THE FRIGID TEMPERATURES didn’t keep some from enjoying a sunny day without school Thursday. From left, Kyle Blazier and Tyler Phelps chase after Logan Phelps as he rides a snowmobile around the Phelps yard on Withworth Avenue in Kingsford. (Theresa Proudfit/Daily News photo)

IRON MOUNTAIN — A warmup is in store for the weekend in the Upper Peninsula after nearly a week of being in the grip of arctic-like cold.

Most area schools resumed classes today despite continued extreme cold, with the North Central School District in Powers being the exception.

The U.S. Postal Service also was expected to make deliveries today after suspending rounds for two days.

The area did shiver through one additional night of temperatures that plunged by double digits below zero. The Kingsford observation station still hovered at about minus 17 this morning, one degree shy of the overnight low of minus 18.

The forecast calls for a low of 10 degrees tonight, with a slight chance of snow falling from mostly cloudy skies, according to the National Weather Service.

Saturday is expected to be partly sunny, with a high of 27 degrees.

But Sunday will see the biggest change, with a high of 38 degrees. Monday brings the same, but with the prospect of freezing rain.

Other parts of the Midwest could see temperatures swing by as much as 80 degrees. Experts say the rapid thaw is unprecedented, and it could create problems of its own — bursting pipes, flooding rivers and crumbling roads.

“I don’t think there’s ever been a case where we’ve seen (such a big) shift in temperatures” in the winter, said Jeff Masters, meteorology director of the Weather Underground firm. “Past record-cold waves have not dissipated this quickly. … Here we are going right into spring-like temperatures.”

Although many places remained painfully cold Thursday, the deep freeze eased somewhat, and the system marched east. In western New York, a storm that dumped up to 20 inches of snow gave way to subzero temperatures and face-stinging wind chills. On Friday, the wind chill in some areas was expected to be as low as minus 20 degrees.

A ban on trucks and commercial buses on the western section of the Thruway was lifted Thursday night. Gov. Andrew Cuomo had vowed to crack down on violators a day after a semitrailer crash near Rochester caused a pileup that injured a state trooper.

In New York City, about 200 firefighters battling a blaze in a commercial building took turns getting warm on buses.

Schools remained closed in Buffalo, New York, but students headed back to school in other parts of the Midwest. Educators in Fargo, North Dakota, were busy Thursday after two days of canceled classes. For Superintendent Rupak Gandhi, who came to Fargo last summer from Colorado, this week has been “a personal new.”

“I’ve had experience with cold and snow days, but negative 50? Absolutely not,” he said.

The dangerously cold weather is suspected in at least 17 deaths, including a man found frozen in his backyard in a Milwaukee suburb and a homeless man whose frozen body was found in a western New York bus shelter. Both men were found Thursday, the same day temperatures plunged to record lows in several Midwestern cities.

But relief from the bitter Midwestern cold is as close as the weekend. Rockford, Illinois, was at a record-breaking minus 31 degrees on Thursday morning but should be around 50 degrees on Monday. Other previously frozen areas could see temperatures of 55 degree or higher.

The dramatic warm-up will offer a respite from the bone-chilling cold that canceled school, closed businesses and halted trains. But potholes will appear on roads and bridges weakened by the freeze-thaw cycle. The same cycle can crack water mains and homeowners’ pipes. Scores of vehicles will be left with flat tires and bent rims.

Joe Buck, who manages Schmit Towing in Minneapolis, said he’s already taking calls for Monday to deal with a backlog of hundreds of stalled vehicles.

In Detroit, where some water mains are almost 150 years old, city workers were dealing with dozens of breaks, said Palencia Mobley, deputy director of the Detroit Water and Sewerage Department.

The thawing of the pipes can sometimes inflict greater damage than the initial freeze. Bursts can occur when ice inside starts to melt and water rushes through the pipe, or when water in the pipe is pushed to a closed faucet by expanding ice.

Elsewhere, a bridge in the western Michigan community of Newaygo, 40 miles north of Grand Rapids was closed as the ice-jammed Muskegon River rose above flood stage. Officials in Buffalo, New York, watched for flooding on the Upper Niagara River because of ice.

In other signs that the worst of the deep freeze was ending, Xcel Energy on Thursday lifted a request to its Minnesota natural gas customers to temporarily lower their thermostats to ease concerns about the fuel supply.

Earlier in the day, several cities set record lows, including Cedar Rapids, Iowa, which set a daily record low of minus 30 degrees.

Chicago’s temperature dropped to a low of around minus 21 degrees on Thursday, slightly above the city’s lowest-ever reading of minus 27 degrees in January 1985. Milwaukee’s low was minus 25 degrees, and Minneapolis recorded minus 24 degrees. Wind chills were lower still.

Masters, from Weather Underground, said the polar vortex was “rotating up into Canada” and not expected to return in the next couple of weeks. If it does return in late February, “it won’t be as intense.”

Still, memories of the dangerous cold were bound to linger.

In Illinois, at least 144 people visited hospital emergency rooms for cold-related injuries over two days. Most of the injuries were hypothermia or frostbite, according to a spokesman for the state Department of Public Health.

The effect on the overall economy was not expected to be that great, in part because there were no widespread power outages such as there are in a hurricane.