Every year, tornadoes, the most violent atmospheric storms, cause severe economic and environmental damage and loss of life in the U.S. Tornado outbreaks, which consist of multiple tornadoes occurring during a single weather event, are associated with the majority of fatalities and economic losses from tornadic events. Scientists can’t prevent tornado outbreaks from occurring, but they can look to long-term trends and climatology to better prepare the Nation for impacts. Although tornado outbreaks pose substantial risks to society, very little is known about current or future trends in tornado outbreak severity.
A new study in Nature Communications, funded by the CPO Modeling, Analysis, Predictions, and Projections program, evaluates how the average annual number of tornadoes per outbreak and spread in the data has changed over time. Michael Tippett and Joel Cohen found that the annual mean number of tornadoes per outbreak in the U.S. increased from 1954-2014, and that the annual spread (variance) increased four times faster than the mean over that time period. The scientists found the two to be related through Taylor’s Power Law, which says that an increase in the variance of tornado outbreak severity is exponentially proportional to an increase in the average tornado outbreak severity. According to the authors, this relationship suggests that the chance of a severe outbreak is increasing more quickly than would otherwise be expected. A greater tendency for very large and more extreme tornado outbreaks has important implications given the background of a warming climate and the potential for more frequent environments that are favorable to severe thunderstorms. This research supports NOAA’s efforts to bridge the gap between weather and climate forecast skill through a seamless prediction capability.
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