Mad rushes to the grocery store usually mean one of two things in the U.S. Northeast: either a holiday or a nor’easter snowstorm is approaching. With the holiday season just behind us, the sparsely stocked supermarket shelves in the region are a sign of the latter—and tonight’s storm in particular is gearing up to be historic.



Nor’easters are born much like other storms. “They’re low pressure systems of the same kind that give us our precipitation in the United States throughout the year,” says Jeff Masters, director of meteorology at Weather Underground. “But they happen to form right off the Northeast coast and in winter.” The winds blowing from the northeast that give these storms their name also push them ashore.



The storms derive their energy from the contrast in air temperature between very cold, dense air from Canada that meets relatively warm, moist air coming off the Atlantic Ocean. “That difference in air masses can be converted to the energy of the winds and precipitation that come with the storm,” Masters says.



These gusts might feel harsh to people in the storm’s path, but they rarely reach the strength of even the most mild hurricane-force winds. Nevertheless, nor’easters can cause major destruction. The now-infamous blizzard of 1978 caused 99 deaths and an estimated $520 million in damage when it hit New England.



Super-severe nor’easters like this one may soon become the norm, according to Kevin Trenberth, senior scientist at the National Center for Atmospheric Research in Boulder, Colo. “It is expected with climate change that snowfalls will increase as long as the temperatures are cold enough [for snow to fall]," he says. On average, the air above the ocean will be warmer than it would have been, "and the atmosphere can hold 4 percent more moisture for every 1 degree Fahrenheit increase in temperature,” he says. Once the cold air from Canada hits this ocean air that is more moisture-laden then usual, an even greater amount of snow than normal will be dumped on the Northeast.



[Read more about climate change’s intensifying effect on storms here]



The net effect of climate change on nor’easters is likely to be even larger than this atmospheric temperature–moisture relationship suggests, however, as these increases can set off a cascade of effects. "The increased moisture in the storm can also feed back and amplify the storm itself," Trenberth says.



No one can conclusively diagnose tonight’s potentially historic nor’easter as a symptom of this phenomenon, but those of us who didn’t stock up on groceries and supplies before the storm may want to start getting into the habit now for future weather events.



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