Trends in Large Hail and Damaging Winds: A New Perspective

The Storm Prediction Center (SPC) maintains a database of reported severe thunderstorm events over the coterminous United States (Schaefer and Edwards, 1999). The data on “severe hail” and “straight-line” severe thunderstorm wind gusts begins in 1955. To be classified as severe, hail must have a diameter of at least ¾” (190 mm) diameter or greater, and wind gust speed must be 58 mph (50 kts or 26 m/s) or greater. These winds can be measured, estimated, or inferred from the damage they produce.

Accuracy of the severe thunderstorm climatology is affected by a number of factors, including: 1) population biases, which are related to the likelihood an event will be observed, 2) diurnal influences, with night time events being more difficult to observe, 3) procedural guidelines used to determine the occurrence of a severe storm (e.g., what constitutes wind damage), 4) the training and scientific understanding of the observer (e.g., estimating hail size), and 5) the ability to conduct accurate storm surveys (e.g., differentiation between tornadoes and straight-line wind damage). If users of the database are to use the information in a meaningful and proper manner they must be aware of the strengths, weaknesses, and biases inherent in the database, and incorporate this understanding into their analysis and interpretation of the results.

An analysis of the trend in the annual number of reported severe thunderstorm events gives an indication of the quality of the data. The number of reports has increased by over an order of magnitude since 1955. In the mid-1950s, there were 700-800 wind reports and about 350 hail reports annually. Currently, there are over 12,000 reports of each storm type per year. This increase was influenced by a number of factors, including: 1) a population increase in many areas of the country and resultant growth of urban structures into previously rural areas, 2) an overall increase in weather awareness by many segments of the population through the combined efforts of the media and many governmental institutions, 3) improved communications as evidenced by the pervasiveness of cellular telephones, 4) the development of trained storm spotter networks, which has increased the likelihood of observing severe storms, 5) the deployment of the national NEXRAD radar network, which has resulted in substantial improvements in the remote identification of severe storms, and 6) the implementation of the national warning verification program, which has resulted in increased accountability of NWS warning products.

The geographic distribution of hail reports and wind reports are shown and compared. Climatological features are related to general meteorological conditions. Additional charts where the events are also sorted by severity (hail of ¾” diameter or larger; hail of 2” (51 mm) diameter or larger; all severe thunderstorm wind gusts; gusts measured or estimated to be 58 mph and higher; gusts measured or estimated to be 75 mph (33 m/s) or greater) are presented and discussed. In an effort to show of the progression of severe thunderstorms during the year, charts by event type and severity for four months (January, April, July, and October) are shown. These charts will be posted on the SPC internet under “SPC Publications” site after the conference.