– Bitter cold is setting in after a major winter storm blanketed a wide swath of the country in snow, sleet and rain this weekend, creating dangerously icy conditions that promise to complicate cleanup efforts and make travel challenging on Martin Luther King Jr. Day.

Some of the lowest temperatures seen so far this season started to set in across the Midwest and Northeast on Sunday. Windchills will bring temperatures into the teens in the New York City area and down to 40 below in upstate New York, the National Weather Service forecast.

In New England, windchills will fall to as low as 20 below zero around Boston and as low as 35 below in parts of Vermont, Maine and New ­Hampshire.

"It's life-threatening," said Ray O'Keefe, a National Weather Service meteorologist in Albany. "These are dangerous conditions that we're going to be in, and they're prolonged right through tomorrow."

The freeze will follow the weekend's run-ins with power outages, canceled trains and planes, overnight stays at the airport and traffic jams.

Utilities in Connecticut reported more than 20,000 customers without power by Sunday afternoon.

"We had more freezing rain and sleet than we expected," Hartford Mayor Luke Bronin said Sunday as public works crews across the state raced to clear and treat major roadways before dangerous black ice could form.

Amtrak canceled trains across the Midwest and Northeast over the weekend but promised full service would resume Monday. Boston's transit system urged commuters to allow 10 to 15 minutes of extra travel time and warned of icy conditions for pedestrians come Monday.

The storm — caused by the clash of an Arctic high-pressure system with a low-pressure system coming through the Ohio Valley — wreaked havoc on air travel and other forms of transportation all weekend.

More than 1,500 flights were canceled nationwide Sunday. Among the hardest hit was Boston's Logan Airport, where stranded passengers lingered Sunday as typically bustling security lines, ticketing counters and baggage claims were largely deserted.

In Kansas, a snowplow driver was killed when his vehicle rolled over, and in southeastern Missouri, slippery conditions caused a 15-vehicle crash on Interstate 55 on Saturday.

One saving grace of the storm: Heavily populated coastal communities from New York to Boston largely escaped major snowfall after days of sometimes dire ­predictions.