What does the new minimum hail size criterion for severe thunderstorms
(1 inch diameter) mean for our region?

Steve Keighton, Science and Operations Officer

Previously, the National Weather Service issued Severe Thunderstorm Warnings whenever a thunderstorm was expected to produce wind gusts to 58 miles per hour (50 knots) or more, and/or hail size 3/4 inch (penny-size) diameter or larger. For the past few years, offices that cover parts of the Central and Western U.S. have experimented using a warning criterion of one inch diameter hail. Feedback from users was positive. Therefore, beginning January 5, 2010, the minimum size for severe hail nationwide increased to one inch (quarter-size) diameter. There will not be a change to the wind gust criterion of 58 mph.


Hail size comparison between three quarter inch and one inch in diameter
Figure 1. Comparison of penny size hail to quarter size hail


This change was motivated by research indicating significant damage does not occur until hail size reaches 1 inch in diameter, and as a response to requests by core partners in emergency management and the media.  It was perceived that 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.

In areas that experimented with changing to the one inch hail criterion, media partners said that their user feedback suggests warnings are now more meaningful. In addition, television networks have received fewer viewer complaints from breaking into programming for non-damaging storms.  The Emergency Management community in those areas agreed that warnings carry more weight, and spotters could now concentrate on the more significant events.

Is this also going to mean fewer severe thunderstorm warnings now coming from the Blacksburg NWS? In the Mid-Atlantic and Appalachian region of the U.S., a high percentage of the storms that produce penny size or larger hail also produce at least minor wind damage.  Part of the reason for this is probably because much more of the Eastern U.S. is covered by trees compared to much of the Central U.S. where these experiments took place, so there is simply more opportunity to find evidence of wind damage.  The images below show the frequency of three-quarter inch hail reports (left image) vs. severe thunderstorm wind reports (right image) across the country, and illustrate this shift toward the eastern part of the U.S. for the severe winds.

Severe Hail Days Per Year 1980-1999Severe Wind Days per Year 1980-1999
Figure 2. Comparison of Severe Hail Reports vs. Severe Wind reports (days per year 1980-1999)

Therefore, based on the climatology, we suspect the number of severe thunderstorm warnings that we issue under the new hail size criterion will not necessarily have a significant impact on the frequency of warnings we issue, except perhaps in those situations where we recognize an environment supportive of primarily large hail.  In other words, the same radar-based thresholds we have always used for severe storm warning decisions will not need to change in most situations.

In order to provide some guidance to local forecasters for those events where large hail is determined to be the primary threat, we reviewed all one inch and larger hail reports from the spring and summer of 2009, and then closely examined the radar characteristics of each of those hail-producing storms for a small time window leading up to the hail report. The goal was to determine if there was any notable difference between these one-inch hail producing storms and the criteria we have used in the past. We did find some subtle differences in the height of the main “core” of the storm (which is related to updraft strength), but the correlations were not especially strong. We collected enough data from this small study to suggest that as a starting point for this season we utilize slightly higher thresholds for the depth and height of the storm core for the new 1-inch hail criterion.  Again, this will only be for the relatively few environments where we expect to experience that are supportive or mainly large hail.  For all other environments, the thresholds for our warning decisions will remain the same. 

Additional data collection during the 2010 season, as well as evaluation of our strategy this year, may result in subtle changes for subsequent seasons, but for now, we expect the increase to the 1-inch hail criterion will have very little impact on the number of severe thunderstorms warnings we issue.  Your reports of all sizes of hail, especially from half-inch diameter to as large as they may come (and we’ve had hail to the size of baseballs in the past) are still very important for us at the NWS, so please continue to call in all reports of hail!