The rash of Super Tuesday tornadoes in Tennessee is already prompting researchers to study just how the twisters formed and how intense they were.
In the wake of the outbreak, WPLN News interviewed several weather experts about what happened, the science of tornadoes and some commonly held misconceptions.
In addition to a few cameos, our experts informing this story are:
- Kelton Halbert, a Nashville native now pursuing his doctorate degree in atmospheric science at the University of Wisconsin
- Krissy Hurley, Nashville’s warning coordination meteorologist for the National Weather Service
- David Drobny, co-founder of storm-tracking service @NashSevereWx on Twitter
For Halbert, some of his earliest memories surround Nashville’s 1998 tornado. He says it’s become his goal to advance our understanding of the devastating events.
How does a tornado form? It’s still mysterious.
Seemingly the most basic of questions — what causes tornadoes? — remains a tough one to answer. And unlike, say, a hurricane, the decisive moment for a tornado can be incredibly sudden.
“It’s definitely still one of those things that’s complicated and being looked into,” Halbert said.
He compares tornado genesis to a cooking recipe: You need a special kind of thunderstorm called a supercell, and those typically form in environments with vertical wind shear — wind that changes speed, direction and height.
Together, those conditions can allow a thunderstorm to rotate and produce a tornado.
In Tennessee’s recent storm, there’s evidence that some of the seven tornadoes that touched down formed quickly.
Krissy Hurley, with the National Weather Service, said she wasn’t on duty that night, but she checked conditions after attending the Nashville Predators game. Nothing seemed exceptional.
“We felt pretty confident that we were going to get reports of large hail,” she said. “That can rapidly change, and that’s what we saw.”
What was remarkable was that the Nashville tornado “went from almost nothing west of [John C. Tune Airport], to almost immediately EF-2 damage, in a span of … 30 to 60 seconds.”
“That’s really impressive for a tornado,” Hurley said.
Less than two hours later, the storm front struck again.
“Once that tornado touched down in Putnam County, it went from EF-0 strength to EF-4 damage in less than 3 minutes,” Hurley said. “That is incredible.”
How unusual were these tornadoes? They stand out in several ways.
Halbert says Tennessee saw multiple tornadoes because a wide geography had the necessary conditions, allowing the supercell to continue to travel.
“The longer a supercell exists, the more opportunities it has to produce a tornado,” he said. “The fact that it happened in the overnight hours makes it extra special because that’s not as common.”
Nashville’s EF-3 tornado shredded a 60-mile path that night, wreaking havoc for an hour. That also qualifies it as a long-track tornado, which make up less than 1% of tornadoes.
“This really is a very extreme event,” he said.
At 60 miles, the tornado that crossed Nashville, Mt. Juliet and Lebanon was the second-longest in Middle Tennessee history, and in the top 10 statewide.
Hurley, with the weather service, said the path of the storm was also unusual in that it moved from west to east, essentially riding on a warm front along Interstate 40.
Tornado-producing storms usually move from southwest to northeast.
The EF-4 tornado in Cookeville, she said, was surprisingly narrow and brief for a twister of such intensity — going 8 miles in eight minutes.
Night tornadoes are especially dangerous. Why do they happen here?
Tennessee ranks among the most deadly states for tornadoes, and meteorologists largely attribute this to the prevalence of overnight storms.
Hurley said a key reason is the amount of warm air that lingers into the night, compared to areas that cool down.
“You can blame the Gulf of Mexico. Because of our location, we’re able to pump that warm, moist air and continue pumping it in even after the sun sets,” she said.
The damage appears inconsistent from home to home. Do tornadoes hop?
Yes, Halbert says. It’s called “cycling.”
“The Nashville tornado picked up and then kind of did some hopping a little bit until it reached Cookeville. And so it was the same supercell.”
He said tornado energy can rise or dissipate while it travels.
As for varying damage, it’s partly about the internal dynamics.
“Not everything inside that … swath is going to have the same damage, because you can have subsection vortices inside the larger circulation and those will tend to have stronger wind speeds and stronger suction ability,” he said.
Hurley, who often evaluates storm damage, said construction methods also play a role, and that she often finds homes that are not properly attached to their foundations.
But, at least for low-end tornadoes between EF-0 and EF-3, “you expect your house to hold up.”
In the latest Tennessee storms, she credits the number of basements in Donelson for the high survival rate.
In Mt. Juliet, she found roofs and entire floors removed.
“That’s why you’ve got to get low, and get as many walls between you and the outside as possible,” she said.
Across the damage path, Hurley worked with a structural engineer to make evaluations. She said they didn’t find “grossly inadequate” construction.
Tornado myths can be persistent. Did these tornadoes disprove any?
In addition to wind and debris, tornadoes can also spin up theories and assumptions.
During a recent broadcast of the WeatherBrains online show, meteorologists outlined several misconceptions that they say they’ve had to work to dispel.
Among them: that tornadoes can’t hit cities, can’t cross rivers and can’t travel into the higher elevation of the Cumberland Plateau.
Those beliefs can create a false sense of security, said David Drobny, of @NashSevereWx.
“This thing jumped the Cumberland River at least a half a dozen times,” Drobny said. “This tornado kept going without regard to topography at all.”
As for the plateau? The weather service database indicates that the deadliest tornado in state history happened on the plateau in Overton County.
“Tornadoes do not discriminate,” said Henry Rothenberg, News Channel 5 meteorologist.
Another local question has also emerged: Is East Nashville part of a small tornado alley?
Halbert says such a “micro-sized” tornado alley is unlikely and could be hard to prove.
“If East Nashville gets hit within the next 5 or 10 years, that’s something where you’re going to see some research coming out of this office,” she said.
Will we learn more from these tornadoes?
Yes. Meteorologists are already calling for research into how the supercell functioned and the precise conditions that led to the tornadoes. In the WeatherBrains show, one described Tennessee’s event as an “unusual … nightmare scenario” that calls for further study.
“There’s always uncertainty,” he said. “The atmosphere is very complex. It’s constantly changing. … What it’s doing on the ground, and what it’s doing 5,000 feet up, are different.”
Drobny said the scientific community is still working on some basic questions about tornado genesis.
“And that’s why we want to express the high amounts of uncertainty for a high-impact, low-probability event.”
Hurley, with the weather service, said one point of inquiry will be how the tornadoes interacted on March 3.
She said there’s an early hunch that the most severe tornado in Putnam County was both created and “killed” by a smaller tornado that briefly touched down to the south.
“That quarter-mile-long, EF-0 tornado likely saved the heart of Cookeville,” she said.