Deadly North Dakota Tornado Was a Rare EF-5, Weather Service Says

The National Weather Service announced on Monday that a deadly tornado that ripped through rural North Dakota this summer has been assessed as a 5 on the enhanced Fujita scale, with wind speeds greater than 210 miles per hour. It’s the first time since 2013 that a tornado has been given the highest rating possible on the scale.

Three people were killed on June 20 when the tornado, part of a series of storms moving through North Dakota and Minnesota, struck in Enderlin, N.D., and more than 30 train cars were derailed. Weather Service officials initially categorized the tornado as an EF3, which can hold winds anywhere from 136 to 165 m.p.h., according to Melinda Beerends, a meteorologist at the Weather Service office in Grand Forks, N.D.

Tornadoes are categorized by analyzing their wind speeds and the level of damage that is left after the event. But the damage to the train — including fully loaded grain hoppers weighing 286,000 pounds each that had been tipped over — suggested to Ms. Beerends that the winds had potentially been stronger. She called in a team of wind damage experts at the Weather Service, along with the Northern Tornadoes Project at Western University in London, Ontario, to investigate further. They used engineering analysis on simulations to understand what wind speed would be required to lift the structures damaged in southeastern North Dakota that day.

One of these experts was Jim LaDue, a lead instructor at the National Weather Service Warning Decision Training Division. Mr. LaDue was part of a quick response team that assessed the Enderlin tornado by looking at damaged trees, houses and the train cars.

At first, the team agreed with the initial assessment that the tornado was probably an EF3 or possibly an EF4, given the damage to some of the properties. But analysis of a lifted tanker car showed winds had to have reached at least 210 m.p.h., according to Mr. LaDue.

Tornado storm ratings can be a particular difficulty for local Weather Service offices, as tornadoes are rated after they occur, rather than other severe weather events like hurricanes or tropical storms, which can be categorized as they are occurring.

“Hurricanes hit such a large area that we are able to actually sense those wind speed estimates,” Ms. Bereends said. She pointed out that meteorologists are able to send measuring tools into a hurricane, but that it was highly unlikely that tornadoes, which often form and dissipate relatively quickly, would move anywhere near tools that would measure wind speed in real time. Additionally, current tools have difficulty withstanding EF5 wind speeds, she said.

There are 28 different damage indicators used to categorize a tornado on the enhanced Fujita scale, and only a handful of them allow for the EF5 rating. Before the Enderlin tornado, the last one in the United States was in May 2013, when dozens of people were killed in Moore, Okla.

Mr. LaDue said that working on a tornado site in 1998 in southern Minnesota helped him understand that the tornado ratings can help local communities make sense of the damage the storms can deliver.