After a relatively mild December, the eastern U.S. might be in for a chilly wake up call late next week, when cold, Arctic air is forecast to plunge south in true winter fashion. Many locations along the Eastern Seaboard will be facing their coldest temperatures of the season so far, and bitterly cold wind chills.

We’ve known for a couple of weeks now that the weather pattern over the U.S. was going to take a wintry shift in early January, transitioning from above average temperatures to bitter cold across much of the eastern U.S. Though it’s still a week away and hard to pin down exactly how cold it might get, forecast models are beginning to paint a picture of next week’s Arctic blast, and the magnitude of the chill appears large enough to warrant a discussion.

The jet stream is forecast to take a deep dive into the Lower 48 starting Tuesday, which will pump cold air south starting in the northern Plains and Upper Midwest. The chill spreads south and east quickly, bringing temperatures anywhere from 15 to 40 degrees below average to much of the eastern U.S. by Thursday morning.

The Midwest will likely endure the worst of the cold, when temperatures could be running up to 20 to 40 degrees below average on Wednesday and Thursday.

After Saturday’s forecast high of 35 degrees, Chicago likely won’t pull itself above the freezing mark through the rest of the week. The National Weather Service is forecasting just eight degrees on Wednesday, after a high of nine degrees on Monday for the city. The forecast high in Minneapolis on Wednesday is a teeth-chattering zero degrees.

In Washington, D.C., highs might not break the 30-degree mark on Wednesday or Thursday as the cold front reaches the Eastern Seaboard. Thursday morning lows will probably be in the teens around D.C., and possibly in the single digits in its cooler suburbs. With winds around 15 mph, the temperature will feel more like zero. The Weather Underground forecast for Washington, D.C., shows the wind chill diving below the freezing mark on Monday, not to return to above-freezing until the following Sunday.

Reagan-National Airport hasn’t recorded a low below 20 degrees since March 4, 2014. The lowest temperature so far this season was 22 degrees on Nov. 19. December’s lowest temperature was 29 degrees on the 31st.

Highs around Boston might be hard pressed to climb out of the single digits on Thursday — also the coldest temperatures of the season for the location.