Empirically Based Probabilistic Tornado Hazard Analysis of the United States Using 1973–2011 Data

Tornadoes are one of the most devastating natural hazards that occur in the United States. Although the average number of tornadoes per year across the country is approximately 1,200, the annual likelihood of experiencing a tornado at a particular location is quite small due to their relatively small size. However, the high consequence of a tornado strike necessitates the determination of geographic tornado hazard. This paper presents a methodology to estimate the annualized probabilistic tornado hazard over the contiguous United States based upon the most recent 38 years of climatological tornado data. Furthermore, with the use of detailed damage surveys after the April 3–4, 1974 and April–May 2011 tornado outbreaks, an empirical method was developed and applied to account for the gradient of wind speed along a tornado’s path length and path width. From this, a probabilistic tornado hazard index can be developed across the United States and can eventually be extended to tornado hazard for any location.