Tornadoes are one of nature's most identifiable phenomena. Perhaps the biggest reason for the interest in tornadoes is the devastation they produce for a feature that is of a tangible scale. The following is a study of significant tornadoes that occurred in the Chicago area between 1855 and 2002.
For the purpose of this study, a significant tornado is defined as being rated F2 or greater, or a tornado that has caused fatalities or injured at least 10 people. The Fujita scale, or F scale, estimates wind speeds based on damage intensity [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]. F scale is 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.
Sources for the data in the study include StormData, the Storm Predication Center, and Significant Tornadoes written by Tom Grazulis.
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
F-Scale Distribution of Significant Tornadoes
F2 tornadoes were most prevalent, while there were about 15 F3 and F4 tornadoes between 1855 and 2002. Only one F5 tornado ever crossed the Chicago area.
Monthly Distribution of Significant Tornadoes
Most of these tornadoes occurred in the spring, between March and June. The drop off is rather significant during the middle of summer, while the fall is another, yet smaller peak. Surprisingly, significant tornadoes were more prevalent than first thought during the month of November.
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/coverage was rapidly increasing, 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 since 1855 in the Chicago area. Putting these four tornadoes aside, there appears to be a general decrease in deaths and injuries since the 1960s. Only three significant tornadoes have occurred in the Chicago area since our Doppler radar was installed in 1993, therefore there is not enough data to show just how important this tool really is.
For the purpose of this study, an outbreak of significant tornadoes is defined as two or more significant tornadoes during the same calendar day across the Chicago area. This area is being studied in order to gain a perspective of the typical rate of occurrence, month of occurrence, and the amount of historical impact that can be associated with outbreaks of significant tornadoes across the Chicago area.
Monthly Distribution of Significant Tornado Outbreaks
14 significant tornado outbreaks have occurred between 1855 and 2002. The overwhelming majority of these outbreaks happened during the months of March, April, and May. An interesting finding is a secondary maximum of significant tornado outbreaks occurred during the month of November.
Decade Distribution of Significant Tornado Outbreaks
64% of all significant tornado outbreaks occurred during the 1950s, 1960s, and 1970s. The last major tornado outbreak was almost 30 years ago. According to trends in Chicago's tornado history, we are due for a significant tornado outbreak in the near future.
Deaths and Injury Statistics During Significant Tornado Outbreaks
About three quarters of all deaths and injuries from tornadoes occurred during outbreaks of multiple significant tornadoes. Although there have only been 14 of these events since 1855, these rather rare events present the most risk to the public. Better planning, communication and newer technology (such as the Doppler radar), may help to alleviate the amount of deaths and injuries during future tornado outbreaks.
Area Coverage of Significant Tornadoes Since 1950
Area maps such as the one above may initially be a little misleading. Non-meteorological factors, such as population density and county size may unfairly skew the data. To limit this, we have considered the time period between 1950-2002, eliminating much of the skewed data towards limited population centers. Note that according to the 2000 Census, Cook and DuPage counties had the highest population densities, followed by Lake (IL), Lake (IN) and Kane counties.
The figure above shows a pattern that may hold meteorological significance. Will county may be a little larger than the other counties, but its population density does not come close to DuPage or Lake (IL) counties. Lake county, Indiana is an average size county with a representative population density for a county within the Chicago Metro area. After considering these facts, the Will county, southern Cook county, and Lake county Indiana may be a local corridor of significant tornadoes. One meteorological reason for this may be Lake Michigan's ability to stall approaching warm fronts from the south, supplying a climatological synoptic pattern that favors significant tornado formation (with the enhanced wind shear and warm surge from the south).
Other Significant Tornadoes Outside 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 (which is already documented in the Chicago stats) 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 injured.
One of the biggest tornadoes of the April 3, 1974 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.
Kankakee - In addition to the 1948 F4 tornado which started in eastern Kankakee County (which is already documented in the Chicago stats) 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 injured.
One of the biggest tornadoes of the April 3, 1974 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.
Trends Since 1950
Jim Allsopp, Warning Coordination Meteorologist, performed all of the research and helped with the data analysis.
Steve Rogowski, Meteorologist, performed data analysis and created the graphs, figures, and this web page.