By Laila Kearney and Brendan O’Brien

NEW YORK (Reuters) -The U.S. Northeast braced on Thursday for a historic deep freeze, with wind chills expected to plunge to the equivalent of -50 degrees Fahrenheit (-46 Celsius) in some spots, while freezing rain in the South left thousands without power and turned roadways into ice rinks.

Dangerous wind chills were likely in an area stretching from northern Pennsylvania to Maine starting early on Friday and through Saturday evening, the National Weather Service said in its forecast.

“The wind chills have the potentially to be once-in-a- generation cold,” the weather service said, urging people to either stay indoors or take precautions against frostbite and hypothermia. The wind chill factor describes the combined effect of wind and cold temperatures on exposed skin.

In New Hampshire’s Mount Washington State Park, atop the Northeast’s highest peak, record-breaking wind chills of -110 degrees and wind speeds topping 100 miles (160 km) per hour were expected.

“It’s definitely wicked cold, you can say that,” said Frances Tarasiewicz, a weather observer for the park.

“Today it’s a seasonal 5 degrees, but it’s coming at us quick,” he said of the cold blast headed for the Mount Washington Observatory, where staff members live on the mountain in eight-day shifts.

For residents of the Northeast, the frigid blast may feel especially jarring after weeks of unseasonably warm weather in most of the region and a dearth of snow except in pockets of western and northern New York State.

Boston Mayor Michelle Wu declared a cold emergency for New England’s biggest city for Friday through Sunday, as the city is expecting below-zero wind chills starting Friday. Boston has opened warming centers so people can get out of the cold.

“I urge all Boston residents to take precautions, stay warm and safe, and check on your neighbors during this cold emergency,” Wu said in a statement.

As the Northeast prepared for the cold blast, Texas and parts of the South were dealing with the aftermath of a winter storm that brought days of freezing rain, sleet and ice accumulation, causing widespread power outages and dangerously slick roadways across the region.

More than 450,000 homes and businesses in Texas, Arkansas, Mississippi and Tennessee were without electricity early on Thursday, according to Poweroutage.us, after half an inch (1.3 cm) of ice in some spots toppled power lines and trees.

Officials across the region also urged motorists to stay off the roads after icy roadways across the region were blamed for eight deaths over the past several days, CBS News reported.

Around the state, Texans took to social media to post images of vehicles sliding sideways on slippery highways and ice-crusted tree limbs bending under the weight. A video posted on Twitter by user Rob Quigley shows a man on ice skates playing hockey on a glazed sidewalk in a Dallas suburb.

The cold blast and ice storms have come as the celebrated groundhog known as Punxsutawney Phil saw his shadow on Thursday, Groundhog Day, a sign of six more weeks of frosty weather in North America.

Even so, mild temperatures are headed back to much of the Northeast starting Sunday, said Bob Oravec, a forecaster with the National Weather Service’s Weather Prediction Center in College Park, Maryland.

“It certainly has been a mild winter in the Northeast, despite this brutal cold snap, which will be relatively short-lived,” he said.