Illinois averages around 54 tornadoes per year. The majority of these tornadoes are relatively small and less powerful, being in the EF0 to EF1 range (65 to 110 miles per hour). The strongest tornadoes, those of winds greater than 200 miles per hour, make up less than 1% of all tornadoes.
Although not yet completely understood, tornado formation most often occurs with squall lines and supercell thunderstorms, although other thunderstorm types can have them as well.
Squall lines are multiple thunderstorm cells lines up side by side. Sometimes this line bows out in the center (a bow echo on radar). As the ends of this line curve inward, rotation can develop.
Supercell thunderstorms are storms where the entire storm is rotating. In some cases an area of more intense rotation may form within the supercell, creating a low hanging cloud called a wall cloud. If even more intense rotation develops in the wall cloud, rapidly rotating air may reach the ground, creating a tornado. In certain situations, multiple tornadoes may develop, all rotating around a central core.
In recent years, the trend of tornado frequency appears to be shifting from the north Texas/Oklahoma/Kansas area to what’s called the Dixie tornado alley, encompassing Mississippi, Alabama, and extending into southern Illinois.