For now, frigid air has retreated to the frozen north, and many areas to the south are basking in springlike warmth.

Since late December, weather over the eastern United States has at times more closely resembled March or even April. Before a rare, seasonably cold day Thursday in Washington and Boston, the cities saw streaks of 17 and 19 consecutive days with higher-than-average temperatures.

Temperatures are once again forecast to spike this weekend, rising 30 degrees or more above normal for this time of year across much of the Interstate 95 corridor, surpassing 60 degrees in the Northeast and flirting with 70 in the Mid-Atlantic.

Some of the highest temperatures, compared with averages in January, have been recorded in the Midwest. Kansas City’s average temperature in January is running 12 degrees above normal, after a record high of 66 degrees Thursday.

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Only fleeting blows of Arctic air have managed to enter the Lower 48.

Judah Cohen, a meteorologist at Atmospheric and Environmental Research, a Verisk company, explains that the polar vortex, the zone of frigid air in the high latitudes surrounded by powerful winds, has been unusually strong. When the polar vortex is intense, the coldest air over the Northern Hemisphere remains bottled up in the Arctic.

Conversely, enduring cold snaps usually overtake the Lower 48 only when the vortex has substantially weakened, stretched or split into different flows.

“I’m very surprised by how strong the polar vortex has been and continues to be,” Cohen said. “It’s hard for cold air to expand across the Northern Hemisphere.”

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Meanwhile, the cold air bottled up over the Arctic is extreme. At the top of the Greenland ice sheet last week, the temperature fell to minus-87 degrees (minus-66 Celsius), the second-lowest reading in the past 11 years, according to Christopher Shuman, a glaciologist at the University of Maryland-Baltimore County and NASA Goddard Space Flight Center. (Shuman cautioned that this temperature measurement is preliminary and still requires validation. He noted that reports on Twitter that it was the lowest on record in Greenland are erroneous.)

It has also been excessively cold in Alaska. In Fairbanks, low temperatures have consistently fallen to near minus-40 over the past week, and the temperature is averaging more than 18 degrees below average during January.

Although the cold lodged over the frozen north is intense, it covers a historically small area for this time of year. Jonathan Martin, a professor of meteorology at the University of Wisconsin, wrote in an email that the size of the “cold pool” over the Northern Hemisphere, which is indicated by temperatures of 23 degrees or lower a mile above ground, ranks as the smallest on record for December and early January.

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Martin noted that winters that begin with small cold pools, which are indicative of warm conditions averaged over the hemisphere, often end up that way. In winters after the 10 warmest Decembers, based on cold pool size, seven have ranked among the top 10 warmest for the hemisphere, he wrote.

Martin’s research has shown the size of the Northern Hemisphere cool pool shrinking over time, an indicator of climate change.

The combination of the strong polar vortex and shrunken cold pool is potentially bad news for winter weather lovers. But it’s not all doom and gloom.

“There is some elongation of the polar vortex going on,” Cohen said, which might help unlock some cold air later this month into early February. But he noted that “it’s not an impressive weakening event” that would lead to more-sustained and severe winter weather.

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Unless the vortex is seriously disturbed in the coming weeks, Cohen said, the pulse of colder conditions later this month into early February “may be all the winter we have.” But he cautioned that predicting the polar vortex is difficult. It could still suddenly become disturbed and unleash another “Chiberia.”

Other long-range forecasters see additional reasons not to count winter out, noting that the polar vortex is not the only pattern that determines winter’s severity.

Commodity Weather Group president Matt Rogers sees a “big driver for a possible pattern change” from what’s known as the Madden-Julian Oscillation, a large overturning circulation in the tropics that propagates east across the globe. He sees it shifting to a colder phase in about seven to 10 days.

The change in this oscillation may help rearrange the configuration of weather systems so that the cold air parked over Alaska and Canada can spill south into the Lower 48.