The Weather Service favors warmer-than-normal conditions for the southern two-thirds of the Lower 48, including the Mid-Atlantic. Only a sliver of the Northern Plains and Pacific Northwest is expected to experience colder-than-normal temperatures.

The mild forecast follows back-to-back lackluster winters across the nation.

Last winter was practically the winter without a winter. Spring arrived weeks ahead of time, and Chicago basked in record 70-degree warmth in February. It ranked as the sixth-warmest winter on record.

The winter before, with the exception of the blockbuster January snowstorm along the East Coast, was quite tame, as well, ranking as the warmest on record. Temperatures surged through the 70s to the Canadian border on the East Coast’s warmest Christmas Eve on record.

Climate warming from rising concentrations of carbon dioxide is exerting an effect on winter temperatures, said Mike Halpert, deputy director of the Weather Service’s Climate Prediction Center. “It does, undoubtedly, play a role,” he said in a call with reporters. “The increase in CO2 factors into our model forecast.”

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While predicting generally warmer-than-normal conditions, Halpert stopped short of forecasting a third straight exceptionally warm winter like the last two. “The odds of seeing three top 10 [warmest winters in a row] is reduced, not eliminated,” he said. “We’re not anticipating the kind of record warmth we’ve seen the last two winters.”

He further said that while winters are warming, on average, natural variability continues to play the dominant role in the ultimate outcome of the season’s temperatures.

Still, Halpert said “there was nothing to indicate” that the nation will have punishing outbreaks of the polar vortex as experienced in the winter of 2013-2014 and, to a lesser degree, the winter that followed. But he wouldn’t totally rule out some penetrating outbreaks of extreme cold, explaining that they usually can’t be predicted until about a week or so in advance.

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Halpert said the “driving force” behind the winter outlook is the forecast development of a weak La Niña event, characterized by a cycling cooling of water over the tropical Pacific Ocean.

“If La Niña conditions develop, we predict it will be weak and potentially short-lived, but it could still shape the character of the upcoming winter,” Halpert said. “Typical La Niña patterns during winter include above-average precipitation and colder-than-average temperatures along the northern tier of the U.S. and below-normal precipitation and drier conditions across the South.”

During La Niñas, the prevailing storm track is usually west of the East Coast through the Ohio Valley, which cuts back the chances of crippling snow events along the Interstate 95 corridor from the Mid-Atlantic to New England. “Usually the Mid-Atlantic sees less snow than normal,” Halpert said.

The areas most likely to contend with major winter storms span from the Northern Rockies into the Great Lakes.

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While the Weather Service is relying on La Niña to provide most of the clues about how the weather will unfold, Halpert agreed that other weather patterns like the Arctic Oscillation could easily alter conditions. He stressed that the Weather Service outlook is probabilistic, meaning it only conveys odds about certain temperature and precipitation outcomes. In other words, there are no guarantees.

For example, the chance that La Niña materializes, the basis for the outlook, is only 55 to 65 percent.