WRAL WeatherCenter Blog

Who says size doesn't matter?

Posted January 4, 2010 4:37 p.m. EST

When it comes to severe storms — at least in deciding which storms qualify as "severe" — size does matter.

Beginning Tuesday, January 5, 2010, penny-sized hail will no longer be sufficient for a thunderstorm to be considered severe.  Thunderstorms will only be considered severe if they produce one or more of the following:

  • hailstones of at least one inch in diameter (basically the size of a quarter)
  • wind gusts of at least 50 knots (58 mph)
  • a funnel cloud
  • a tornado

This change will impact how the National Weather Service issues severe thunderstorm warnings.  Whereas before the change, a thunderstorm producing penny-sized hail would have prompted a warning to be issued, now, that storm will not prompt a warning.  This larger minimum size should result in fewer warnings, and that should translate into fewer program interruptions and fewer crawls for those warnings.  To wit, Jeff Orrock, Warning Coordination Meteorologist with the NWS' Raleigh office, says only 38% of hail reports since 1994 have been of quarter-sized hail or larger.

The NWS is making the change based in large part on research that suggests hailstones smaller than one inch in diameter do not cause "significant damage".  They will continue to issue severe thunderstorm warnings when a storm is producing or is expected to produce wind gusts of at least 58 mph, funnel clouds, or a tornado. The body of research suggests these, along with the larger-sized hail, are sufficient to cause significant damage or risk to life and limb.

Services that depend on the National Weather Service's warnings, such as WRAL WeatherCall and e-mail alerts, will also adjust automatically with the change. That means if you've signed up to receive severe thunderstorm warnings by phone or e-mail, you will only be alerted for storms that meet the new set of criteria.

This change was tested in Kansas for a number of years, and that test was expanded to much of the central and western parts of the US last year.  The NWS says that in those areas, "media partners stated their user feedback suggests warnings are now more meaningful" and that there were "fewer viewer complaints from breaking into programming for non-damaging storms."  Emergency managers in those areas also said they felt severe thunderstorm warnings carried more weight and that people were more likely to act on them.

What do you think?  Is adjusting the definition of a "severe thunderstorm warning" a good idea?  Have you experienced damage due to hail smaller than one inch in diameter?