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Pinedale Online > News > March 2009 > NWS changes hail size criterion for severe thunderstorms
NWS changes hail size criterion for severe thunderstorms
by National Weather Service
March 24, 2009

Beginning Wednesday, April 1, NOAA’s National Weather Service forecast offices serving Wyoming will use a new hail size criterion for determining a severe thunderstorm. The change comes after several years of operational testing and research into damage caused by falling hail.

Over the past several years, NWS offices serving the state of Kansas have experimented with the use of one inch diameter hail as the criterion for a severe thunderstorm as opposed to the current criterion of 3/4 inch diameter hail. The demonstration was such a resounding success among key partners and customers, including emergency management and media, that the NWS has expanded this policy to include states throughout the central portion of the country. Beginning April 1, this will include all counties in the state of Wyoming.

Severe thunderstorm warnings will now be issued if hail size is expected to equal or exceed one inch in diameter. The wind gust criterion of 58 mph for the issuance of a severe thunderstorm warning will remain unchanged. The Riverton NWS office will issue a Special Weather Statement that will detail movement and expected impacts for those thunderstorms producing hail and wind below these severe limits.

"The scientific research and archived damage reports increasingly support property damage occurring with hail one inch in diameter or larger," said Chris Jones, Warning Coordination Meteorologist at the Riverton NWS office. "Raising the hail size criterion to a one inch threshold will likely result in fewer warnings, which we believe will lead to stronger confidence that a genuine threat exists when a warning is issued."

Additional information regarding the hail size criterion change, an online customer survey feedback form, and a schedule of classes for volunteer storm spotters is available on the Riverton NWS website at or by calling the Riverton office at (800) 211-1448.

Pinedale Online > News > March 2009 > NWS changes hail size criterion for severe thunderstorms

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