Study Finds Link Between El Nino and Tornado Frequency in US

By Mark Leberfinger, AccuWeather.com Staff Writer

April 6,2015; 8:17PM,EDT

 

 

It's been known that El Niño and La Niña can influence year-to-year variability of atmospheric moisture and temperature, which in turn affects the weather that occurs in the United States.
A Columbia University study, published in Nature Geoscience in March, has found that these patterns also have an impact on severe weather in Tornado and Dixie alleys.
Based on the current weak El Niño, researchers are forecasting a 60 percent chance of normal frequency of severe weather, 30 percent chance of below normal and 10 percent chance of above normal for the region encompassing Oklahoma, northern Texas, Arkansas, northern Mississippi and Louisiana, which suggests perhaps a relative quiet period ahead for the Plains.
Two tornadoes approach Pilger, Nebraska, on June 16, 2014. The twin tornadoes wreaked havoc in Pilger, killing two people and injuring dozens. (AP Photo/Eric Anderson, File).
"It's important to remember, though, that even the most quiet seasons still produce 800 tornadoes," John Allen of the International Research Institute for Climate and Society in the Earth Institute of Columbia University said.
Once a relationship has been derived with the observations, it can be used to estimate a climatology of the number of days favorable to the development of hail and tornadoes over the United States, based solely on environment, Allen said.
"This doesn't mean you will definitely see tornadoes or hail, just that the environment is more favorable to their occurrence - even the most favorable environment doesn't always produce a tornado or hail," Allen added.
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The two phenomena are part of El Niño Southern Oscillation, or ENSO.
ENSO's strongest signal occurs over the Northern Hemisphere's autumn and into the winter months, but the signal tends to persist into the spring, thereby modulating the environments and conditions that produce hail and tornadoes, Allen said.
When a pooling of warm Pacific water extends along the equator from the South America coast on west through the international date line for more than three consecutive months, there is an El Niño.

"In El Niño years, winds that usually bring warm, moist, thunderstorm-inducing air into the Plains are weakened," AccuWeather.com Meteorologist Ben Noll said. "Storm systems have a tendency to track across the southern tier of the United States, which can lead to more instances of severe weather across the Southwest, but fewer severe weather risks across the Central states or in Tornado Alley."
Due to the frequency of storms across the South during El Niño, there can be an uptick in severe weather along the Gulf coast, Noll said.
In developing their forecast, researchers also analyzed environmental factors such as the potential spin in the atmosphere and instability to estimate the monthly frequency of hail or tornado events, Allen said.
In a typical La Niña weather pattern, which is classified by cooler-than-normal water in the central and eastern equatorial Pacific Ocean, large areas of atmospheric energy called upper lows tend to track out of the Rockies and into the Plains.
"This can lead to a spring spike in severe storms across the mid-South, including states like Arkansas, Tennessee, Missouri, Oklahoma and Kansas," Noll explained. "Due to a variable Pacific jet stream, instances of severe weather in the mid-Atlantic and Southeast are more likely during La Niña events."
El Niño is budding across the equatorial Pacific at present, and it should mean that the quieter-than-usual severe weather season across the traditionally hardest-hit spots will continue, though seasonal variation will lead to an uptick in April, regardless of large-scale weather patterns like El Niño, Noll said.
"Over the next month or two, look for a zone that includes much of the mid-South to be targeted frequently for gusty thunderstorms," he said.