chicago and surrounding areas are under the coverage of National Weather Service Chicago, Illinois, a National Weather Service office located in Romeoville on the grounds of Lewis University Airport. The National Weather Service also operates a Center Weather Service Unit in Aurora for the purposes of aviation. The forecast office, when fully staffed, has 13 or 14 employees, but only have 2 or 3 at any given time, who monitor Chicago-area weather all hours of the day. The number of active forecasters can be increased before a significant event. The National Weather Service works alongside local emergency management to relay warnings and other relevant information, and data from the service is used by broadcast meteorologists on local television across the region.WGN forecaster Mark Ratzer determined in 2018 that areas under the jurisdiction of the Chicago National Weather Service office are on average put under tornado watches on 6 days every year, with a further 15 days having a severe thunderstorm watch.
Cook County's hazard mitigation plan discusses the hazard of tornadoes, discussing their potential to have significant impacts in the "Safety and Security" and "Health and Medical" fields. Cook County had 15,345 manufactured homes, which are the structures most vulnerable to the impacts of a tornado. The total exposure of all structures vulnerable to a tornado was over $893 billion dollars. The Federal Emergency Management Agency assigned Cook County a "Very High" social vulnerability score for tornadoes, and assigned a risk index of 100 for all tornadic events.
Sirens
As of 2025, Chicago has a total of 112 warning sirens across the city; these are split into 12 siren zones. Sirens may be activated individually, by zone, or city-wide. Sirens are tested for 30 seconds on the first Tuesday of every month at 10:00a.m. local time. Sirens are sounded during tests and tornado warnings, alongside earthquakes, biological hazard incidents, extreme wind events, and general severe weather.In 2022, F Newsmagazine wrote that the sound of tornado sirens in Chicago is distinct compared to many others, containing both rising and falling tones alongside a third tone drop. It has been compared to a malfunctioning ambulance siren, with the host of News Center Maine stating that
f there is anything creepier than a tornado it’s this.The sound is produced by the Federal Signal Modulator model of siren's "alternate wail" mode. It was chosen to distinguish the sound of warning sirens from those of emergency vehicles.
Tornadoes in the city
1781 Portage tornado
A potential tornado occurred in the Chicago Portage region in the modern-day southwest of Chicago in 1781, where a traveller reported significant tree damage near Mud Lake. This may have been the first tornado in the Chicago region, however, this event is officially not acknowledged by the National Weather Service.1876 tornado
A tornado struck downtown Chicago on May 6, 1876, killing 2 and injuring 35 across its path. Structures damaged or destroyed include the Reaper Mansion, a candy factory, a county hospital, and a freight depot. A reporter with the Chicago Tribune published a description of the tornado's multiple vortex structure, well ahead of accepted scientific theories on the subject:While workers at the freight depot and the county hospital were trapped, they survived the tornado. WGN meteorologist Tom Skilling brought up the 1876 tornado as a response to a query about a tornado striking high-rises in downtown Chicago, despite its occurrence well before the construction of Chicago's high-rise buildings.
1967 Oak Lawn tornado
The Oak Lawn tornado occurred on April 21, 1967 and was described as the "worst storm of the day" during the 1967 Oak Lawn tornado outbreak. The tornado struck Oak Lawn where numerous homes were leveled. At the intersection of 95th Street and Southwest Highway, traffic led to numerous stopped cars being lofted into the air and dispersing in all directions. The tornado leveled a roller skating rink and mobile home park before striking Hometown and Evergreen Park. The tornado moved through Chicago's South Side, where the tornado widened and moved onto Lake Michigan near a water filtration park at 78th Street, where a wind gust of was recorded.Following the tornado, President Lyndon B. Johnson deployed 800 National Guard troops to Oak Lawn to aid in search-and-rescue operations. A later analysis of interviews following the tornado found that, while a majority of people in houses were sheltering in basements or in the process of doing so, many of those affected by the tornado had been away from home, in places where they would be unable to receive any warning of the tornado. Ahead of the storm, following the issuance of a tornado warning for McHenry county for the F4 Belvidere tornado, the Weather Bureau's warnings were extended for DuPage, Kane, and Cook counties, 24 minutes before the Oak Lawn tornado touched down, in what was described as "an outstanding example of the Weather Bureau’s Tornado Warning System"
July 2024 tornado event
A series of storms, including a potent derecho, impacted the Chicago region on July 14 and 15, 2024, producing two notable tornado events concentrated in the region, with as many as 5 tornadoes ongoing concurrently. Notably, 6 tornadoes directly impacted the city of Chicago; in addition to the numerous others from the Chicago National Weather Service office, the totals stood at 32 from the July 15 event; 35 on the calendar date of July 15, including those from overnight on July 14; 38 in the 24-hour period from 10:20p.m. from July 14 through July 15; all records for the Chicago National Weather Service office's area of responsibility. Similar records were broken for the entire state of Illinois during the event. The anomalously high tornado figure was noted as far exceeding the longstanding record of 32 in a single calendar year, a figure broken the previous year, 2023, in which 58 tornadoes were recorded to strike the region. In addition, the average amount of tornadoes to strike the region in the month of July was only 1, using 10 to 20-year climate records. Storms of a similar magnitude to the July 15 derecho were described as being such that the Chicago region would experience them every 5–10 years or so.The record-breaking amount of tornadoes during the event was attributed to three primary factors: increased understanding of tornado-producing weather systems; better detection of tornadoes via radar; and better surveying and volunteer reporting methods. The Chicago National Weather Service office conducted storm surveys for weeks after the event and determined the figure of 32 from the July 15 event primarily consisted of EF0 and EF1 tornadoes, with only one EF2 tornado occurring during the entire event.