Monday, December 7, 2009

Severe hail criteria changing in January 2010

How big does hail have to be to be considered "severe?" Well, the official definition that most people know well will be changing on January 5, 2010 nationwide.

Following a public comment period and operational testing in the Central and Western Regions of the National Weather Service (NWS), the NWS will officially raise the criteria required for hail to be considered severe from at least 0.75" (penny-sized) to at least 1.00" (quarter-sized) in about a month. The criteria for thunderstorm wind to be considered severe (50 knots, or 58 mph) is not changing. During the public review and comment period, fully 86% of the comments favored raising the criteria.

As to the expected impact, the NWS estimates that approximately 40% of Severe Thunderstorm Warnings will no longer be required - that many are based solely on hail of less than quarter size. (In fact, Eastern Region testing indicated that only 35% of their hail reports are of quarter-size or greater (Steinbugl, 2009).) In addition, damage assessments indicate that hail must typically be 1.00" or larger to produce damage to structures or automobiles (Marshall, et al.). In the tested locations, the public, media, and emergency managers liked the change.

My opinion is this: The reduction in number of warnings due to the new criteria should make for more meaningful warnings and should eventually (after a period of education) cause the warnings that are issued to carry more weight. The fact of the matter is, currently the vast majority of the public is complacent and has become de-sensitized when a Severe Thunderstorm Warning is issued. That is due in large part to the fact that, much of the time, the weather doesn't get strong enough in Joe Q. Public's eyes to justify taking action when a "severe thunderstorms" rolls through. We've gotten used to these marginal warnings and have let our guard down. I am in favor of the change and hope that this change, in addition to the recently-implemented storm-based warnings (issuing a thunderstorm warning not by county, but along the path of the storm), will eventually cause heightened awareness of, and greater credibility to, thunderstorms that pose a threat to life and property.

Marshall, Timothy P., et al, Haag Engineering Co. "Hail Damage Threshold Sizes for Common Roofing Materials."
Steinbugl, Matt, NWS/State College, PA. "Issues with Transitioning to a New Severe Hail Criteria," 2009.
Runk, Kim, NWS/Central Region. "One-Inch Hail Podcast," 2009.

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John M. said...

I think that severe thunderstorm warnings in general should be looked at for their effectiveness. Entirely too often a warning is issued when hail is marginal at best, and wind speeds are marginal at best.

I hope for the day that severe thunderstorm warnings are only issued in truly 'severe' storms. Currently it appears as though these warnings are issued out of fear that we may "miss one". That's never a good reason to alert the public. The "Significant Weather Statement" does it's job very well, and in my opinion could replace about 50-75% of the "Severe Thunderstorm Warnings" that are issued.

MWN/Erik said...

Thanks for your comments John. I think the new criteria will help in this regard a great deal. You're right that the SVS does a good job, and it will probably be used more for the 3/4" hail events in the future. I'm not sure that warnings are neccessarily issued out of "fear," but I do think the change will make for more meaningful warnings.

Thanks again!