The worst type of tornado hasn't hit the US for over a decade, perplexing experts
May 20, 2013, marks the last time the United States was hit by the worst type of tornado.
On that date, a monster EF5 tornado tore a 17-mile path through the Oklahoma City suburbs, creating "40 minutes of terror" for people on the ground, as the headlines read the next day. It killed dozens and injured hundreds of people.
But similarly damaging and strong tornadoes have touched down in the past decade, including one on Dec. 10, 2021, when a horrific EF4 tornado tore across western Kentucky, killing scores of people and leaving behind unimaginable heartbreak and damage.
A study has found the likelihood of a decade-long absence of EF5 tornadoes is low and offers an explanation for the phenomenon, according to NOAA meteorologist Anthony Lyza, lead author on the study. The past 11 years is the longest “drought” in the worst type of tornadoes since the beginning of official records in 1950.
Researchers found little evidence that tornadoes were getting weaker. Instead, it appears that the subjective tornado grading system – which is overseen by the National Weather Service – has gotten tougher on big tornadoes.
"There is no evidence that the occurrence of intense tornadoes has changed, but rather that higher ratings are being applied more strictly," Lyza told Paste BN.
Lyza, lead author on the new study about the EF5 tornado drought, said that although these monstrous tornadoes are admittedly the rarest of beasts, the odds of going that many years without a single one are a paltry 0.3%.
Since 2013, a few EF4 tornadoes have been reported, including several with wind speeds estimated in the 190-200 mph range.
Thus, he and other researchers say that some of the EF4 tornadoes of the past few years should probably have been classified as EF5s.
What is the Enhanced Fujita scale?
Tornadoes are classified on the Enhanced Fujita scale, which ranges from EF0 to EF5. Originally created by famed tornado researcher Ted Fujita, the scale takes into account estimated wind speeds, observed damage and damage verified in weather service surveys after tornadoes.
The original "F-scale" was in use from the 1970s to 2007, while new "Enhanced Fujita (EF) scale" has been in use since 2007.
Unlike hurricanes, which are based on the wind speed of a storm as it progresses, tornadoes are only rated after they've roared through a region.
What is an EF5 tornado?
An EF5 rated tornado has wind speeds of 201 mph or greater. According to the weather service, some of the descriptions of damage include: "Well constructed homes are swept away, steel-reinforced concrete structures are critically damaged, trees are usually completely debarked, stripped of branches and snapped."
Why are EF5 tornadoes so rare?
EF5 tornado ratings are rare for two reasons: The first is that tornadoes of that intensity are uncommon events. Most tornadoes have wind speeds well under the 201 mph threshold for EF5, Lyza said.
The second reason is that ratings are based on damage, and the number of structures that can withstand winds up to EF5 strength are few and far between.
Basically, most buildings would be blown away by EF4 tornadoes, so there are few left standing to encounter EF5 damage.
What's the difference between the old F5 damage and current EF5 damage?
Compared with the older Fujita scale, the Enhanced Fujita (EF) scale makes it much harder to qualify as an EF5. "It is more difficult to attain an EF5 rating as the EF scale is designed right now than it was to attain an F5 rating on the legacy Fujita scale," Lyza said.
"As we illustrate in the paper, every single tornado of the EF-scale rating era that has been given a peak wind speed estimate in the top 10 mph of the EF4 wind speed range (190–200 mph) has had that rating assigned based off of a site-built home being swept off of its foundation.
"It’s likely that several of these tornadoes would have been rated F5 using the legacy Fujita scale."
Thus, the lack of EF5-rated tornadoes in the past decade is "less due to a weakening of tornadoes and likely attributable to (a) stricter application of the Enhanced Fujita scale."
He said there is a group of experts from the American Society of Civil Engineers and American Meteorological Society that's working to revise the Enhanced Fujita scale.