The eastern U.S. is on track for a frigid week as a polar vortex stretches south, bringing below-normal temperatures to Maine.

Starting Monday, much of the eastern two-thirds of the country will see temperatures 12 to 25 degrees colder than usual for this time of year.

In Portland, temperatures were only expected to be about 6 degrees below normal Sunday and Monday, said Jon Palmer, a meteorologist with the National Weather Service in Gray.

The interior parts of Maine will see the chilliest conditions, with lows in the single digits and below zero in some spots.

“We’re not really expecting really, really, really cold temperatures (in Maine) per se, but we could see some colder nighttime lows, especially more in the interior part of the state,” Palmer said. “The coast will stay a little bit warmer.”

Lesley Eshelman, right, and Tamara Sheen bundled up to walk along the Eastern Prom Trail together on Sunday. Temperatures in Maine are expected to drop even more in the coming days. Brianna Soukup/Portland Press Herald

The coldest air was expected to come in Sunday night, and temperatures will slowly warm up through Friday.

“Especially if you’re going up towards Rangeley and Fryeburg, really up into the mountains, they’re really going to see very, very cold air over the next week,” Palmer said.

The weather phenomenon is known as the polar vortex — a large area of low pressure and cold air that always exists near the poles. In the winter, the vortex can strengthen, sending cold air south and resulting in frigid temperatures and wind chills, according to the weather service.

Chilly temperatures come as a major winter storm moves from the Central Plains to the mid-Atlantic this weekend.

There was heavy snow between Kansas and Indiana, with up to 14 inches accumulating in some spots. Severe travel disruptions were expected to continue as the snow moves into the Ohio Valley this week.

The storm is set to track south toward Virginia and Washington, D.C., early this week, Palmer said, so New England won’t be impacted. Instead, the polar air will likely keep Maine dry for the rest of the week.

This story contains information from the Associated Press.