Tennessee gets more tornadoes at night than any other state. Here’s why
Overnight tornadoes strike Tennessee more than anywhere else in the United States.
The Volunteer State also has the most nighttime tornado fatalities in the country. Spotting tornadoes after dark, especially if they are rain-wrapped, and waking people up to alert them factor into that statistic, National Weather Service Nashville meteorologist Krissy Hurley said. The state’s population density also plays a part.
In March 2020, overnight storms spawned seven tornadoes, including an EF3 in Nashville and an EF4 in Putnam County. The tornadoes killed dozens and left more than 100 injured statewide on
March 2 and 3.
A swath of Easter Sunday tornadoes soon hit the Chattanooga region, killing 10, and by the end of 2020, Tennessee led the nation in overall tornado deaths.
But what causes nearly half of Tennessee’s tornadoes to strike after the sun goes down?
Storms passing through the
South don’t dissipate as quickly in the evening as they might in the Plains, where less moisture is available to insulate warmth. Hurley said lingering moisture from the Gulf of Mexico can be a tipping point for storms in Tennessee.
“It’s an ample source of moisture. It’s never going away — it doesn’t matter if we hit sunset, it doesn’t matter if it’s in the middle of the day,” she said. “Once it makes it up here, it tends to stay here.”
Atmospheric scientist and disaster geographer Walker Ashley said states like Texas and Oklahoma in “tornado alley” typically see most tornadoes in May and June. But in Tennessee and across the Mid-South, that season stretches from November to April.
“That happens to be a time of year where day length is at a minimum,” said Ashley, who is a professor at Northern Illinois University. “So naturally, your odds of a tornado at night are much higher.”
He also pointed to the prevalence of quasi-linear convective system — or QLCS — events in Tennessee. Simply put, they are lines of fast-moving storms that are not perfectly straight and can cause tornadoes. QLCS events include things like squall lines and bow echoes, which can both pack powerful winds.
Ashley said QLCS events tend to occur in the evening and overnight hours, and the region from the Ohio River valley down to the Tennessee River valley tends to have more QLCS events than the rest of the U.S.
“It’s like a potpourri of things that can create tornadic environments,” he said.
Kelsey Ellis, a professor at the University of Tennessee in Knoxville and an expert on hazardous weather climatology, said it’s important to understand what it takes to form a tornado to get a clearer picture of what’s happening in Tennessee.
Instability, or air that wants to rise, is a key ingredient to form a strong thunderstorm and possible tornado.
“If you’re standing near a strong thunderstorm, you may feel warm air at your back flowing into and feeding the storm,” Ellis said. “Greater instability helps make that happen.”
Wind shear, which is how wind speed and direction change with height, is another key factor. There is more wind shear in the South than the Plains, according to Ellis. That allows nighttime and cool-season tornadoes to happen even without substantial surface heat and energy.
Given the likelihood of severe storms and tornadoes, especially at night, weather experts and public officials emphasize the importance of weather awareness.
It’s a good idea to have multiple ways to receive weather updates, like a storm alert app or a weather radio that will wake you up. You can also formulate an emergency plan and designate a “safe room” to shelter from storms.
“If you get separated from family, make sure you have a place to meet or have an out-of-town contact to let know you are OK,” the weather service advises.
Keeping a bag packed with supplies like food, water, extra clothing, shoes, necessary medicine, flashlights and extra batteries is also a good practice. If a storm knocks out the power, familiarize yourself with carbon monoxide safety measures before firing up a generator.
“Weather may not be on the forefront of what’s going on in your life,” said NWS Nashville meteorologist Scott Unger. “But, especially in the South, it needs to be.”