Wednesday is the first full day of spring in the Northern Hemisphere, but you wouldn’t know it on the U.S. East Coast. A huge, ponderous snowstorm is lurching its way up the Atlantic seaboard, dumping snow from Washington to Boston.

More than two inches per hour are falling in some places. At Washington’s Reagan National Airport, it hasn’t snowed this much, this late in the season, in more than 50 years. And the storm will not be a passing event: The entire system will meander up the northeast, snowing all the while, for almost two days straight.

Blame the groundhog, perhaps, who predicted six more weeks of winter ... seven weeks ago. But for residents of the region, it’s starting to feel more like a different kind of Groundhog Day. This storm is the fourth nor’easter in three weeks. After a brief warm stretch in late February that led this publication to ask if it was already spring, the country’s densest corridor has gotten pounded by windstorms, snow squalls, and persistently chilly temperatures.

As one longtime resident recently asked me: Why are we getting all of these nor’easters all of a sudden? What even is a nor’easter? And what is up with that name?

Louis W. Uccellini is the right man to answer those questions. Uccellini knows what a textbook nor’easter, or northeast snow storm, looks like—because he cowrote Northeast Snowstorms, the two-volume textbook about them. “Paul Kocin and I spent about 20 years working on that book,” Uccellini told me Wednesday. “It’s our passion. It’s what brought us to work everyday.”

Uccellini, a longtime government meteorologist, is now the director of the National Weather Service.

Big northeast snowstorms simply don’t form very often, Uccellini said: When he and his coauthor studied the half-century of weather between 1949 and 2003, they only found 47 storms that could be classified as nor’easters.

But it does make sense that the eastern U.S. has seen so many nor’easters in the last few weeks, he said. If the atmosphere is in the mood to produce a nor’easter, it doesn’t stop after making just one.

“One of the things we emphasized in the book is the episodic nature of these storms. They come in batches,” Uccellini told me. Northeast snowstorms can only emerge from a very specific set of circumstances. When those circumstances are achieved, storms can follow one after another, walloping the coast week after week.

Uccellini posed the follow-up question: “So what’s the background state that makes these storms? Why now?”

Why indeed? Some meteorologists have suggested that the recent weak La Niña in the Pacific Ocean—a phenomenon that makes that ocean’s surface cooler than normal—contributed to the sudden outbreak of storms. Uccellini disagreed: While La Niña may have nudged some nor’easters farther into New England, the pattern is not driving the current outbreak of storms.

Rather, he said, northerners should look to a recent change in the nearer ocean.

Just as the Pacific Ocean passes through different stable states over the course of months or years—like El Niño—so too does the Atlantic Ocean. One of the ways that scientists measure the Atlantic’s changes is with an index called the North Atlantic Oscillation, or NAO, which compares the air pressure in Iceland with the air pressure in the Azores. In other words, it combines a measurement taken just south of the Arctic with one taken 1,800 miles away, just north of the tropics.

Those may sound like two arbitrary spots in space—how does the weather in Reykjavik affect the weather in Raleigh?—but the NAO matters immensely for the eastern United States. It measures what Columbia University researchers call “a large-scale seesaw” in the atmosphere. In essence, that seesaw works like this: If the NAO is positive, there’s a powerful low-pressure system over Iceland, which draws in storms and helps them cross the northern Atlantic. If the NAO is negative, the Icelandic low-pressure system is weak, making it harder for storms to cross the sea. Storms tend to linger around North America as a result.

“The NAO was positive all the way until the end of February, and since then it’s been negative,” Uccellini told me. He believes this suddenly negative NAO is helping to drive the current spurt of storms. When he and his coauthor studied 35 different winter storms while writing Northeast Snowstorms, they found that only one large-scale weather phenomenon was correlated to them.

“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.

Edgar Comee, an octogenarian Mainer, agreed. “The use of nor’easter to describe a northeast storm is a pretentious and altogether lamentable affectation, the odious, even loathsome, practice of landlubbers who would be seen as salty as the sea itself,” he wrote to The New Yorker in 2005. In the same note, Comee declared himself chairman of the “Ad Hoc Committee for Stamping Out Nor’easter.”

Even some professional linguists take that same side. Mark Liberman, a linguist at the University of Pennsylvania and an author of the popular blog Language Log, once deemed nor’easter “a literary affectation ... like e’en for even and th’only for the only.The Oxford English Dictionary cites usages of nor’easter dating back to 1837. But many of those examples originate from British, not American, writers, and Liberman rejected them on the whole as “not very [impressive].”

The battle rages on—even if its participants are not always known. At writing, the Wikipedia entry for nor’easter lists no fewer than four independent appearances of the contracted term in the 19th century, all of New England vintage. If accurate, they represent an impressive find—especially since they come from an etymologist apparently content to exist in online anonymity. The oldest of these examples, which is unverifiable without independent archival work, claims to hail from The Hartford Times in December 1839.

“We Yankees had a share of this same ‘nor’easter,’” groaned that unknown Connecticut journalist. Almost 180 years later, we Yankees can look back at the four storms we’ve endured this month and say: We’ve had more than our share of nor’easters at this point, thank you very much. Spring can’t come soon enough.