While tornadoes can happen any time of the year, the tornado season really starts to ramp up in March in Illinois (see plot below) and remains active through the spring and on into summer. April, May, and June are the three standout months historically. So far 2015 has been very quiet with no severe weather reported in Illinois so far, according to NOAA.
The Midwestern Regional Climate Center, located at the Illinois State Water Survey, recently released a new tornado tracking tool. You can choose the range of years, and the tornado intensity (EF scale), as well as zoom in for more details (screen shot below). BTW, the tool covers the entire US, not just the Midwest. It is another great tool developed by our talented GIS expert Zoe.
Speaking of tornadoes, here is the monthly tally of tornadoes in Illinois for 2014. Half of our tornadoes this year occurred in February while March through July have been very quiet.
That is about the opposite of the tornado climatology for Illinois, which shows that winter months are typically much quieter than the spring and early summer months.
Tornado risk has been on a lot of minds after the events in Moore, OK. For the first time that I can recall, the local TV station (Champaign) ran an advertisement for pre-constructed storm shelters.
In 2011, we produced county-level tornado track maps for all counties in Illinois to measure the historical risk of tornadoes across the state. These maps were based on data from the NOAA Storm Prediction Center for the period of 1950 to 2010. For example, below is the map for Cook County that shows tornado tracks and touchdowns, color-coded by (E)F-scale. To me, the strongest reminder that a strong tornado can pass through Chicago is the April 21, 1967, Oak Lawn F-4 event that killed 33 and injured 500. Here are some pictures of from the Oak Lawn Public Library of that event that look a lot like the damage in Moore, OK.
You can see the rest of the maps on the Tornado Maps page at the Illinois state climatologist website.