Climate: Are tornado outbreaks becoming more frequent?

A new NOAA study tracks the occurrence of seasonal tornadoes across the U.S.
Severe tornado outbreaks across the U.S. are increasing. Photo courtesy NOAA.

Average number of tornadoes per outbreak has increased by 50 percent since 1954

Staff Report

Forecasting tornadoes has never been easy, and when you add global climate change to the equation, it becomes even murkier. In some studies, researchers say they haven’t been able to pinpoint any long-term trends in tornado activity, while other research suggests tornado seasons are becoming more variable.

In one recent 10 year period, tornadoes in the United States resulted in an average of 110 deaths per year and annual losses ranging from $500 million to $9.6 billion, so trying to establish patterns and improve forecasting models is not just an idle mathematical exercise.

A new examination of tornado records during the past 60 years indicates that tornado outbreaks (six or more tornadoes during a limited time) have become more frequent. Such outbreaks result in the largest numbers of deaths and injuries, as well as the most property destruction. The study, published recently in Nature Communications, also shows that mathematical models show the chance of such extreme events is growing over time.

Analyzing data from the National Oceanic and Atmospheric Administration, the researchers concluded  the average number of tornadoes per outbreak increased by about 50 percent from 1954 to 2014. But the total number of tornadoes annually has stayed about the same during that time, suggesting that tornadoes are increasingly clustered in outbreaks.

The study also tried to track patterns in the variability of tornado outbreaks. In a press release explaining the study findings, Cohen said both the mean and variance of the number of tornadoes per outbreak were increasing.

“An increase in both of these values over time is noteworthy. if the mean number of tornadoes per outbreak is larger each year, and the range of the values is more widespread each year, it suggests that outbreaks with an extreme number of tornadoes are more likely in the future,” he said.

When they looked at the number of tornadoes per outbreak, the researchers found that the variance is increasing four times faster than the mean … suggesting that something new is happening with tornado outbreaks.

“Variance is growing faster than I would have guessed, and we don’t know why it’s so different from what we find elsewhere,” said Cohen. “If I were to speculate, I would say that certain physical drivers are accelerating, leading to increased energy in the atmosphere, which affects the forces behind tornadoes. Our results do not directly link climate change to the increasing severity of outbreaks, but we’ve found an indicator of change that’s hard to explain otherwise.”



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