“The only climate signal was that the NAO had to be negative,” he told me.
How does a negative NAO lead to more big winter storms? It has to do with how nor’easters spin into existence in the first place. Big winter snowstorms form when a high-altitude layer of cold air comes to sit on top of a low-altitude layer of warm air. The contrast between those two air masses generates wind, and the change in pressure between them creates a kind of upward suction, as warm air is drawn into higher altitudes. Combine that wind and that upward drift with the rotation of the Earth and you get a powerful winter storm: an extratropical cyclone.
But remember those two starting ingredients: A low-altitude layer of warm air and a high-altitude layer of cold air. Uccellini told me that there is basically always warm air lingering around the East Coast. “We know we’ll always have warm air, because the Gulf Stream is sitting right there” off the southeastern coast, he said.
“It’s a matter of how you lock in the cold air,” he said.
Enter the negative NAO. The negative NAO basically traps cold air over North America.“With a negative NAO, you get a blocking pattern that creates a trough over southeastern Canada, and that locks in the delivery of cold air into the East Coast states,” he said.
“You need the cold air–warm air contrast, horizontally” to get a nor’easter, he said. “The negative NAO really lends itself to getting that process set up.”
Large nor’easters can still form even if the NAO is in a positive phase. The blizzard in early January of this year dropped two feet of snow in some places, yet it came together while the NAO was positive.
But it was “the exception that proves the rule,” Uccellini told me. A negative NAO helps storms develop more slowly, letting them linger offshore and gather strength. By contrast, the January 2018 blizzard formed very quickly: It was first forecast on January 1; it explosively intensified on January 2 and 3, when it also began snowing along the coast; and it dissipated by January 6.
Finally: What’s up with that name? After all, Uccellini and Kocin titled their textbook Northeast Snowstorms, not Nor’easters. And scientists don’t regularly name storms after their geography. Hurricanes often strike the Gulf Coast, for instance, but American meteorologists don’t call them gul’coasters. (And sou’wester, perhaps the word in American English most similar to nor’easter, does not describe a weather system, but a type of hat. Though a smart dresser may wear a sou’wester in a nor’easter.)
But nor’easters are named for wind direction, not where they strike. During a storm event, it’s the northeast wind that brings the strongest gusts and precipitation to New England.
That doesn’t make the name any less a source of controversy. In 2003, a Boston Globe writer alleged that nor’easter was a fake folksy-ism. “From 1975 to 1980, journalists used the nor’easter spelling only once in five mentions of such storms; in the past year, more than 80 percent of northeasters were spelled nor’easter,” wrote Jan Freeman, the paper’s language columnist. “It’s no more authentic than ‘nucular’ for nuclear or ‘bicep’ for biceps.”