El Niño, La Niña Predict Severity of Tornado Season, Study Says

You’ve probably heard the phrases El Niño and La Niña, likely in the context of weather patterns and your local forecast. What are they, exactly? “In the Pacific Ocean, around about the equator, there is either an upwelling of cold water, which goes westward, or there’s warm water. The warm water condition is called El Niño and the cold water condition is called La Niña,” John Allen, a scientist at the International Research Institute for Climate and Society, explained to weather.com.

Forecasters have previously looked at these phenomena as one factor in making drought, hurricane and flooding predictions. Now Allen and colleagues at Columbia University’s Earth Institute and the Lamont-Doherty Earth Observatory suggest El Niño and La Niña can help anticipate tornado and hailstorm frequency in parts of the United States. Projections for this year show an average or below average season, according to Allen, though it’s likely still too early to tell whether those will bear out.

“The kind of winter that we’ve had this year, for example, so far has been one that was in more or less of a neutral phase,” said Dr. Greg Forbes, severe weather expert for The Weather Channel. In fact, March 2015, a month that typically sees an average of 80 tornadoes, has been cyclone-free, according to the National Climatic Data Center. This hasn’t happened since 1969, according to Forbes. 

To look for this new connection, the researchers studied the ability of tornadoes and hail storms to form under certain environmental conditions like variable air temperatures, moisture and wind shear (how winds change at different altitudes). They then looked at when El Niño and La Niña happened.

What they found is that with the latter comes an atmospheric pattern more favorable to tornadoes and hailstorms, whereas the former does the opposite. “Oklahoma, northern Texas, Louisiana, Arkansas, southern Missouri, southern Kansas into western Tennessee, Mississippi, that region, that sums up where the biggest influences from El Niño and La Niña are,” Allen said. Outside that region, the influence is much weaker, the study said.

Why not just tally up actual tornadoes instead of focusing on the weather conditions that spawn them? Historically, tracking tornadoes wasn’t scientific or systematic, said Forbes, who was not involved in the research. “In decades past, sometimes even the official records of tornadoes got made by the National Weather Service looking at newspaper clippings,” he explained. “So a lot of the weak tornadoes in the ’50s and ’60s and ’70s didn’t get reported like they are right now.”

Allen, who has a background in meteorology and climatology, agrees with that assessment, but said this could help with disaster preparedness. “It’s useful to know if you’re going to get a 2011 sort of event where you get these enormous tornado outbreaks,” he said. “It’s good to have some forewarning so you can make sure your resources are prepared.” During a single four-day period in April of that year, more than 200 tornadoes swept across the southeast, taking hundreds of lives. 

One point Allen makes is that this research cannot pinpoint when a tornado or severe thunderstorm will happen. And even the data isn’t always spot on. “Many of the big tornado events have actually occurred in years when we may not have expected,” he said, adding a note of caution: “It only takes one tornado to track through a city and you end up with a disaster.”

The researchers published their work Monday, Mar. 16, 2015, in the journal Nature Geoscience. 

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