Was the US cluster of tornadoes Linked to climate change?

The calendar said December, but the warm, humid air screamed spring. Add an eastbound storm front guided by a La Niña weather pattern in that mismatch and spawned tornadoes that killed dozens more than five US states.

Tornadoes in December are unusual, but not unheard of. But the ferocity and path length of Friday night tornadoes it probably puts them in a category of their own, meteorologists say. One of the tornadoes, if confirmed to be just one, likely broke a nearly 100-year record for how long a tornado stayed on the ground on a path of destruction, experts said.

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

Warm weather was a crucial ingredient in this tornado outbreak, but whether climate change is a factor is less clear, forecasters say.

Scientists say figuring out how climate change is affecting the frequency of tornadoes is tricky and understanding is still evolving. But they do say that the atmospheric conditions that give rise to such outbreaks are intensifying in the winter as the planet warms. And the tornado alley is moving further east, away from the Kansas-Oklahoma area, toward the states where Friday’s killers struck.

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

WHAT CAUSES A TORNADO?

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Tornadoes are rotating vertical columns of air that form from thunderstorms and extend to the ground. They travel at ferocious speed and destroy everything in their path.

Thunderstorms occur when thicker and drier cold air is pushed onto warmer, more humid air, conditions that scientists call atmospheric instability. As that happens, an updraft is created when the hot air rises. When winds vary in speed or direction at different altitudes, a condition known as wind shear, the updraft will begin to rotate.

These changes in the winds produce the necessary spin for a tornado. For especially strong tornadoes, changes in both wind speed and direction are needed.

“When there is considerable variation in the wind in the lower few thousand feet of the atmosphere, it is possible for ‘supercellular storms’ to occur that produce tornadoes,” said Paul Markowski, professor of meteorology at Pennsylvania State University. “That’s what we had yesterday.”

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

But typically, there’s not much of the instability in winter needed for tornadoes because the air isn’t as warm and humid, Gensini said. This time there was.

WHAT CONDITIONS PRODUCED STORMS OF THIS SCALE?

Some factors, which meteorologists will continue to study.

Spring temperatures across much of the Midwest and South in December helped bring in warm, humid air that helped form thunderstorms. Some of this is due to La Niña, which generally brings warmer-than-normal winter temperatures to the southern US But scientists are also expecting outliers, warm weather in the winter to become more common as the planet warms.

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

Once the storm formed, exceptionally strong wind shear appears to have kept the tornadoes from dissipating, experts say. Tornadoes are believed to die when updrafts from storms lose energy.

Tornadoes generally lose energy in a matter of minutes, but in this case it was hours, Gensini said. That’s part of the reason for the exceptionally long track of Friday’s storm, which traveled more than 200 miles (322 kilometers) or so, he said. The record was 219 miles (352 kilometers) and was set by a tornado that struck three states in 1925. Gensini believes this will surpass it once meteorologists finish analyzing it.

“To get a really long route, you need to have a really fast moving storm. This storm was moving at more than 50 miles (80 kilometers) per hour for most of its life, ”Gensini said. That is not the speed of the winds, but the general movement of the storm.

“You’re talking about highway speed storm movements,” Gensini said.

HOW RELATED IS CLIMATE CHANGE TO TORNADO OUTBREAKS?

It’s complicated. Scientists are still trying to resolve the many conflicting factors about whether human-caused climate change is making tornadoes more common, or even more intense. About 1,200 tornadoes hit the US each year, though that number can vary, according to NOAA’s National Severe Storms Laboratory. No other country sees so many.

Attribute a specific storm like the one on Friday to the effects of climate change remains a great challenge. Less than 10% of severe thunderstorms produce tornadoes, making it difficult to draw conclusions about climate change and the processes that lead to them, said Harold Brooks, a tornado scientist at the National Severe Storms Laboratory.

However, scientists have observed changes in the basic ingredients of a thunderstorm as the planet warms. Gensini says that overall, extreme storms “are becoming more common because we have much warmer air masses in the cold season that can withstand these types of severe weather outbreaks.”

More tornadoes are likely in the United States in the winter, Brooks said, as national temperatures rise above the long-term average. Fewer events will be held in the summer, he said.

Furtado of the University of Oklahoma said that tornado alley, a term used to describe where many tornadoes hit the US, has drifted east into the Mississippi River valley. That change is due to increases in temperature, humidity, and shear.

“Bottom line: people in the Mississippi River Valley and the Ohio River Valley are becoming increasingly vulnerable to more tornadic activity over time,” he said.

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