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Why One Inch Hail Criterion?

Figure: The minimum size hail criterion for severe thunderstorms changes from 3/4 inch (penny-size) to 1 inch (quarter-size) nationwide on January 5, 2010.

Previously, the National Weather Service (NWS) issued Severe Thunderstorm Warnings whenever a thunderstorm is forecast to produce wind gusts to 58 miles per hour (50 knots) or greater and/or hail size 3/4 inch (penny-size) diameter or larger. For the past few years, offices that cover areas of Kansas have experimented using a warning criterion of one inch diameter hail. During the spring and early summer of 2009, this experiment expanded to other areas in the Central and Western U.S. Beginning January 5, 2010, the minimum size for severe hail nationwide increases to one inch (quarter-size) diameter. There will not be a change to the wind gust criterion of 58 mph.

This change is based on research indicating significant damage does not occur until hail size reaches 1 inch (quarter-size) in diameter, and as a response to requests by core partners in emergency management and the media. Particularly in areas of the Central U.S., the frequency of severe thunderstorm warnings issued for penny-size and nickel size hail might have desensitized the public to take protective action during a severe thunderstorm warning 

In areas that experimented with changing to the one inch hail criterion, media partners stated their user feedback suggests warnings are now more meaningful. In addition, television networks receive fewer viewer complaints from breaking into programming for non-damaging storms.  The Emergency Management community in those areas agreed that warnings carry more weight, and spotters now concentrate on the more significant events.


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Page Author: WDTB Webmaster
Page last modified: January 5, 2010 13:29

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