A new study completed this summer has shed some light on strong-violent tornado trends across north central and northeastern Illinois and northwestern Indiana.

Data was collected on all 202 F2-F5 (EF2-EF5) tornadoes to have occurred across the current National Weather Service Chicago/Romeoville forecast area since 1880. A brief summary of the findings is listed below.

1. Of the 202 F2-F5 tornadoes since 1880, 134 were F2, forty-three were F3, twenty-four were F4, and one was F5, the Plainfield tornado of August 28^th, 1990.
2. F2-F5 tornadoes have been responsible for 289 fatalities and 3,751 injuries since 1880.
3. F2-F5 tornadoes have an average path length of 13.0 miles and an average path width of 197 yards.
4. The period from 1956-1978 saw an exceptionally active period of strong-violent tornado activity across the Chicago/Romeoville Forecast Area. The decades of the 1960s and 1970s each saw exactly three dozen F2-F5 tornadoes, eleven more than the next highest decade, the 1950s. All other decades saw sixteen or fewer, with the least being five in the 1900s.

    

   

5. Our peak daytime period for strong-violent tornadoes is from 3 PM CST – 6 PM CST, with no strong-violent tornadoes having occurred during the midnight CST, 4 AM CST, and 10 AM CST hours. 

    

   

6. April is the peak month for strong-violent tornado activity. May and June follow closely behind, with August rounding out the top four months. 

    

   

7. DuPage County features the highest number of strong-violent tornadoes per square mile, though this may be somewhat tainted by population bias. An interesting maxima of activity lies across Grundy, Will, Kankakee, Lake IN, Porter, Newton, and Benton Counties, where population bias does not play as much of a role. 

    

   

8. The largest outbreak of strong-violent tornadoes since 1880 in the Chicago/Romeoville Forecast Area was on April 21^st, 1912, when seven F2-F4 tornadoes struck southern portions of the forecast area.

    

   Date	EF2 Tornadoes	EF3 Tornadoes	EF4 Tornadoes	EF5 Tornadoes	Total S-V Tornadoes
   April 21st, 1912	3	3	1	0	1
   May 18th, 1898	1	0	4	0	5
   November 12th, 1965	4	1	0	0	5
   April 21st, 1967	2	0	3	0	5
   March 12th, 1976	2	2	1	0	5
   June 5th, 2010	3	2	0	0	5
   May 25th, 1896	1	2	1	0	4
   July 17th, 1903	1	3	0	0	4
   March 28th, 1920	2	1	1	0	4
   June 7th, 2008	4	0	0	0	4

    

9. 79% of all fatalities due to strong-violent tornadoes were caused by tornadoes rated F4 or F5, or violent tornadoes, while only 12% of strong-violent tornadoes were rated F4 or F5. This once again shows that F4-F5 tornadoes, though rare, are by far the greatest threat to life and property.
10. Major tornadoes and tornado events appear to occur in cycles for the Chicago/Romeoville Forecast Area. Increases in strong-violent tornado activity are evident in the data from the 1890s to the late 1910s, and again from the late 1950s to the late 1970s. There is no evident reason as to why this is the case, nor is there enough data to establish a true scientific pattern yet.
11. Of the eight wintertime tornadoes to have occurred on record in the Chicago/Romeoville forecast area, five of them were rated F2 or greater, with the strongest being the EF3 tornado that struck Boone and McHenry Counties on January 7^th, 2008. 
     

    Wintertime Tornadoes 8 wintertime (Dec-Feb) tornadoes have struck the Chicago/Romeoville County Warning Area since 1950 January 26th, 1950 –F2 December 6th, 1951 (2) –F1, F3 December 3rd, 1955 –F2 December 8th, 1966 –F2 December 4th, 1973 –F1 February 16th, 2006 –F1 January 7th, 2008 –EF3

     

12. Boone, Cook, and McHenry counties, as well as Lake County Indiana have seen the most violent (F4-F5) tornadoes, with each having each seen four violent F4 tornadoes since 1880. Only Grundy and Livingston Counties have not recorded violent tornadoes.

     

    County	Number of Violent Tornadoes
    Boone	4
    Cook	4
    Lake IN	4
    McHenry	4
    Lake IL	3
    Lee	3
    Ogle	3
    Will	3
    Winnebago	3
    Jasper	2
    Kankakee	2
    Newton	2
    Porter	2
    Benton	1
    DeKalb	1
    DuPage	1
    Ford	1
    Iroquois	1
    Kane	1
    Kendall	1
    La Salle	1
    Grundy	0
    Livingston	0

     

13. F2/EF2 tornadoes have a return frequency of once per year. F3/EF3 tornadoes have a return frequency of once every three years. F4/EF4 tornadoes have a return frequency of once every five and a half years. Overall, the Chicago/Romeoville Forecast Area sees a strong-violent tornado once every eight months, or three times every two years.

 More information, including graphics and a list of all 202 F2-F5 tornadoes to strike the Chicago/Romeoville Forecast Area, can be found here. 

T. Lyza   12 August 2010