The painfully cold weather system that put much of the Midwest into a historic deep freeze was expected to ease Thursday, though temperatures still tumbled to record lows in some places.

But the crazy weather ride isn't over yet.

The Twin Cities fell to about 25 below zero Thursday morning but could rocket up to 45 degrees above zero by Sunday — and then plummet again to about 10 below by Wednesday night, the National Weather Service office in Chanhassen said Thursday.

That works out to a temperature swing of about 125 degrees within seven days.

While Sunday may feel like shorts and sandals weather, it's still a few days away. And the problems brought on by the frigid temperatures are lingering.

Disruptions caused by the past few days of cold will persist, including natural gas outages.

Schools throughout Minnesota remained closed, but students in eastern North Dakota headed back to school Thursday as the weather was forecast to crawl out of double-digit subzero temperatures.

As temperatures bounce back into the single digits Thursday and into the comparative balmy 20s by Friday, more people were expected to return to work.

The blast of polar air that enveloped much of the Midwest on Wednesday closed schools and businesses and strained infrastructure with some of the lowest temperatures in a generation. The deep freeze cracked a Blue Line light rail track in Minneapolis, forcing trains to share the remaining track for a few hours.

The Postal Service took the rare step of suspending mail delivery in many places, and in southeastern Minnesota, even the snowplows were idled by the weather.

The bitter cold was the result of a split in the polar vortex, a mass of cold air that normally stays bottled up in the Arctic. The split allowed the air to spill much farther south than usual.

Officials in dozens of cities focused on protecting vulnerable people from the cold, including the homeless, seniors and those living in substandard housing.

The cold had already been a concern before Tuesday. A Rochester, Minn., man apparently died from exposure during last weekend's cold snap.

Ali Gombo, 22, was apparently locked outside his home without a key to get inside, police said, and officers found him dead on a deck when they responded Sunday morning. A preliminary autopsy found Gombo died of hypothermia, according to Capt. Casey Moilanen.

Moilanen said Gombo had been out at a bar Saturday night. Overnight low temperatures going into Sunday were minus 15 in Rochester.

This week, the cold was cited in the death of an elderly Illinois man who was found several hours after he fell trying to get into his home and a University of Iowa student found behind an academic hall several hours before dawn.

Aside from the safety risks and the physical discomfort, the system's icy grip also took a heavy toll on infrastructure, halting transportation and knocking out electricity and natural gas.

Xcel Energy asked all its Minnesota customers to conserve natural gas and set thermostats to 63 degrees through Thursday morning. The request came as the utility struggled to restore gas service in some communities north of the metro area, including around Princeton, where Xcel booked hotels for people who'd lost service.

Xcel said it hopes to restore gas to all affected homes by Friday.

MPR News reporter Cody Nelson contributed to this report.