NWS Increases Severe Hail Threshold

January 8, 2010 · 0 comments

If you step outside in a thunderstorm and get bonked on the head with penny size hail, don’t blame your misfortune on a severe storm. On January 5, the National Weather Service changed the criteria for severe thunderstorms by upping the minimum size hail from ¾ to 1 inch—quarter size. The wind threshold—50 knots, or 58 mph—remains the same.

Hail size penny vs. quarter

Comparison of the diameter of a penny to a quarter, and an actual quarter atop hailstones to show relative sizes.

The reason for the change, according to a statement issued by the Fire and Public Weather Services Branch of the NWS, is that research reveals “significant damage” doesn’t occur from hail smaller than an inch. Hailstones the size of quarters or larger are the ones most destructive to cars, homes, buildings, and crops.

Over the years, and particularly in the Plains states, the statement reads, “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.” Too many warnings for events that were not damaging garnered complaints and made the warnings somewhat meaningless.

Experimental warnings for 1-inch hail in Kansas the last few years expanded in the central and western United States in 2009. Emergency managers and media outlets in the areas that previously made the changes noted that people seem to take warnings for severe hail more seriously now as they carry more weight.

Of course observing hailstones 1 inch or larger and forecasting hail size are two different things. But, just as research has supported increasing the severe hail threshold, scientists are making strides toward more accurately predicting hail size from radar observations of severe thunderstorms. A poster that will be presented by Matthew Kramar of the NWS office in the Washington, D.C. area, et al., on Wednesday afternoon at the Annual Meeting (January 20, 2:30-4:00 PM, Exhibit Hall B2) reveals results of correlating radar hail cores to hail size for the Mid-Atlantic region—a study that piggybacks on successes in establishing operational hail prediction in the Plains states.