What is a "Severe" Thunderstorm?
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.)
The National Weather Service conducted a demonstration in the state of Kansas and adjoining County Warning Areas over the past four years, utilizing a hail size criterion for issuance of Severe Thunderstorm Warnings of 1” in diameter, rather than the historical ¾” threshold.
The basis for the change...
Beginning in the spring to summer of 2009, this demonstration will expand to include 14 states in the central US, inlcuding Iowa, illinois, Missouri, Wisconsin, and Minnesota.