No April Fools: Severe Thunderstorm Warning Hail Criteria Changing

The National Weather Service (NWS) Office in La Crosse, Wisconsin is changing the hail size criteria used for Severe Thunderstorm Warnings to 1-inch in diameter starting April 1, 2009. In the past, ¾ inch was the criteria used. The criteria used for thunderstorm wind gusts will remain unchanged - 58 mph (or 50 knots).

This change will be happening at most NWS forecast offices across the upper Midwest this spring based on expansion of a successful demonstration during the past four years in and near Kansas. Research has shown that hail stones of at least 1-inch diameter are required to cause property damage and warnings issued for marginal hail sizes may desensitize the public

The NWS will continue to track all thunderstorms very closely but it is expected that fewer Severe Thunderstorm Warnings will be issued. For “sub-severe” or strong thunderstorms, the NWS will instead release Special Weather Statements that detail threats of small hail, wind gusts of 40 mph or higher, and/or frequent cloud-to-ground lightning to the public

The La Crosse NWS office estimates that 46% of all documented large hail since 1955 was under an inch in diameter.

An informational video (Windows Media File format/WMV) is available at:

To summarize: the Severe Thunderstorm Warning criteria starting April 1, 2009 will be 1-inch or larger diameter hail, or thunderstorm wind gusts of 58 mph (50 knots) or higher.

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