Regional Variations in Vulnerability to Tornado Outbreaks in the Eastern United States

Significant regional variations exist in the vulnerability of society to tornadoes. These variations are tied to a number of factors, including geographical variations in the frequency of tornadoes, the strength of housing structures and the diurnal timing of tornadoes. In this study, regional variations in tornado vulnerability are assessed across the eastern two-thirds of the U.S. through a GIS-based analysis of demographic and land cover characteristics that are tied tornado vulnerability.

Tornado outbreaks are identified from all F/EF-1 level and greater tornadoes reported in the Storm Prediction Center database from 1954 to 2010. An 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. The intensity of each outbreak is estimated by multiplying the track length of each tornado by its maximum F/EF level and summing up these values across all tornadoes in the outbreak. Census data at the block group level is used to identify mobile home and population densities along each tornado track. NASA's Global Land Cover data is accessed to obtain the percent tree cover along the path of each tornado. This information is uploaded into a GIS, and a 1-km buffer is created around each tornado path. Zonal statistics are then calculated within this buffer to determine mobile home and population exposure as well as the amount of tree cover found along the path of each tornado. These three vulnerability variables are then related to the numbers of fatalities and injuries. The strength and character of these relationships are examined across different regions and different spatial scales. Preliminary results indicate that regional variations in vulnerability are significantly related to regional differences in population, mobile home and tornado density. Populated areas with more tree cover are found to be at a relatively greater risk for death and injury, and are tied to the occurrence of increased airborne debris and reduced visibility.