The extreme weather that has hammered California with runaway wildfires and hit Louisiana with its most powerful hurricane in 160 years may be about to get even worse.

La Nina -- a phenomenon that occurs when the surface of the Pacific Ocean cools -- has officially formed, the U.S. Climate Prediction Center said Thursday. It triggers an atmospheric chain reaction that stands to roil weather around the globe, often turning the western U.S. into a tinder box, fueling more powerful hurricanes in the Atlantic and flooding parts of Australia and South America.

"We're already in a bad position, and La Nina puts us in a situation where fire-weather conditions persist into November and possibly even December," said Ryan Truchelut, president of Weather Tiger LLC. "It is exacerbating existing heat and drought issues."

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