EXPLAINER: Which came together to create the deadly Alabama tornado

DENVER (AP) — A La Nina weather pattern, warm, humid air from an unusually warm Gulf of Mexico likely juiced by climate change, and a decade-long eastward shift of tornadoes converged to form the unusually early and deadly storm system generate that hit Alabama Thursday, weather forecasters said.

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

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

Gensini said his concern is primarily based on historical patterns and changes in atmospheric conditions that will occur if a La Nina, a natural cooling of parts of the Pacific that is changing weather around the world, dissipates as forecast in a few months.

A NEEDED COMBINATION

In order for tornadoes to form, two important ingredients are required, which are often not high enough at the same time: wet storm instability and wind shear, which is a difference in wind speed and direction at different altitudes.

At this time of year, “shear is guaranteed,” said Harold Brooks, principal scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory. “What happens is when you get moisture you can have a (storm) system. That’s the ingredient that’s usually missing at this time of year.”

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

“If you get tornadoes in January, this is the type of setup that’s going to produce them,” Gensini said.

Even without moisture there are no tornadoes.

WARM MOIST AIR

Humidity readings in the air in Alabama were about twice what they were this time of year and more like May’s in Tornado Alley, an area that stretches from Texas to South Dakota and is known for being tornado-prone, Gensini said . That’s more than enough for a tornado.

The warm, humid air is coming from the Gulf of Mexico, and he said, “It’s a signal of climate change.”

Pointing to NOAA measurements of water temperature throughout the Gulf on a computer screen, Gensini said: “Look at that number. 70 (21 degrees Celsius). 70. 70. This is ridiculous. This is well above average” for this time of year. The warm water nearby filled the air.

“This is a La Nina-type system that one would expect but is being enhanced by unusually warm sea surface temperatures in the Gulf of Mexico,” Gensini said.

The warm, moist air meets the cold front and rises like a ramp, and the mixing that creates tornadoes begins, Gensini said.

TORNADOS STRIKE EAST

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

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

Tornado activity is increasing most in Mississippi, Arkansas, Tennessee, Louisiana, Alabama, Kentucky, Missouri, Illinois, Indiana, Wisconsin, Iowa, and parts of Ohio and Michigan. The largest drop in tornadoes has been in Texas, but despite the drop, Texas still gets the most tornadoes of any state.

Gensini said his lab is working this summer to figure out why.

MORE VULNERABILITY

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

In Tornado Alley, a tornado can travel miles and hit nothing and not be a problem, Brooks said. But that’s not really the case in the East. People and buildings are in the way.

And the people in the way are more vulnerable.

“There’s more poverty in the Southeast, there’s a larger population of RVs,” which is one of the most dangerous places in a tornado, Brooks said.

Also due to storm tracks, or the routes storms follow due to wind and weather conditions, the further east they hit, when people are sleeping or not listening to warnings, the more likely tornadoes are to hit later in the day and even at night. said Gensini.

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