The way violent tornadoes in the United States are rated has changed over time, resulting in no EF5-rated tornadoes since 2013, according to researchers from the NOAA National Severe Storms Laboratory in a paper published in the Bulletin of the American Meteorological Society.

After a tornado occurs anywhere in the US, NOAA experts examine or “survey” the damage and use specific criteria to give each tornado a rating from one to five on the Enhanced Fujita Scale. For nearly a dozen years, no tornadoes have been given the scale’s highest rating of EF5 by the NOAA National Weather Service. This is the longest period without an EF5-rated tornado since official records began in 1950. 

“We were curious about why the US is in this historically long period without an F5 or EF5-rated tornado and how likely this period was due to natural causes as opposed to changes in tornado surveying practices,” said lead author and research scientist Anthony Lyza, NSSL research meteorologist. “Our findings suggest the lack of EF5 tornadoes is highly unlikely to be rooted in natural causes.”

The authors examined this lack of EF5 tornadoes from an historical perspective. A key breakpoint occurs when we switched from using the legacy Fujita (F) scale, developed in the 1970s, and started using the newer EF scale in February 2007, and how they handle the complete destruction of single-family homes. The typical rating for a home being swept away from its foundation on the EF scale is EF4, whereas it was F5 on the F scale. This is likely the fundamental cause of the lack of EF5 tornadoes in recent years.

The researchers suggest a few simple adjustments of the wind speeds accompanying the damage indicators on the current EF scale would lead to consistent 5-level rating assignments from 1880 to present day. These findings have been shared with a group within the American Society of Civil Engineers and American Meteorological Society, which is working on revisions to the EF scale.