The extreme weather that’s 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.” The effects are already evident. Rising temperatures and an extreme mega-drought across the U.S. West are fueling fires from Washington to Arizona. California is having its worst fire season on record, torching an unprecedented 2.5 million acres. And in the Atlantic, a record number tropical storms have formed by September, including Hurricane Laura, which killed more than a dozen people across the Caribbean and the U.S. last month.

La Nina doesn’t just mean more heat. It also raises the chances for a colder winter across the northern U.S. and increases the prospects of floods in northern Australia and more rain in Indonesia and in Brazil’s three southern most states. California’s rainy season typically starts by early winter, and can counter the fire-spreading Santa Ana winds that are starting now. But if La Nina gets in the way, it could have dire consequences for the state where fires have already charred more than 2.5 million acres, the most on record, and 80% of the land is abnormally dry.

“It can create a ripple effect over North America,” said Michelle L’Heureux, a forecaster with the Climate Prediction Center in College Park, Maryland. In the Atlantic, having La Nina develop in the last three months of hurricane season could mean more storms of greater power. This year’s 17 named storms make for the quickest that tally has been reached in data going back to 1851, well above the annual average of 12.

La Nina can lead to more more powerful hurricanes in the Atlantic because it chokes off turbulent winds that blow at varying speeds, directions and altitudes. Those gusts, known as wind shear, can lop to the top off a growing tropical storm or hurricane — weakening it or destroying it altogether.