AP EXPLAIN: What came together to unleash tornado in Alabama

A La Niña weather pattern, warm, humid air from an unusually warm Gulf of Mexico—probably the result of climate change—and a decades-long shift of tornadoes into the eastern United States combined to create the unusually early and deadly storm system that hit Alabama on Thursday, forecasters said.

And it could be the start of a bad tornado year, one expert fears.

Early signs, which could change, “indicate that the overall pattern remains favorable for an above-average tornado year,” said Victor Gensini, a professor of meteorology at Northern Illinois University who studies tornado patterns.

Gensini explained that his concern is based primarily on historical patterns and changes in weather conditions that occur when La Niña, which is a natural cooling of parts of the Pacific Ocean that affects weather around the world, dissipates as expected in a few months.

A COMBINATION TO FORM

For tornadoes to form, two main ingredients are needed that are often not at high enough levels at the same time: wet storm instability and wind shear, which is a difference in wind speeds and directions at different times. altitudes.

At this time of year, “shear is guaranteed,” said Harold Brooks, a senior scientist at the National Severe Storms Laboratory at the National Office of Oceanic and Atmospheric Administration (NOAA, for its acronym in English). “What happens is that when you have moisture you can have a (storm) system. That is the ingredient that is usually missing at this time of year.”

The cold front was following a classic ripple in the jet stream — the atmospheric rivers that move weather systems — seen in La Niña winters, Gensini explained. La Niña winters tend to produce more tornadoes, and NOAA said this week that preliminary numbers show 1,331 tornadoes in 2022 — which was a La Niña year — 9% more than average.

“If you’re going to have tornadoes in January, this is the type of configuration that’s going to produce them,” Gensini added.

However, without moisture there are no tornadoes.

HOT MOIST AIR

The humidity level in the Alabama air was about double what it should be at this time of year, and more like May in Tornado Alley, a region that stretches from Texas to Dakota. South known for being prone to tornadoes, Gensini explained. That’s more than enough for there to be one.

The warm, moist air is coming from the Gulf of Mexico, and “that’s a sign of climate change,” he said.

Gensini pointed to a computer screen showing NOAA measurements of water temperatures for the entire Gulf of Mexico and noted, “Look at that number. 70 (Fahrenheit, 21 degrees Celsius). 70. 70. That’s ridiculous. That’s way above average” for this time of year. That nearby warm water increased the humidity in the air.

“This is very much a La Niña type of system that one would expect, but it is being augmented by abnormally warm sea surface temperatures in the Gulf of Mexico,” Gensini said.

Moist and warm air collides with the cold front and rises like a ramp, and in the mixture that forms, tornadoes begin, he added.

TORNADOES COME TO THE EAST

A new pattern of tornado activity has emerged in recent decades.

There are fewer tornadoes in Tornado Alley and more east of the Mississippi River in the Southeast, a 2018 study by Gensini and Brooks found.

Tornado activity is increasing the most in Mississippi, Arkansas, Tennessee, Louisiana, Alabama, Kentucky, Missouri, Illinois, Indiana, Wisconsin, Iowa, and parts of Ohio and Michigan. The largest drop in the number of tornadoes has occurred in Texas, but even despite the decline, that state still produces the largest number of tornadoes in the entire United States.

Gensini said his lab will work this summer to try to figure out why.

MORE VULNERABILITY

An unpleasant side effect of tornadoes moving further east is that they move from less populated areas to more crowded ones, Brooks and Gensini said.

In Tornado Alley, a tornado can travel for miles and miles and not hit anything or anyone and therefore not be a problem, Brooks added. But that’s not really the case in the eastern United States. There are people and constructions in your way.

And people on the road are more vulnerable.

“There is more poverty in the Southeast, there is a greater population living in mobile homes,” which are one of the most dangerous places to be during a tornado, Brooks said.

Additionally, due to storm tracks, or the paths storms follow due to wind and weather conditions, the farther east tornadoes strike, the more likely they are to strike later in the day and even at night. when people are asleep or don’t listen to warnings, Gensini said.

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Seth Borenstein is on Twitter as: @borenbears

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The Associated Press’s climate and environmental coverage is supported by several private foundations. The AP is solely responsible for all content.

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AP EXPLAIN: What came together to unleash tornado in Alabama


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