Trends in Tornado Vulnerability across the United States

While the annual number of tornado fatalities in the U.S. has shown a declining trend over the last 30 years, secular changes have been observed in the risk factors associated with tornadoes, including the geographic distribution of the population, the number of people living in mobile homes, and the number of tornadoes observed. The goal of this study is to examine these changes at both a national and regional scale and ascertain their influence on tornado vulnerability. Using a GIS, demographic and land cover characteristics are identified in the immediate vicinity of all tornadoes observed between 1980 and 2010. The diurnal timing and intensity (F/EF-level) of each tornado is recorded from the national Storm Prediction Center database. Mobile home and population densities are estimated along each tornado track using Census data from 1980, 1990, and 2000 at the tract level, which is available across metropolitan areas. 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 along the path of each tornado. Secular trends are identified in the character of tornadoes, mobile home density, and population exposure, which are then related to trends in the number of fatalities. Analyses are carried out both nationally and across different regions of the country. Preliminary results reveal significant changes in tornado vulnerability over the last 30 years, especially in the number of mobile homes across various regions of the country.