Potentially record cold will inundate most of the United States this week—following a relatively warm November, according to forecasters.

“This is definitely a huge cold air outbreak for the western U.S.,” Jason Furtado, an atmospheric scientist with the University of Oklahoma, told Mashable. “We’re talking temperatures 12 to 15 degrees Celsius below average for a few days in the West.”

“This is going to be a real shock to the system.”

Currently, Alaska is experiencing frigid temperatures, and some of that weather system might be sent into the western U.S. The temperature is minus-31 degrees Fahrenheit in Fairbanks, representing the coldest day in the city in nearly two years, the Washington Post reported. Other Alaska towns—like Bettles—saw even lower temperatures.

Arctic air will make inroads to the Lower 48 this week after its current visit to Alaska. Slips down into the western U.S.
A ‘Tale of Two Continents’ mid-late this week. Extreme cold air outbreak in Western North America but mild in the East.

Between the start of 2016 and October of this year, Alaska was having its warmest year on record.

Forecasters say the cold will start in the western U.S., but it will eventually move east progressively.

“It’s gonna be a whiplash for sure,” Furtado told Mashable, referring to the plunge in temperatures.