(Formerly P10.1) Possible impacts of the Enhanced Fujita Scale on United States tornado data

The Enhanced Fujita (EF) Scale became the operational standard for rating United States tornadoes in February 2007. Some wind speed differences were apparent between the same F and EF categories, given the former's lone explicitly stated damage indicator (DI) – well-constructed houses – and the related DI (one- or two-family residences) in the EF Scale. Further, the presence of numerous non-house DIs in the EF Scale lent more precision to the rating process, both spatially along paths and in terms of degrees of damage (DoD) for individual DIs. Hypothetically, this finer resolution of the EF Scale should boost the rating of some events that formerly would have been classified using only available damage to “well-built homes”, or merely as F0 from a lack of any DI at all. In reality, how have the rating tendencies changed since the implementation of the EF Scale? Have EF2s increased at the expense of EF0s and EF1s? Can any secular trends or artifacts in tornado rating be inferred from the relatively short record since the switch?

Herein, we examine tendencies in U. S. tornado records before and after the implementation of the EF Scale. Tornado rating data are compared over various time periods for F/EF Scale ratings, multi-level rating bins (e.g., significant tornadoes ≥F2 and ≥EF2 ), and other measures. It appears that changes in assigned ratings that are not associated with the change from F to EF are at least as large as those associated with the formal policy change. The question of how much of this is a result of changes in practice and differences in meteorological events cannot be answered fully, but it appears that a significant fraction of the change comes from non-meteorological factors.