Rachel Wegner|Nashville Tennessean
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.
FATALITIES: Tennessee led nation with highest number of tornado deaths in 2020
In March 2020, overnight storms spawned 10 tornadoesin Tennessee, including an EF-3 in Nashville and an EF-4inPutnam County. The tornadoes killed 25 people and injuredhundredsstatewide on March 2 and 3. By the end of that year, Tennessee also led the nation in overall tornado deaths.
But what causes nearly half of Tennessee's tornadoes to strike after the sun goes down? Three experts offera look at the science behind it all.
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Experts: A'potpourri' of factorsdrive night tornadoes in TN
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.”
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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 Midsouth, 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.
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Ashley said QLCS events often occur in the evening and overnight hours, and the region fromthe Ohio River valley down to the Tennessee River valley tends to have more QLCS events than the rest of the country.
"It's like a potpourri of things that can create tornadic environments," he said.
Kelsey Ellis, aprofessorat theUniversity 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 akey ingredientto 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 is another key factor, which is how wind speed and direction change with height. There is morewind shear in the South than the Plains, according to Ellis. That allows nighttime and cool season tornadoes to happen even without substantial surface heatand energy.
How to stay weather aware in Tennessee
Given the likelihood of severe storms and tornadoes in Tennessee, especially at night, weather experts and public officials emphasize the importance of weather awareness.
It's a good idea to havemultiple ways to receive weather updates, like a storm alert appor 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 NWS said.
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Keeping a bag packedwith 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 monoxidesafety 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."
Find reporter Rachel Wegner at rawegner@tennessean.com or on Twitter @rachelannwegner.