A brutally cold Arctic air mass invaded the nation’s midsection this weekend and sent temperatures to levels normally associated with January. The nation’s official low temperature at midday Sunday was a nippy minus-44 degrees in Mosby, Montana. The National Weather Service warned that temperatures between Montana and Kansas could be 50 degrees below normal.

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A fast-moving storm system has swept west to east across the country. The system incited severe thunderstorms, flash flood warnings and tornado watches in the Deep South on Sunday.

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Officials in Lee County, Ala., reported that at least 22 people died during the severe weather, in which pounding winds, rain and tornadoes downed power lines and damaged buildings. Search and rescue teams were scouring the damage Sunday evening for other possible deaths or injuries.

At the same time, prodigious amounts of snow began falling in West Virginia and other parts of the central Appalachians as the storm system rolled northeast across Pennsylvania and toward New England. Officials in Philadelphia, New York City and Boston prepared for a disruptive blast of winter weather, along with possible power outages and air traffic delays.

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Boston, which was buried in more than 100 inches of snow four years ago but largely has been spared this year, could see six to eight inches of wet snow before the storm races into Maine and then enters the Canadian Maritimes.

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The District remained in its familiar position Sunday, close to the rain-snow line with every possibility on the table — rain, sleet, snow, a major snowfall, a dismaying wintry mix, or just a mild dusting that nervous school officials would surely decide was apocalyptic. On Sunday, the Capital Weather Gang reported “precipitation types may alternate.”

It was raining in downtown Washington, while the precipitation was frozen to the city’s northwest; residents in Frederick, Md., were still expecting four to eight inches of snow.

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The Arctic blast in the nation’s midsection is a repeat of what happened in January, when the polar vortex — the very cold air mass that normally circulates in the Arctic — broke into pieces, with a fragment hurtling south and creating dangerously cold conditions in the Lower 48 states.

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The quirky behavior of the polar vortex is caused by a dip in the jet stream and is associated with a warm air mass that pulsed north into Alaska, said Jason Furtado, an assistant professor of meteorology at the University of Oklahoma.

The timing is unfortunate.

“I think people this year are less prepared for this kind of cold wave, and especially in some areas where things started to bud and blossom,” Furtado said.

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In addition to being cold and stormy, America is soggy. Tennessee, for example, is the land of flooded basements. And the big storm is sending more water down the dangerously swollen Mississippi River. The U.S. Army Corps of Engineers in recent days has diverted water from the river to ease the flooding hazards in New Orleans and other cities.

Greg Carbin, a meteorologist with the National Weather Service, said February set a record in the continental United States for the proportion of the country that received more than 10 inches of rain. That is a huge amount of rain in just four weeks, and by his tabulation, 5.5 percent of the country reached or exceeded that mark.

He noted that the records date only to 1969, but it is remarkable that the past three Februaries have been historically very wet.

His summation of the weather now: “It’s wet and miserable.”