A Study of the Chicago Areas Significant Tornadoes

Tornadoes are one of nature's most awesome forces. Throughout history, many tornadoes have ravaged the Chicago metro area, and despite popular myth, several have struck within the city limits of Chicago. The following is a study of significant tornadoes that occurred in the Chicago area between 1855 and 2008.

For the purpose of this study, a significant tornado is defined as being rated F2/EF2 or greater, or any tornado that has caused fatalities or injured at least 10 people. Weaker tornadoes (F0 and F1) are more common but not as well documented as strong (F2 and F3) and violent (F4 and F5) tornadoes, especially before about 1950. The significant tornadoes account for most of the deaths, injuries, and property damage from tornadoes.

The Fujita scale, or F scale, estimated wind speeds based on damage intensity, where F0=40-72 mph, F1=73-112 mph, F2=113-157 mph, F3=158-206 mph, F4=207-260 mph, and F5=261-318 mph. The F scale was determined by conducting ground and air surveys of the damage path. The F scale was developed in the 1970s and was later applied to historical data, based on written accounts and photographs of tornado damage. Therefore the F scale ratings are subjective and may not accurately reflect the wind produced by the tornado.  The F scale was replaced by a new Enhanced F scale in February, 2007. Estimated wind intensity with the scale was adjusted, but the historical database will not change.  EF0=65 to 85 mph, EF1=86-110 mph, EF2=111-135 mph, EF3=136-165 mph, EF4=166-200 mph, EF5=>200 mph. To learn more about the enhanced F scale, go to http://www.spc.noaa.gov/efscale/.

Sources for the data in the study include StormData, the Storm Predication Center, Significant Tornadoes 1680-1991 written by Tom Grazulis and local data.

For the purpose of this study, the Chicago area is defined as McHenry, Lake, Kane, DuPage, Cook, Kendall, Will, and Lake (IN) counties.

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.

F-Scale Distribution of Significant Tornadoes 



F2 tornadoes were most prevalent, while there were about a dozen F3 and F4 tornadoes between 1855 and 2008. Only one F5 tornado ever crossed the Chicago area.

Monthly Distribution of Significant Tornadoes 


Most of the tornadoes occurred in the spring, between March and June. The drop off is rather significant during the middle of summer, while late summer and fall is another, yet smaller peak. Surprisingly, there have been as many significant tornadoes in November as in July.

Diurnal Distribution of Significant Tornadoes

For the purpose of this study, the hour indicated is the hour in which the tornado began. For example, if a tornado began at 155 PM and ended at 205 PM. It will be considered as occurring in the 100 PM hour. Time is local time. Most tornadoes occur in the afternoon and evening hours, with a strong peak showing up during the 5:00 PM hour. Tornadoes are extremely rare in the early morning hours. Solar radiation heats the ground, which heats the lower atmosphere. The atmosphere becomes most unstable during the warmest time of day, which is typically in middle to late afternoon. The atmosphere gradually cools and stabilizes after dark, diminishing the threat of severe storms and tornadoes. But tornadoes can occur anytime of day or night. 

Decade Distribution of Significant Tornadoes 


An obvious peak of significant tornadoes occurred in the 1950s, 1960s and 1970s. Although additional data was ingested from SPC starting around 1950, and the local population density was rapidly increasing in the 1950s through 1970s, this doesn't account for the return to pre-1950 trends starting again in the 1980s. Therefore, there is a good deal of certainty that most of the influence on this data is meteorological, a significant finding.

Deaths and Injuries from Tornadoes 


From this graph, it is evident that 4 tornadoes have been responsible for most of the deaths and injuries in the Chicago area; the Maywood-Melrose Park Tornado of March 28, 1920, the Belvidere and Oak Lawn Tornadoes of April 21, 1967, and the Plainfield Tornado of August 28, 1990.

It’s not surprising that 85% of the deaths and 75% of the injuries have resulted from the most violent tornadoes (F4s and F5s), even though these are rather rare across the Chicago area, accounting for only 16% of the significant tornadoes. If all tornadoes are considered, including weak, non-killer tornadoes, the violent tornadoes would only comprise about 2% of all tornadoes. However, a significant percentage of deaths and injuries were caused by smaller tornadoes (F0-F3). All tornadoes, regardless of size, have the potential to cause deaths and injuries.

Multiple Significant Tornado Days

When conditions are favorable for the formation of significant tornadoes, sometimes several significant tornadoes occur in the same area on the same day. Since 1950, when more reliable record keeping began, there have been ten days that produced multiple significant tornadoes in the Chicago area. The worst was April 21, 1967, when six significant tornadoes occurred, including three violent F4 tornadoes. There was a total of ten tornadoes including four F1s. The most recent event was June 7, 2008 when four significant tornadoes occurred. Before 1950 there were five multiple tornado days documented. There is probably not enough data to draw any conclusions. There have been two in March, four in April, three in May, one in June, one in July, one in August, one in September, none in October, and two in November.

County by County Breakdown

To look at the frequency of significant tornadoes by county, one must first level the playing field since counties vary greatly in size. Tornado frequency was calculated per hundred square miles. Doing this, Lake County Indiana tops the list with, followed by DuPage. Will County, popularly known by some as the Chicago area’s tornado alley, surprisingly only ranks sixth out of the eight counties in the study. Lake County Indiana also leads, along with Cook, with four violent (F4 or greater) tornadoes.

Other Significant Tornadoes Near the Chicago Metro Area

Rockford - On September 14, 1928 an F4 tornado had a 26 mile path from 8 miles south southwest of Rockford through the southeast side of the city and ended in Boone County. 14 people were killed, 100 injured, and $1.2 million in damage was done. This was part of a large outbreak. There were several other tornadoes in southern Wisconsin. 

Kankakee - In addition to the 1948 F4 tornado which started in eastern Kankakee County and passed through Lake County Indiana (which is already documented in the Chicago statistics) there was an F4 tornado April 17, 1963. It had a 70 mile path from 3 miles northwest of Essex to just west of Medaryville, IN. It affected Kankakee, Newton, Jasper, and Pulaski Counties. There was near F5 damage with several houses swept away in Kankakee County and again in Jasper County. The worst damage was in the Bradley-Bourbonnais area north of Kankakee. 1 person was killed and 70 were injured.

One of the biggest tornadoes of the April 3, 1974 super-outbreak began in rural Benton County, but the most intense damage was at Monticello in White County where the damage path was 1/2 mile wide. This F4 tornado had a 121 mile path but may have been a family of 3 tornadoes.
A deadly F3 tornado struck the small Illinois River town of Utica in LaSalle County April 20, 2004, killing 8 people.

Trends Since 1950

• 6 days with F4 or greater tornadoes - once every 9.8 years
• 13 days with F3 or greater tornadoes - once every 4.5 years
• 47 days with F2 or greater tornadoes - once every 1.3 years
• 10 multiple significant tornado days - once every 5.9 years
• It’s been 18 years since the last F4 or greater tornado.  

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.

Detailed Information for each of the 92 Tornadoes (PDF Format)