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At 6 p.m. CST on Monday, January 4, 2010, the criteria for hail that the National Weather Service uses to issue severe thunderstorm warnings will change from ¾ of an inch to 1 inch. The Central and Western Regions of the NWS used the 1 inch threshold for this past severe thunderstorm season after a successful test over the past five years or so in Kansas. In the Kansas experiment, the number of severe thunderstorms was reduced by forty percent. The response from emergency managers and the public has been overwhelmingly positive.
The criteria for severe wind is not changing. It will remain 50 knots, or 58 mph. Many people have been clamoring for the severe wind threshold to be increased to 65 mph.
There is widespread support for the idea that there are too many severe thunderstorm warnings and a corresponding concern that too many warnings lead to complacency on the part of the general public when they hear them. Further, engineering studies show that damage to structures and autos from hail does not really begin until the stones reach one inch in diameter.
The NWS Memphis conducted a verification study in 2008. The 1 inch criteria resulted in 25% fewer severe reports. In other parts of the United States, changing to the new hail size is expected to reduce severe reports by 20 to as much as 100 percent.
The reduction will allow forecasters to put more focus on damaging winds in their warnings. This means that they will have to have a better understanding of damaging winds and their signatures.
This is a significant policy change on the part of the National Weather Service. It is probably second only to the decision to start using storm based warnings, or polygons.
