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.) 

Photo couresy Jason Hicok

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...

  • Core partners shared concerns about the public becoming desensitized by numerous warnings being issued for marginal hail sizes.
  • A Texas Tech research study, “Hail damage threshold sizes for common roofing materials” (Marshall, T.P., Richard F. Herzog, and Steven K. Smith, 2002:, 21st Conference on Severe Local Storms, San Antonio, TX) cited laboratory tests of numerous types of shingles.   These tests showed hail damage to shingles and other roofing materials begins at 1-inch hail diameter.
  • Customer responses in Kansas have indicated high satisfaction with adoption of the 1-inch hail criterion. 

Expanded Implementation
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. 

What does this mean for me?
The bottom line is...fewer Severe Thunderstom Warnings.  And when Severe Thunderstorm Warnings are issued, the storms will have the potential to cause damage.



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