NORMAN, Okla. – Research recently published in the Bulletin of the American Meteorological Society suggests that the record-long drought of EF5 tornadoes is due to stricter use of damage ratings rather than any changes in the weather.According to a NOAA database, 59 EF5/F5 tornadoes have occurred since 1950, but it has been nearly 12 years since the last event, when an EF5 devastated Moore, Oklahoma in 2013.Researchers from the University of Oklahoma delved into the mystery surrounding the apparent disappearance of these powerful twisters and found that while such tornadoes still occur, what has changed is the way damage is surveyed after a tornado strikes.The original Fujita scale, developed in the 1970s, was used until 2007, when it was updated and replaced by what is now known as the Enhanced Fujita (EF) scale.The numerical ratings of tornadoes were not changed; they are still ranked from EF0 to EF5 on the scale, but damage indicators were added, which are used to identify the strength of tornadoes.The lack of damage indicators on the original Fujita scale was considered to be one of its many weaknesses, but the adjusted criteria may be too stringent to allow for the strength of tornadoes to accurately be depicted.For example, a well-built home that was swept off its foundation would have received an F5 rating on the Fujita scale pre-2007, but under the Enhanced Fujita scale, a similar home would likely fall under the EF4 category today.”Consequently, under the strictest application of the EF scale, to attain an EF5 rating from a single-family home being swept off its foundation, the home must technically be built above building code, which is a fundamental shift from the F scale and will inherently reduce the number of EF5 damage indicators (DIs) found in surveys,” researchers stated.HOW ARE TORNADOES RATED? THE ENHANCED FUJITA SCALE EXPLAINEDUsing data from 1880 to 2023, researchers calculated the probability of at least one F5/EF5 tornado occurring in any given year to be 41%, with the chance of 10 consecutive years passing without a tornado at just 0.3% – odds that suggest something is amiss with the recent lack of activity.Researchers noted that there have been many EF4 candidates, where, if the damage had been classified differently, the tornadoes would have been rated as EF5s. Examples include Rolling Fork, Mississippi, in 2023; Mayfield, Kentucky, in 2021; Shawnee, Oklahoma, in 2013; and Tuscaloosa, Alabama, in 2011.Researchers identified over a dozen events since 2011 where, if alternative damage indicators had been used, the tornadoes could have been classified as EF5s.”If all EF5-candidate tornadoes were instead rated EF5, the calendar-year probability of observing an F5/EF5 tornado from the 1880–2023 dataset would be 55.6%, nearly equivalent to the probability of an F5/EF5 tornado being observed in a calendar year as derived from the 1880–2013 dataset (56.0%) prior to the ongoing 10-year EF5 gap,” the study stated.APRIL KICKS OFF AMERICA’S MOST ACTIVE TIME OF YEAR FOR TORNADOESResearchers emphasized that it all comes down to what meteorologists want the EF5 category to represent. The group determined that lowering the current EF5 wind threshold from 201 mph to 190 mph would create more continuity based on previous climatology, without adjusting damage assessments.  While changing the EF scale could lead to other issues, none appeared to outweigh the uncertainty surrounding the current rating products.The United States averages over 1,200 tornadoes every year with nearly 90% that are either determined to be an EF1, EF0 or an EFU, with not enough damage to determine a rank.