Climatology of Tornado Outbreaks in the Eastern United States

Previous research has shown that the majority of tornado-related fatalities occur in association with outbreaks in which tornadoes are clustered in space and time. While case studies of several notable tornado outbreaks have been conducted, few studies have been published on the climatological and geographical characteristics of tornado outbreaks. In this presentation, a climatology of tornado outbreaks across the eastern two-thirds of the U.S. is constructed from all F/EF1 level and greater tornadoes reported in the Storm Prediction Center database from 1954 to 2010. A tornado outbreak is defined as a sequence of six or more tornadoes with no more than a six-hour period between consecutive tornadoes in the sequence. All tornadoes in the sequence must also be associated with a common synoptic-scale weather system. Seasonal, diurnal, secular, and spatial patterns in outbreak occurrence are described and compared to those patterns associated with non-outbreak tornadoes.

Two metrics are developed to objectively assess and rank the intensity of each tornado outbreak in the climatology: 1) Work: The wind force of each tornado, estimated by its maximum F/EF level, multiplied by its track length and summed up across all of the tornadoes in the outbreak; 2) Fujita Miles: the track length of each tornado multiplied by its maximum F/EF level and summed up across all of the tornadoes in the outbreak. An adjustment factor is applied to each metric to account for variations in the intensity of each tornado along its track. Preliminary analyses indicate that the adjusted wind force of the tornadoes in each outbreak is highly correlated with the total adjusted Fujita Miles. These metrics are also used to assess the intensity of several recent tornado outbreaks from 2011 and 2012 and see where they rank among other major outbreaks in the historical record.