Clustering Severe Storms Across the United States

The definition of severe weather in the meteorological community is quite well known, hail three-quarters of an inch in diameter or larger, convective wind gusts greater than 58 mph or damage, and/or tornadoes. The severe weather is considered significant if large hail is two inches in diameter or greater, convective winds are 75 mph or greater, or if a tornado creates F2 or greater damage. Reports of severe storms have been increasing greatly since 1988 with an average of 26,000 reports a year.

Tornadoes, large hail and convective winds between 1999 and 2003 were calculated within a cluster if a report occurred within 180 nautical miles of another report. If a tornado, hail or damaging wind report occurred outside of the 180 nm limit, then the report was considered to be an “isolated” report. The data was then divided into “pairs,” groups of three and groups of four or more. It was found that just over 4,014 tornadoes (68%) occurred in clusters of four or more, and only 14% were isolated.

The presentation will include how useful this information can be in issuing accurate watches and warnings across the United States.