May’s tornadoes largely missed homes and structures, yielding relatively little impact and sparing the country of any tornado fatalities for the first time during the month since 2014.
May is ordinarily the peak month of tornado season, and it’s known for the seemingly routine barrages of twisters that spin up across the Great Plains and central United States.
May 2021 proved exceptional for its lack of these higher-end twisters. Reliable bookkeeping on tornadoes dates back to around 1950. It’s probable that many lower-end tornadoes were missed early in the record as radar technology wasn’t available to detect the weaker twisters, and there were far fewer storm spotters, but most severe tornadoes were probably captured.
Most of the tornadoes during the past month were short-lived and weak, and classic tornado setups were less frequent than in years past. The National Weather Service Storm Prediction Center issued 25 tornado watches, the eighth-fewest since 1970.
All told, 288 tornadoes developed in May, only eight of which even reached EF2 strength. The rest were EF0s, EF1s or “unrated” EFUs. Tornadoes are not assigned an EF rating if there’s insufficient damage evidence to rate them.
The lack of EF3 or greater ratings assigned during the month does not mean that no tornadoes of EF3 strength or greater occurred. In fact, it’s almost certain they did. However, the EF scale, which is used to classify tornadoes, is a damage scale. This means that, without damage, higher-end ratings cannot be assigned.
On May 26, for instance, more than a dozen tornadoes — some probably with winds perhaps cresting in the EF3 range — swirled across extreme southern Nebraska near the Kansas border. Most were rated EFUs, because they passed over open rural landscapes with nothing to hit.
“We issued the ‘considerable’ tornado warning for both the tornadoes north of Benkelman, Nebraska, and the one around Herndon, Kansas, on the 26th,” said Ryan Husted, a meteorologist at the National Weather Service in Goodland, Kan.
“We had confirmed reports from chasers, law enforcement and emergency management saying a large tornado was ongoing, but the way we rate tornadoes is on the damage they produce. It’s not the size. It’s not on the velocity on radar. It’s just on the damage.”
When he and his team went to inspect for damage, however, there was little to be found.
“It seems in both cases the tornadoes produced damage at the start of their life cycles, and the tornadoes then proceeded to move into open country, where there’s not much to hit,” Husted said.
Current rating practices are both a blessing and a curse to meteorologists and statisticians trying to categorize tornado strength. Because direct observation of tornadic winds are virtually impossible, engineers have helped create metrics that can allow meteorologists surveying damage to estimate a tornado’s intensity. However, that requires structures be affected — something that’s not always the case in Middle America. That makes it challenging to evaluate long-term tornado trends, because ratings are as much a product of where a tornado occurs as they are how strong it is.
“I surveyed the Herndon tornado, and it hit a farmstead and the historic schoolhouse … but otherwise a few power lines and trees,” Husted said. “There’s just nothing there in fields. Could the tornado have been rated an EF3? Perhaps — if there had been stuff in that area. But we’ll just never know.”
The other strong tornado he surveyed near Herndon, Kan., was assigned an EF2 rating based on tree damage. Otherwise, the tornado danced over wide-open spaces.
Studies conducted using field-based mobile Doppler radar have revealed that tornadoes are often rated with intensities below their maximum wind speed, simply because a structure failed in lower winds or structures weren’t present to begin with. Karen Kosiba and Josh Wurman of the Center for Severe Weather Research estimate that 1 in 5 tornadoes born from rotating supercell thunderstorms is capable of EF4 or EF5 damage.
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June 12, 2021 at 10:08PM
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No severe tornadoes hit the U.S. this May for first time on record - The Washington Post
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