When are severe thunderstorm warnings issued?

As of April 1, 2009, the criteria for a Severe Thunderstorm Warning is 1 inch hail (the size of quarters) and/or 58 mph wind for several states in the midwest including Iowa, Illinois, Missouri, Wisconsin, and Minnesota. (Previously and for other parts of the county, the criteria was 3/4 inch (penny size) hail and/or 58 mph winds.)