In what has been a generally benign and gentle winter around here, January is about to end on a decidedly blue note. Thursday is likely to be the coldest last day of January in Philadelphia since record-keeping began during the administration of President Ulysses S. Grant 146 years ago.

The region will be spared the ferocity of the Arctic invasion in the Midwest that snapped rail lines and forced the cancellation of hundreds of flights in Chicago, where the low Wednesday was 23 below zero. That was the temperature, not the wind chill. It hadn’t been that cold out there in 25 years. Minneapolis got to 27 below; Sioux Falls, S.D., minus 25. And in Grand Forks, N.D., it was 25 below at 6 p.m. Wednesday, going down to minus-30 overnight.

But it won’t be a day at the beach around here either, and given the paucity of extreme cold this winter, the air likely will have a particular sting. The National Weather Service has issued an advisory for wind chills as low as 11 below.

From a single-digit low — the first of the winter — temperatures will struggle to reach 20 in the afternoon, with winds gusting past 20 mph. If the forecast holds, the day would set Jan. 31 records for lowest maximum reading — the record is 21, set in 1935 — and daily average temperature.

The Blues

With a low and single digits and high in the upper teens, Thursday is expected to be the coldest Jan. 31 on record inPhiladelphia. Here are records for low temperatures, average temperatures and lowest maximum temperatures for January.

SOURCE: National Weather Service
Staff Graphic

“This is a dangerous thing that’s going on," said Patrick Bokovitz, director of the Chester County Department of Community Development, who expected all of his county’s homeless shelters to be filled.

“Code blue” declarations were posted by counties across the region as officials opened “warming centers” and stepped up efforts to get the homeless into shelters.

For thousands of SEPTA riders, the morning commute might well be preceded by longer-and-colder-than-usual waits.

Although the agency probably won’t set fires near the tracks to keep the trains running, as Chicago transit officials did, SEPTA said freezing temperatures can cause tracks and overhead wiring to contract — and that can lead to breakdowns on Regional Rail and trolley fleets.

SEPTA is assigning personnel to be available in case there are track problems or power line breaks, and will have buses in place along rail routes to retrieve passengers stranded by train failures. It also will watch for freezing in air lines on trains and buses.

Meanwhile, the Camden City School District announced that all schools will be closed Thursday due to expected extreme cold. The central office will remain open.

Downtown Chicago streets on Wednesday were largely empty, as were trains and buses, after most offices told employees to stay home. The hardiest commuters ventured out only after covering nearly every square inch of flesh against the chill, which froze ice crystals on eyelashes and eyebrows in minutes.

The cold air was modified as it crossed land masses on its journey eastward. In the Philadelphia region, the vanguard of the Arctic outbreak arrived dramatically Wednesday afternoon, as the potent front set off snow squalls that briefly caused whiteout conditions.

The squalls were brief but furious — winds gusted to 52 mph at Philadelphia International Airport — and the arrival times were problematical. A massive pileup was reported on Route 222 near Route 422 in Wyomissing, Berks County.

Another nuisance snow is possible on Friday, said Dave Dombek, a meteorologist with AccuWeather Inc. “There’s a wake clipper that’s passing through. It could be an inch of snow. The trends in the last day or so have been a little more definite.”

Temperatures are forecast to reach the single digits in the city on Friday morning, and could drop below zero in areas to the north and west that have snow cover as a result of Tuesday’s snowfall and Wednesday’s booster shot.

On Thursday, Philadelphia will be considerably chillier than parts of Alaska, and that’s not an unusual juxtaposition. The cold is the result of the southward intrusion of a piece of the polar vortex that has migrated to the mid-latitudes. That has drained some of the cold air from the Arctic regions.

While the global temperature is about 1.6 degrees Fahrenheit higher than it was in the 20th century, according to the National Centers for Environmental Information, the warming wasn’t evenly distributed. The Arctic is warming at twice the rate of the rest of the globe, thus, it has been exporting some of its cold.

In a paper published in March, a group of researchers, including Rutgers University’s Jennifer Francis and Judah Cohen with the Atmospheric & Environmental Research firm, documented a correlation between winter extremes in the middle latitudes, especially in the East, and Arctic warming.

The forecasts are calling for the Arctic air to head back to where it came from. On Wednesday afternoon, temperatures were in the mid-30s in Anchorage; it’s heading to 10 below there on Saturday morning.

In Philadelphia, Friday’s high will be in the mid-20s, rising to the mid-30s on Saturday and 50-plus early next week.

“Our turnaround here is going to be pretty dramatic,” Dombek said. “It’s doubtful that the mild air is going to hold for a long period of time, but it could be several days.”

And the next reassertion of winter isn’t in sight, said Dean Iovino, a meteorologist at the National Weather Service office in Mount Holly.

“I didn’t notice any strong signal for anything returning,” he said.

Staff writers Melanie Burney, Joseph A. Gambardello, and Jason Laughlin contributed to this article, which also contains information from the Associated Press.