ALERT: Large-Scale Tornado Outbreak Becoming More Frequent in the US

A new study revealed that the frequency of large-scale tornado outbreaks, or sequences of tornadoes that occur in close succession, have been increasing in the U.S.

The study, published in the journal Science, also showed that extremely powerful tornado events are also becoming more frequent in the U.S. However, contrary to previous climate models, the study found that the increasing frequency of tornado outbreaks does not appear to be the result of the warming climate.

"What's pushing this rise in extreme outbreaks, during which the vast majority of tornado-related fatalities occur, is far from obvious in the present state of climate science," said Joel E. Cohen, the Abby Rockefeller Mauzé Professor at Rockefeller University and Professor of Earth and Environmental Sciences at Columbia University, in a press release.

For the study, the researchers analyzed observation-based meteorological estimates associated with tornado outbreaks, in addition to datasets provided by the National Oceanic and Atmospheric Administration (NOAA) using new statistical tools. One of the tools used by the researchers is the so-called extreme value analysis, which deals with deviations.

Using all the available data, the researchers were able to estimate the number of tornadoes in the most extreme outbreak in a five-year interval. The researchers found that the number of extreme tornado outbreaks in a five-year interval has doubled in the past century, which means that in 1965 the worst outbreak expected over five years would have had about 40 tornadoes, while in 2015 would have had about 80 tornadoes.

The researchers also discovered that the warming climate is not responsible for the upward trend of tornado outbreaks' frequency. Previously, climate scientists thought the increasing convective available potential energy or CAPE due to global warming could increase the frequency of tornado outbreaks by creating more favorable conditions. However, the current study showed that the outbreaks were more likely to be driven by the storm relative helicity, which is the measure of vertical wind shear and not been projected to increase under a warming climate.

Tornado outbreaks usually last one to three days and could feature several thunderstorms and six or more tornadoes in close succession.