(This story has been updated.)

November has been ridiculously mild over much of the Lower 48. But, slowly, some signs are emerging that more wintry weather will chip away at this warm pattern — starting this week. By early December, there’s some chance much of the nation will be in the grips of a very cold, snowy pattern.

Before we talk about winter weather, it’s worth reviewing how mild it has been to this point.

On Wednesday, the National Oceanic and Atmospheric Administration reported that snow cover in the Lower 48 reached the lowest extent on record for the entire month of November. This is remarkable, considering we’re at the month’s midpoint.

Snow covered the ground over a pathetic 0.2 percent of the nation.

Meanwhile, scores of weather stations west of the Mississippi River are having their warmest November on record to date.

The weather station in Lincoln, Neb., which is pretty representative of conditions in the central United States, shows the extreme magnitude of the temperature difference from normal so far this month. It has averaged about 13 degrees above normal:

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The weather in Grand Forks, N.D., has more closely resembled what is expected in Albuquerque during the first two weeks of November.

On Wednesday, dozens of locations set record highs from the Rockies through the Plains.

With the first half of November behind us, will the warm pattern relax during the second half? Somewhat, said Matt Rogers, long-range forecasting specialist for the Capital Weather Gang.

“The warm pattern is starting to break down already,” Rogers said. “The second half doesn’t look nearly as warm as the first half but will still probably be warmer than normal.”

This week, for example, a significant early winter storm is developing in the Rockies and will spread snow through the High Plains into northern Minnesota. The National Weather Service has issued a blizzard watch for eastern South Dakota and southwest Minnesota for late Thursday into Friday as the combination of wind gusts to 45 mph and 3-6 inches of snow may create whiteout conditions.

The storm will drag a cold front through the central and eastern United States, with temperatures plummeting between late this week and early next week, at least for a couple of days.

“Now we’re getting transient cold pushes that we weren’t before,” Rogers said.

But even with the temperature plunge in the forecast for the second half of this week, models’ forecasts still favor milder-than-normal conditions through Thanksgiving for much of the nation. The lone exception may be the East Coast, where temperatures may be slightly lower than normal.

Beyond Thanksgiving into early December, some forecasters are intrigued by the GFS model, which is advertising a radical change in the weather pattern over North America.

It suggests a major destabilization of the polar vortex and for the North Atlantic Oscillation to plunge into the negative territory. When this oscillation is strongly negative, it promotes the flow of Arctic air into the eastern United States, and the pattern is often favorable for snowstorms.

“Time to gas up the snow blower across the U.S.,” tweeted Judah Cohen, a seasonal forecaster for Atmospheric and Environmental Research. “IF (big if) pattern verifies, that is as favorable a pattern for snowstorms that I can recall.”

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In a follow-up phone call with Cohen, he stressed the uncertainty in the specifics of any pattern change, that is where the cold air and any storminess will focus and the exact timing. “When polar vortex goes through this disruptive state, it’s very difficult for the models to simulate,” he said.

Rogers said he thinks the GFS model may be too aggressive in its representation of the pattern, noting that the European model forecasts a pattern less extreme — though still more favorable for cold and snow than the current one.

Rogers added that he wouldn’t be surprised if the North Atlantic Oscillation eventually tanked and brought about a wintry stretch. He pointed out that the oscillation reached its most negative level on record in October, beating out October 2009. In many areas of the Mid-Atlantic, the winter following October 2009 was the snowiest on record.

Cohen said there’s not “a huge correlation” between the North Atlantic Oscillation in October and wintry weather, generally, but that in years in which the oscillation had been strongly negative in October, active winter weather often followed.

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