The Great EF5 Tornado "Drought": A Shift in Grading, Not a Shift in Strength?
For over a decade, the United States has experienced an unusual absence of the most catastrophic type of tornado: the EF5. The last time the nation witnessed the devastating power of an EF5 tornado was on May 20, 2013. On that fateful day, a monstrous twister carved a 17-mile path of destruction through the Oklahoma City suburbs. The event lasted a harrowing 40 minutes, leaving a trail of death, injury, and widespread devastation in its wake. Dozens perished, hundreds were injured, and the landscape was forever scarred.
While that day remains etched in the memories of many, the subsequent years have been marked by a peculiar absence of tornadoes reaching that same level of intensity. There have been instances of other tornadoes creating substantial damage and loss of life. For example, on December 10, 2021, a powerful EF4 tornado ripped through western Kentucky. It inflicted devastating losses and leaving behind unimaginable heartbreak and damage. But still, the elusive EF5 rating has not been assigned to any tornado in over a decade.
This prolonged "drought" of EF5 tornadoes has sparked curiosity and concern among meteorologists and the public alike. Is it a statistical anomaly? Are tornadoes somehow becoming weaker? Or is there another explanation for this phenomenon?
A recent study led by NOAA meteorologist Anthony Lyza sheds light on this intriguing question. The study suggests that the likelihood of a decade-long absence of EF5 tornadoes is statistically low. It proposes that the perceived "drought" may not be due to a decrease in the intensity of tornadoes, but rather a shift in how they are classified.
Lyza’s research indicates that the subjective tornado grading system, overseen by the National Weather Service, has become more stringent in its assessment of major tornadoes. "There is no evidence that the occurrence of intense tornadoes has changed, but rather that higher ratings are being applied more strictly," Lyza explained.
In other words, tornadoes may still be reaching EF5 intensity, but they are not being classified as such under the current grading system. Lyza and his team estimate the odds of going so many years without a single EF5 tornado at a mere 0.3%. This suggests that something is amiss in the way tornadoes are being evaluated.
The Enhanced Fujita (EF) scale, which is used to classify tornadoes, ranges from EF0 to EF5, with EF5 representing the most violent and destructive tornadoes. The scale considers estimated wind speeds, observed damage, and damage verified in weather service surveys.
Unlike hurricanes, which are rated based on their sustained wind speeds, tornadoes are only rated after they have passed through an area. This post-damage assessment is crucial to assigning a tornado its EF rating. An EF5 tornado is defined as having wind speeds of 201 mph or greater. The damage associated with an EF5 tornado is catastrophic: well-constructed homes are swept away, steel-reinforced concrete structures are critically damaged, and trees are completely debarked and stripped of branches.
However, the rarity of EF5 ratings is not solely due to the rarity of such intense tornadoes. Another factor is the limited number of structures that can withstand winds of that magnitude. According to Lyza, "Most tornadoes have wind speeds well under the 201 mph threshold for EF5." Furthermore, "ratings are based on damage, and the number of structures that can withstand winds up to EF5 strength are few and far between."
Essentially, even if a tornado generates EF5-level winds, it may not be rated as such if it does not encounter structures capable of exhibiting EF5-level damage. Most buildings are simply blown away by EF4 tornadoes, leaving little evidence of the even more extreme forces that an EF5 tornado can exert.
The transition from the original Fujita (F) scale to the Enhanced Fujita (EF) scale in 2007 has also played a significant role in the apparent "drought" of EF5 tornadoes. The EF scale is more rigorous and demanding than its predecessor. "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 stated.
To illustrate this point, Lyza explained that every tornado in the EF-scale era that has had a peak wind speed estimate in the top 10 mph of the EF4 wind speed range (190–200 mph) has been assigned that rating based on a site-built home being swept off of its foundation. It is 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 may be less a reflection of a weakening of tornadoes and more attributable to a stricter application of the Enhanced Fujita scale. Lyza also mentioned that a group of experts from the American Society of Civil Engineers and American Meteorological Society is currently working to revise the Enhanced Fujita scale, potentially leading to future changes in how tornadoes are classified.
In conclusion, while the absence of EF5 tornadoes in the United States for over a decade is a noteworthy and statistically unusual phenomenon, it does not necessarily indicate a decrease in the intensity of tornadoes. Instead, it appears that the stricter application of the Enhanced Fujita scale, coupled with the limited availability of structures capable of exhibiting EF5-level damage, has contributed to the perceived "drought." The debate continues on weather tornados that are in the 190-200 mph range should be rated as a EF4 or EF5. The current criteria make the difference between the two. This is where there is subjectivity from the raters.
