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By Elizabeth Chuck

A brutal cold snap hit the East Coast on Tuesday, sending temperatures plunging well below freezing for tens of millions of residents.

The cold front came after a record warm December — and forecasters say the Arctic air could become even colder and more widespread later in the month.

Temperatures were the lowest they've been all season from the Deep South up to Maine on Tuesday, with punishing wind chills. In Buffalo, New York, the wind chill made single-digit temperatures feel like negative 3 degrees on Tuesday morning. In Boston, it felt like negative 6. And in Bangor, Maine — even without the wind chill — it was negative 5 degrees.

Most places will warm up in the latter part of the week, but the relief will be short-lived, according to The Weather Channel. By mid- to late next week, a cold air mass is forecast to build south from the Arctic across parts of Canada, then move to much of the United States — not just the East Coast.

Related: Sick of El Niño? You Ain't Seen Nothing Yet, Warns NASA

Early forecasts indicate daily highs in the Upper Midwest and northern Plains will struggle to get out of the single digits or teens, The Weather Channel said.

A woman bundled against the wind and cold walks across the Rachel Carson Bridge over the Allegheny River in downtown Pittsburgh, Monday, Jan. 4.Gene J. Puskar / AP

The frigid temperatures follow a relatively balmy December, in which many along the East Coast enjoyed a Christmas Eve with temperatures peaking in the 70s, in part due to El Niño.