EXPLAINER: Was tornado outbreak related to climate change?

WASHINGTON - The calendar said December, but the warm, humid air cried out for spring. Add a storm front heading east directed by a La Nina weather pattern and it created tornadoes that killed dozens across five US states.

December tornadoes are unusual, but not uncommon. But the severity and path length of Friday night's tornadoes likely put them in a category of their own, say meteorologists. One of the twisters - if confirmed it was just one - likely broke a nearly 100-year-old record for how long a tornado stayed on the ground on a path of destruction, experts said.

“One word: remarkable; another would be incredible, ”said Victor Gensini, meteorology professor at Northern Illinois University. "It was really a setup in the late spring of mid-December."

Warm weather was a determining factor in this tornado outbreak, but whether climate change is a factor is not entirely clear, say meteorologists.

Scientists say figuring out how climate change affects the frequency of tornadoes is complicated, and their understanding is still evolving. But they say the atmospheric conditions that lead to such eruptions intensify in winter as the planet warms up. And Tornado Alley is moving further east, away from the Kansas-Oklahoma region and into states where the killers were hit on Friday.

Here's a look at what is known about Friday's tornado outbreak and the role of climate change in such weather events.

WHAT CAUSES A TORNADO?

Tornadoes are swirling, vertical columns of air that form from thunderstorms and extend to the ground. They travel at breakneck speeds, devastating everything that comes in their way.

Thunderstorms occur when denser, drier cold air is forced over warmer, more humid air, conditions that scientists call atmospheric instability. This creates an updraft when the warm air rises. When winds vary in speed or direction at different heights - a condition known as wind shear - the updraft begins to turn.

These changes in wind generate the spin necessary for a tornado. In the case of particularly strong tornadoes, changes in both wind speed and wind direction are necessary.

"If there are significant wind fluctuations over the lowest few thousand feet of the atmosphere, tornado-producing 'supercell storms' are possible," said Paul Markowski, professor of meteorology at Pennsylvania State University. "We had that yesterday."

Because of the large difference in temperature and air pressure between the equator and the Arctic, there is usually a lot of wind shear in winter, Gensini said.

But usually in winter there isn't much of the instability required for tornadoes because the air isn't that warm and humid, Gensini said. This time the time had come.

WHAT CONDITIONS LEAD TO storms of this magnitude?

A couple of factors that meteorologists will investigate further.

Spring-like temperatures in December in much of the Midwest and South helped transport the warm, moist air that contributed to the formation of thunderstorms. Some of this can be attributed to La Nina, which generally brings warmer than normal winter temperatures to the southern United States. However, scientists also expect that atypically warm weather will become more common in winter as the planet warms up.

“The worst-case scenario has occurred. Warm air in the cold season, in the middle of the night, ”said John Gordon, a meteorologist for the National Weather Service in Louisville, Kentucky.

After the storm formed, exceptionally strong wind shear appears to have prevented the tornadoes from breaking up, experts say. It is believed that tornadoes die off when updrafts from thunderstorms lose energy.

Tornadoes usually lose energy in minutes, but in this case it was hours, Gensini said. That is in part why the storm was unusually long on Friday, which is more than 20 miles or so, he said. The record was 219 miles (352 kilometers) and was set by a tornado that struck four states in 1925. Gensini believes this will surpass him once the meteorologists finish their analysis.

“To get a really long walk, you have to have a really fast moving storm. This storm moved well over 50 miles per hour for most of its life, ”said Gensini. That is not the speed of the winds, but that of the entire storm movement.

"You speak of storm movements at freeway speed," said Gensini.

WHAT IS CLIMATE CHANGE RELATED TO TORNADO OUTBREAKS?

It's complicated. Scientists are still trying to clarify the many conflicting factors as to whether human-made climate change is making tornadoes more common, or even more intense. According to the NOAA National Severe Storms Laboratory, about 1,200 twisters hit each year in the US - although that number can vary. No other country sees so many.

Attributing a particular storm like Friday's to the effects of climate change remains a major challenge. Less than 10% of severe thunderstorms produce tornadoes, making it difficult to draw conclusions about climate change and the processes leading to it, said Harold Brooks, a tornado scientist at the National Severe Storms Laboratory.

However, scientists have observed that the basic components of a thunderstorm change as the planet warms up. In summary, Gensini says that extreme storms "are becoming more common because we have much warmer air masses in the cooler months of the year that can support these types of storms".

The U.S. is likely to experience more tornadoes in winter, Brooks said, as national temperatures rise above the long-term average. There will be fewer events in the summer, he said.

Furtado of the University of Oklahoma said Tornado Alley, a term used to describe where many twisters hit the US, has shifted east into the Mississippi River Valley. This shift is due to increases in temperature, humidity, and shear.

"Bottom line, the people of the Mississippi River Valley and the Ohio River Valley are becoming increasingly prone to tornadic activity over time," he said.

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Follow Suman Naishadham on Twitter @sumannaishadham and Seth Borenstein at @borenbears

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Associated Press is supported by the Walton Family Foundation for reporting on water and the environment. The AP is solely responsible for all content. For all of AP's environmental reports, visit https://apnews.com/hub/environment

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