The 2011 tornado season forced everyone to reassess our understanding of public awareness, warnings, responses, and vulnerabilities. Previous researchers have defined “hazard” as a potential threat, “risk” as the measure of a hazard's frequency for a given location, and “vulnerability” as the likelihood that a place will experience harm from the events. Hence, it is possible to experience hazards and possess elevated risk while avoiding vulnerability.

More than 60 years of tornado path, tornado fatality, and population data are combined to assess and rank U.S. counties by their vulnerability to tornado deaths. Tornado mortality rates are calculated at the county level, normalized by the population of the respective counties. These values are then normalized by the cumulative tornado path length within the county. High values suggest that an area may have decreased public awareness and warning infrastructure (sirens and shelters). Tornado risk is considered to be the probability of an area experiencing a tornado. Risk is calculated using a kernel density estimation method and it is combined with the normalized tornado path mortality rate to identify areas that are most vulnerable or likely to experience killer tornadoes. Emphasis will be placed on metropolitan areas that are particularly vulnerable.