LAFAYETTE, Tenn. (AP) -- Winter tornadoes are not uncommon. The peak tornado season is late winter through midsummer, but the storms can happen at any time of the year with the right conditions.
But this batch Tuesday was the nation's worst in a 24-hour period since May 3, 1999, when some 50 people died in Oklahoma and Kansas. The death toll ranks among the top 15 from tornado outbreaks since 1950.
The tornadoes could be due to La Nina, the cooling of the tropical Pacific Ocean that can cause changes in weather patterns around the world. It is the opposite of the better-known El Nino, a periodic warming of the same region.
Recent studies have found an increase in tornadoes in parts of the southern U.S. during the winter during a La Nina.
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