I lived in central-south Illinois for eight years, commuting by bicycle for most of those years. For six of those years I commuted over 40 miles round trip mostly through flat corn and soybean fields, sharing county roads with combines and old Camaros.
Thunderstorms can pop up with frightening speed in the U.S. Midwest, and I’ve been caught out in the open more than once with tornadoes and other extreme weather. In 1998 I actually watched a tornado come toward me, bounce up less than a hundred yards away, fly over my head, and then touch down on the other side of the road. I’ve been in earthquakes, an airliner near-collision, hurricanes and typhoons and my close encounter with a twister was easily the scariest experience of my life.
Gene Bisbee posted some decent tips for cycling tourers who get caught out during a tornado; this information might be useful to bike commuters also.
Personally, I think the danger from lightning is even more important than a tornado. Large hail also hurts like crazy. Your best bet is to check the weather forecast and avoid riding during inclement weather.