For the past several months, an El Niño of “super,” “mega,” and “monster” proportions has been making headlines. Climate scientists say this event will likely lead to an overall mild winter for the U.S., but a look at the polar vortex reminds us that El Niño won’t be the only factor shaping the season.

Forecast models show that the polar vortex could undergo an “unprecedentedly” early large disruption in the first weeks of November, according to Judah Cohen, a climatologist at the Massachusetts Institute of Technology and a leading expert on this phenomenon. He tracks the polar vortex and issues forecasts via his blog, and his latest outlook suggests it could soon make history.

“This level of polar vortex disruption is most common in January and February,” Cohen told Gizmodo in an email. “Occasionally it has happened in December, though none so far this century.”

In fact, there are no polar vortex disruptions of this magnitude in November in the satellite record, and there are only two documented in the pre-satellite record, which is much less reliable, Cohen said. Now, “the models are predicting one almost three weeks earlier than the earliest-known large polar vortex disruption, which is a remarkable record in my opinion.”

S. weather remains uncertain. While other forecasters agree that a polar vortex disruption—also known as a “sudden stratospheric warming” (SSW) event—is likely on the way, that doesn’t necessarily mean we’ll get an early taste of winter.

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