May snaps long-standing streak for strong tornadoes in US
During the month of May, the United States saw 288 preliminary reports of tornadoes, above the May average of 276 tornadoes. None of these storms were rated as stronger than an EF2, marking the first time there has not been a tornado of EF3 strength or stronger in May since 1950, when record-keeping began. There were also no deadly tornadoes in the U.S. in May, making this the first year without a killer tornado in May since 2014.
May 2021 tornadic activity was a far cry from the unprecedented pace that unfolded just two years ago. In May 2019, there was a nearly two-week stretch during which tornado activity felt nonstop in the U.S., with a record 13 straight days on which at least eight or more tornado reports were issued. That month featured 556 tornadoes in total.
This May saw the eighth fewest tornado watches on record and just eight significant (EF2+) tornadoes.
"This year has been a quiet one for severe storms in general," said AccuWeather Senior Weather Editor and Meteorologist Jesse Ferrell.
"So far, 2021 has seen the second-fewest issuances of severe thunderstorm and tornado warnings since 1995, when radar data became routinely available. When it comes to tornado reports, 2021 is currently in the bottom 25th percentile, meaning this year has seen fewer tornado reports than 75% of the years since 1950," Ferrell said.
A map of NWS office regions ranked by the number of severe thunderstorm & tornado warnings from Jan. 1 to June 4 over last 20 years. Much of the Midwest is ranked first (least) in warning issuance. (NWS data via Iowa Environmental Mesonet)
In terms of the lack of EF3 tornadoes in May, AccuWeather Senior Storm Warning Meteorologist Rich Putnam said common upper-level dynamics in the atmosphere that are typically present during severe weather season have been lacking this year.
A summertime pattern has taken hold of the Central states earlier than usual, Putnam explained, citing the recent 100-degree temperatures in the Dakotas. Tornado numbers typically dip in July or August and tornadoes that do form during that time tend to be less strong than compared to earlier in spring.
"It's pretty unusual," Putnam said of the lack of EF3 tornadoes in May.
The U.S. is experiencing a separate tornado-related drought — one that may delight those who fear some of nature’s most powerful storms. No EF5 tornadoes have been recorded in the U.S. for more than eight years, eclipsing the previous record, which was the period between the May 3, 1999, Moore F5 tornado and the May 4, 2007, Greensburg, Kansas EF5 tornado.
All tornadoes after Feb. 1, 2007 are rated with the Enhanced Fujita (EF) Scale instead of the original Fujita (F) Scale. The scale was changed to better align wind speeds with observed storm damage, according to the National Weather Service.
An EF5 tornado moves past homes in Moore, Okla. on Monday, May 20, 2013. (AP Photo/Alonzo Adams)
The most recent EF5 tornado tore through Moore, Oklahoma, on May 20, 2013. No tornadoes of EF5 force have been recorded since.
EF5 tornadoes are the most powerful storms on the Enhanced Fujita Scale, which allows weather officials to use damage indicators to make a judgment of a tornadoes’ likely wind speeds. EF5 tornadoes have maximum wind speeds of more than 200 miles per hour and can cause "complete destruction."
EF5 tornadoes are extremely rare — there have only been 59 EF5 or F5 tornadoes since 1950. These powerful storms often seem to come in clusters, with the major tornado outbreaks of 1974 and 2011 comprising 11 out 59 of these terrifying storms alone.
The end of April marks the anniversary of the 2011 Super Outbreak, one of the most devastating and deadly tornado outbreaks in US history. Between April 25 and 28, severe thunderstorms spawned tornadoes in over a dozen states in the eastern U.S. This video was shot by Reed Timmer, who was stormchasing at the time.
It is unclear what has led to the lack of EF5 tornadoes over the past eight years. It is almost certain that the EF5 drought is due in part to sheer luck, but there are alternative theories, including the idea that the current system for rating tornadoes may be flawed.
Some scientists believe that many tornadoes produce higher wind speeds than the damage indicators would suggest. A tornado that churns through an open field and hits no structures is unlikely to be rated an EF5, given that tornadoes are rated based on the damage they cause.
A research paper published earlier this year from Joshua Wurmana, Karen Kosibaa, Trevor White and Paul Robinson — all of whom are colleagues at the Center for Severe Weather Research — found that wind speeds measured from the Center’s Doppler on Wheels Program were an average of 43 mph stronger than the National Weather Service’s EF ratings suggest.
The report concluded that most tornadoes are in fact capable of producing “very severe damage, with >20% capable of causing catastrophic EF4/EF5 damage,” suggesting that many tornadoes are significantly underrated by the National Weather Service.
One tornado that some meteorologists believe was underrated was the 2013 El Reno tornado, which, with a width of 2.6 miles, was the widest tornado on record. The storm killed eight people, including storm chasers Tim Samaras, Paul Samaras and Carl Young, and injured more than 150 others.
Mobile Doppler radar measured the tornado’s wind speeds to be just under 300 mph, which would make the tornado one of the strongest tornadoes ever recorded. The storm was rated an EF3 due to a lack of damage supporting a higher rating.
The weather experts AccuWeather spoke with did not have a conclusive reason for why there hasn't been an EF5 tornado in eight years. However, a tornado does not have to be an EF5 to be extremely dangerous, and experts recommend having a severe weather plan and paying attention to watches and warnings issued by the National Weather Service.
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Correction: This story previously reported unprecedented tornado activity in May 2020, and it has been updated to reflect that this occurred in May 2019.
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