May 16, 2019

Climate change may subtly shift Tornado Alley

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Adapted from Gensini and Brooks, npj Climate and Atmospheric Science 2018. DOI: 10.1038/s41612-018-0048-2. Data: NOAA; Map: Harry Stevens/Axios

While scientists prowl the Plains in search of monster storms, others are looking at broader-scale trends that show tantalizing clues about how Tornado Alley may be shifting both geographically and temporally as the climate changes.

Why it matters: The U.S. has the greatest number of tornadoes of any nation on Earth, and where they occur affects emergency management preparations, insurance markets and individual decisions on whether to build a storm shelter. If, as global warming continues, Tornado Alley migrates, or outbreaks become more massive, this would shift the risk distribution.

Details: According to Harold Brooks, a senior researcher at the National Severe Storms Laboratory in Oklahoma, the overall number of tornadoes of EF-1 intensity or greater touching down in the U.S. each year has not changed in a statistically significant way, averaging around 500.

  • However, his research and that of others has shown an increase in tornado risk in parts of the mid-South and a slight decrease in what is more traditionally considered Tornado Alley in the Plains.
  • A 2018 study showed regional shifts in tornado frequency, with an uptick in tornadoes east of the Mississippi River and a slight decline to the west.

Between the lines: Some of the emerging trends are seemingly contradictory. Tornadoes are occurring on fewer days per year, but major tornado outbreaks are spawning more EF-1 or greater twisters than used to occur in a typical large event.

A warming climate does opposing things to supercell thunderstorms.

  • It provides more energy in the form of higher air temperatures and greater amounts of water vapor in the atmosphere, a combo meteorologists refer to as convective available potential energy.
  • But it also may reduce the availability of wind shear, which is another crucial ingredient for tornadoes.
  • In a warming world, there may be fewer days with both ingredients present to produce tornadoes, but when these ingredients do combine ... watch out.

What they're saying: Brooks says it's not yet known what physical mechanism is causing the big tornado days to get bigger, while fewer days with a small number of tornadoes occur. Nor is there a clear cause for the spike in activity in the mid-South.

  • "That’s a big question that I would love to know the answer to," he said, noting that research is underway to find out.

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