The Washington Post Sunday
Busy spring tornado season projected for southern U.S.
La Niña climate pattern could bring active period from late March to April
Due in part to a La Niña climate pattern in the tropical Pacific Ocean, scientists are warning that tornado season could be unusually potent in parts of the South and Southeast, ramping into overdrive during late March into April.
It could be the third consecutive year to feature a particularly active April. Both April of 2019 and 2020 bore witness to more than 270 twisters that carved up much of the mid-South and Southeast. Experts fear this April could be equally action-packed. Such an active period would come in stark contrast to a year that has only featured 23 tornadoes to date, barely a quarter of the typical average count by now.
What to expect
Already, atmospheric scientists are looking ahead at a season that could boom with spurts of intense tornado activity during March and April. Some of the country’s worst tornado outbreaks in the South and Southeast have occurred during La Niña years, such as spring 2011.
“I think you’re going to see severe weather return as early as the beginning of March to the southern Plains and the southeastern United States,” said Victor Gensini, a professor of atmospheric sciences at Northern Illinois University and the creator of Extended-Range Tornado Activity Forecasts, or ERTAF. He’s credited with predicting the May 2019 Great Plains tornado outbreak nearly four weeks in advance.
His long-term forecasts are broad-brushed and still deemed experimental, but in past years some have verified with impressive accuracy.
“There are some signals that suggest early March could be interesting,” said John Allen, a professor at Central Michigan University and a research colleague of Gensini’s.
States bordering the Gulf of Mexico may be at particular risk, including those in the Mississippi and Tennessee river valleys.
Into May, tornado risk generally shifts toward the Plains. Longterm signals are murky at best, and storms over the Plains often feed on smaller-scale features that can’t be sorted out as far in advance.
Making long-range predictions
While predicting a single tornado outbreak is impossible more than a few days in advance, experts can make seasonal tornado forecasts the same way they make long-range projections about the Atlantic hurricane season. Doing so hinges on the state of several broad-scale weather features. The most important driver is La Niña.
La Niña is the opposite of El Niño. It’s characterized by anomalously cool waters in the equatorial tropical Pacific. The presence of this water shuffles other features in an atmospheric jigsaw puzzle, making conditions in the southern United States more favorable for tornadoes, in part by affecting the location and strength of the jet stream.
Gensini pointed to La Niña as the biggest predictive indicator of what may lie ahead. The current La Niña is moderate in intensity, and it is not expected to weaken significantly before the heart of severe weather season. But that doesn’t mean it has played by the rules so far.
“It hasn’t been a typical La Niña by any stretch of the imagination,” said Allen, noting that some of the weather patterns usually associated with such a climate cycle haven’t been present.
“We haven’t yet seen this warmth in the southern states. We also usually see relative cold over the Canadian prairies and higher snowfalls, but we haven’t seen that either,” Allen said.
But more recent changes in the weather pattern across the Lower 48 in the wake of the severe cold snap in the central United States look to help realign the country into a more traditional La Niña pattern, according to Gensini.
“Right now, the equatorial waters off the coast of Peru and Ecuador in the Pacific Ocean are pretty cold, and the atmosphere, especially in the wake of the [central U.S. cold], is more classic La Niña,” Gensini said in an interview. “In the past, that translates to higher [tornado] counts . . . that tends to favor tornado outbreaks more frequently. La Niña tips the scales to greater counts.”
In addition to favoring “significantly above average activity,” Allen notes that “tornado season tends to be earlier and more active” with a La Niña.
According to Allen, springs during a La Niña episode tend to bring peak tornado activity four to six weeks earlier than non-La Niña seasons.
How La Niña spins up tornadoes
The primary role of La Niña in boosting tornado counts across the southern United States stems from its influence on the jet stream. The jet stream, which is a river of swiftly moving winds in the upper atmosphere, meanders south in the wintertime and north in the summer. It helps determine the storm track, energizing and shuttling storms across North America.
The jet stream also induces wind shear, or a change of wind speed and/or direction with height. That’s a key ingredient for rotating thunderstorms. During La Niña seasons, the polar jet stream is shunted farther south over the central United States, bringing more disturbances and more wind shear. The Pacific jet stream, meanwhile, lifts farther north — allowing warm, moist air to overspread much of the United States and set the stage for storms.
“La Niña tends to favor large waves in the [jet stream], usually boom or bust,” Allen said. “You’re not going to accumulate [tornadoes] slowly. There tends to be a lot of tornadoes in a short amount of time, and then nothing. It’s partly a virtue of how La Niña works.”
Other factors at play during March and April
There are other factors that will also influence whether tornado outbreaks occur in March and April.
For one, Gensini mentioned the Madden-Julian Oscillation, or MJO. It’s a large overturning circulation that circulates around Earth’s tropics every 30 to 60 days, enhancing thunderstorm activity where air rises on its leading edge and bringing sinking air and clear skies in its wake.
He noted that, this far out, La Niña is the best starting point for a forecast, but that the MJO comes into play as potential severe weather events approach.
“I like to think of all these [atmospheric features] as different instruments in an ensemble. One might be playing louder than the others, and that’s what you hear,” Gensini said.
Gulf of Mexico water temperatures play a part in influencing tornado seasons across the South and Southeast, too. Unusually mild waters can contribute to a milder and moisture-loaded atmosphere, fueling storms.
“The Yucatán Straits and Caribbean Sea,” said Allen. “That’s where the moisture is coming from.”
Sea surface temperatures in the Caribbean are just under a degree above average currently, with average water temperatures in the Gulf of Mexico. That can fluctuate considerably deeper into the season.
Yet another factor the pair of researchers is considering? Drought in the Southwest. That abundance of dry air can be drawn eastward, forming what’s called an elevated mixed layer, or a cap of warm, dry air that suppresses storm growth during much of the day. During the evening, that cap can suddenly break like a bottle being uncorked, allowing explosive thunderstorm growth.
The bottom line
All told, indications are for a busy tornado season, especially early on in the South, particularly across East Texas and Oklahoma, Arkansas, Mississippi, Alabama, Tennessee and Georgia.
Some publications have run articles comparing this year to the historic and deadly season of 2011, something that Allen says is “very premature.”
“In any given year, the likelihood of a tornado outbreak in one area is rather low,” he said. “We’re sort of weighting the dice at this point. We can say the U.S. may be a bit more likely than normal, but we can’t say anything about one specific place. But it only takes one tornado to cause damage to your home.”
Gensini and Allen will be continuing to make long-term probabilistic tornado forecasts up to four weeks in advance. Their hope is that, down the road, the product can be of use to emergency managers, insurance companies and the government, as well as the general public. Currently, it’s considered more experimental.
Gensini said a team of about 15 people at various government forecasting centers are helping to see if long-range severe weather forecasting is feasible and skillful.
With about $470,000 in new funding from the National Science Foundation, Gensini hopes this season, and the next several through 2024, may help his team build on a footprint that already hints toward promising future results.
“We need a lot more research,” he said.
“I think you’re going to see severe weather return as early as the beginning of March to the southern Plains and the southeastern United States.” Victor Gensini, professor of atmospheric sciences at Northern Illinois University and the creator of Extended-Range Tornado Activity Forecasts