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It’s going to be warm across the eastern US this week

The 8-to-14-day temperature outlook from the National Weather Service’s Climate Prediction, issued Dec. 9.The National Weather Service’s Climate Prediction Center

It’s December, but you might want to pull your spring clothes back out of storage. This weekend, much of the United States — including the Northeast — was downright balmy, and meteorologists expect that the mild weather will stick around.

The eastern two-thirds of the United States — basically everywhere east of the Rockies — will probably see above-average temperatures over the next week, according to an 8- to 14-day outlook that the National Weather Service’s Climate Prediction Center released Dec. 6.

La Nina, a natural weather oscillation where Pacific Ocean surface temperatures drop, is a major contributor to the unseasonably warm weather. Climate change has also led to increasingly warmer winters.

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Several cities and states could set records for the warmest December on record, and many communities could see record-breaking high temperatures for this time of year. Many cities, including Boston, St. Louis, Kansas City,New York City, and Washington, D.C., have a near-zero chance of seeing a white Christmas.

Across a large swath of the country, including the mid-South, Tennessee and Ohio River valleys, southern Great Lakes, and interior Mid-Atlantic, the probability of above-average temperatures is at least 90 percent, the National Weather Service says. Brian Brettschneider, an Alaska-based climatologist, said that is “really quite remarkable.”

“It basically means that for nearly every model, for every range of outcomes, they’re all pointing toward much, much, much warmer than normal temperatures over a large fraction of the country,” he said. “To have that kind of confidence is just really uncommon, so it’s quite noteworthy.”

Brettschneider said the Climate Prediction Center’s scientists haven’t been this confident in such extensive above-average temperatures eight to 14 days into the future at any other time in the past 6 years.

Back in May, the US National Oceanic and Atmospheric Administration announced new 30-year averages for temperature and precipitation in the United States, which serve as a baseline for comparing the weather to recent history. The averages from 1991 to 2020 were, unsurprisingly, warmer than the previous set of mean temperatures taken from 1981 to 2010. But even compared to those new averages ,there’s a high probability that the temperatures will be higher than average in many areas.

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What’s behind this unseasonably warm weather? One major factor is La Nina, which formed this past autumn. The oscillation is characterized by abnormally chilly waters swirling in the eastern tropical Pacific and winds blowing warmer waters westward, which affects atmospheric conditions and weather around the globe.

One atmospheric current affected by La Nina is the jet stream: bands of air that moves from west to east. That ribbon of wind is set to move south, which could bring rain and snow to the western United States.

But then the jet stream will shoot upward toward Canada. As the stream moves northward, an area of high atmospheric pressure will form to the east, trapping warm temperatures beneath it like a lid on a pot.

“High pressure actually causes warming,” said Brettschneider. “Air starts sinking and getting compressed, and as it compresses, it warms.”

The compressed air will also prevent clouds from forming, meaning more of the sun’s rays will pour down through the atmosphere, heating the air up even more.

Though naturally occurring patterns are important, the climate crisis also plays a crucial background role. In the United States, winter — which meteorologists consider the period from December to February — is the most rapidly warming season. A 2020 analysis found that 98 percent of 242 cities across the nation have seen average winter temperatures increase since 1970.

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December has already been unusually warm across much of the country. And according to the National Oceanic and Atmospheric Association, the United States just saw its third-warmest autumn on record.

The Northeast just experienced its fourth-warmest autumn on record, according to the University of Massachusetts Amherst Climate System Research Center.

In Massachusetts, 2020 and 2021 have been the only two years in which the average temperature between June and November has averaged over 62 degrees Fahrenheit. Though this is striking, these soaring temperatures are consistent with the rise in average temperatures over the past several decades.

There has been a “clear trend upward, particularly over the past 50 years,” said Michael Rawlins, a researcher at U Mass. Amherst’s Climate System Research Center, on Twitter.

Warm winter temperatures can take a major toll on Massachusetts industry, making it difficult for local ski resorts to operate, while also threatening ice fishing and agriculture. Without urgent action to curb planet-warming greenhouse gas emissions, Massachusetts winters will continue to get warmer.


Dharna Noor can be reached at dharna.noor@globe.com. Follow her on Twitter @dharnanoor.