Severe weather is possible in the Midwest all 12 months of the year. However, there is clearly a peak season for it from May through early August. On average, the Great Lakes and Ohio Valley regions see anywhere from around 40 to 50 thunderstorm days per year (could see multiple storms for each of those days), with the lower end of that range toward Chicago and the higher end into Indiana and Ohio.
This peak in activity is due to warmer air and moisture surging northward from the Gulf of Mexico. This creates an unstable atmosphere that is conducive for thunderstorm development. This warm and moist air rises, then condenses in the colder air aloft, helping to create a towering clouds which has the capability of producing heavy rain, hail, wind, and even tornadoes. In the spring, this process is further assisted by stronger dynamics within the atmosphere, allowing for sharper cold fronts and upper-level winds, which lead to more intense lift in the air above the surface.
Speaking of tornadoes, there is also a peak season for that as well. Unfortunately, twisters are not uncommon for the area and are formed by the turning of winds with height and lift. They are most common in May and June, and the above is a graph shows the average number of tornadoes per year for Illinois (47), Indiana (24), and Ohio (19).
Data from the Storm Prediction Center between 2003 and 2012 actually further breaks down each criteria for severe weather by days in which each occurs within a 25-mile radius of a certain point on the map. On average, the Midwest will generally see 1 - 3 tornado days per year from Ohio to Illinois. Furthermore, this number then increases for hail and winds, with 5 - 10 severe hail days and upward of 15 severe wind days per year. In total, the region generally experiences around 20 - 25 severe weather days each year.
