Study uncovers possible factors leading to decrease in violent EF5 tornadoes, contributing to drought » Yale Climate Connections

A new analysis of tornado ratings on the Enhanced Fujita Scale is shedding light on a peculiar 12-year absence of EF5-rated U.S. tornadoes. Instead of natural variability or long-term climate change, the lack of top-rated twisters may be due to the way house destruction is now measured.

Before the Fujita scale was updated in 2007, a well-built house swept clean of its foundation was a key indicator of an F5 tornado. However, in the enhanced scale, the wind speed that could produce a clean sweep now falls into the EF4 range instead of reaching EF5. As a result, since the new scale was adopted, at least 13 high-end EF4 tornadoes have not received EF5 ratings despite causing significant damage to site-built, well-constructed houses.

“Where Have the EF5s Gone? A Closer Look at the ‘Drought’ of the Most Violent Tornadoes in the United States” is now available in the Bulletin of the American Meteorological Society. The three authors, Tony Lyza, Harold Brooks, and Makenzie Krocak, are affiliated with the NOAA National Severe Storms Laboratory, with Brooks and Krocak also at the University of Oklahoma.

The authors propose that a slight adjustment in the EF4/EF5 wind boundary would align the original and enhanced scales better, resulting in a more consistent record of top-tier tornadoes over the long term.

Introduced in 1971 by tornado scientist Theodore “Ted” Fujita and meteorologist Allen Pearson, the original Fujita scale revolutionized wind speed classification for tornadoes. It categorized winds into 12 groups, with the F1 to F5 ranges representing the most severe winds, up to 318 mph (512 km/h). Using available evidence at the time, Fujita and Pearson linked wind speeds in each range to expected damage levels.

After the scale’s debut, efforts were made to rate past tornadoes in the National Weather Service database dating back to 1950. Meteorologist Tom Grazulis spearheaded a project to identify and document all F2 tornadoes back to 1880. The widespread adoption of the Fujita scale made it well-known, especially after the 1996 film “Twister.”

Despite its success, the Fujita scale had some flaws. Engineering studies revealed that peak tornado winds rarely exceeded 250 mph (400 km/h). Additionally, tornadoes did not require extremely high winds to cause significant damage, especially to poorly constructed homes. Some tornadoes given F5 ratings in the late 1990s caused extensive damage even at wind speeds below the F5 range. For example, a destructive tornado in La Plata, Maryland in 2002 was initially rated EF5 but later downgraded to EF4.

The limitations of the Fujita scale spurred the development of the Enhanced Fujita Scale. One objective was to align wind ranges with physical reality while maintaining a consistent relationship between the old and new scales. The enhanced scale includes 28 different damage indicators, each with its own severity scale corresponding to estimated wind speeds. The highest estimated wind speed from the indicators typically determines the tornado’s EF rating.

According to the new study, a few key decisions in developing the EF scale, along with a rounding issue, contributed to the EF5 “drought.”

Based on engineering studies, the highest level of damage – “destruction of engineered and/or well-constructed residence; slab swept clean” – was set to correspond to an expected peak wind gust of 200 mph (322 km/h). However, due to rounding brackets in the EF range, the calculated EF4 range shifted to 166-200 mph, placing the wind gust needed for a well-built home to be considered EF5 at the top end of the EF4 range.

The study emphasizes that under strict EF scale application, achieving an EF5 rating from a single-family home being swept off its foundation requires the home to be built above building code standards, significantly reducing the number of EF5 DIs noted in surveys.