Frequency Analysis of EF-Scale Tornado Damage Indicators

The EF-Scale consists of 28 different damage indicators (DIs) with different degree of damage (DOD) levels that are used to rate tornado intensity. Analysis of the frequency of DIs that are used to rate tornadoes has not been made since the EF scale was adopted by the NWS in 2007. In order to focus resources on improving the EF scale rating process and the estimated windspeeds, it is important to understand which DIs are most commonly used.

This paper presents an analysis of data from the NWS Damage Assessment Toolkit (DAT) to compute the frequency statistics of each EF scale DI. It was found that many of the DIs, paths, and contours in the DAT were not labeled with a unique event ID number. A systematic analysis of the DAT data requires each tornado in the DAT and its associated DIs to have a unique event ID number, which allows a user to know which DIs belong to which tornado, without displaying the data using a geographic interface. In order to be able to systematically use the data, the DIs and tornado paths/contours were geographically plotted in ArcGIS, and DIs, paths, and contours belonging to a single event were manually given the same unique event ID number.

Results include plots and tables of the percentage of events that include each type of DI, the frequency of each DI across all tornado events, statistics on the number of DIs used to rate tornadoes, and the number of DIs in a tornado with ratings equal to the tornado rating. For example, the analysis revealed that the most commonly used DIs in the DAT are trees, small barns or farm outbuildings, one- or two- family residences, and manufactured homes. This data is being used in ongoing efforts to model tornado damage on structures and to further improve the EF-Scale DIs and their associated windspeeds.