What Exactly Is a Nor'easter?

1-26-15 Nor'easter
A winter storm approaches the eastern United States in a satellite image released January 26, 2015. The National Weather Service has issued a blizzard warning for New York City and surrounding areas between coastal New Jersey and Connecticut, beginning 1 p.m. EST on Monday. The storm will worsen overnight into Tuesday morning. NOAA/Reuters

New Yorkers, Philadelphians and other East Coasters are hunkering down and waiting for an impending winter storm expected to hit Monday night. The incoming blizzard has the potential to be a "crippling" and "historic" nor'easter, according to the National Weather Service. But what exactly is a nor'easter?

Short for northeaster, it gets its name from the direction from which winds move along the coastline, according to the National Oceanic and Atmospheric Association, and is most likely to occur between September and April.

A classic nor'easter results when an existing low-pressure system moves across the country from west to east and then spawns a new storm off the East Coast, usually between the Carolinas and New Jersey, explains Keith Seitter, executive director of the American Meteorological Society. The initial storm may not have produced a huge amount of precipitation as it moved over land, but the new storm goes through an "explosive development," says Seitter, and becomes a "monster storm" that absorbs the original one.

"The center of the storm stays out to sea but moves just southeast of southern New England," he says. Once it's off the coast, the storm can tap into the ocean's moisture and produce a huge amount of precipitation, which at current temperatures falls as snow. Winds spiral around the center of the storm counterclockwise—with New England sitting at 10 o'clock, says Seitter—so the winds are hitting the coast from the northeast. Hence, "nor'easter."

Seitter says nor'easters come along maybe a dozen times a year and result in rain or snow, depending on the season, with perhaps two or three big ones each winter. But the storm that's approaching now is "truly a rare event," he says. "It could be one of the worst 10 storms we've had in a 100 years."

1-26-15 Blizzard 2006
A plow clears snow along Park Avenue in Manhattan on February 12, 2006. Keith Bedford/Reuters

The storm hitting the Northeast this week first hit Chicago, slid across the U.S. and redeveloped off the Carolinas, Seitter says. It will blanket not only New England but also parts of the Mid-Atlantic, from New Jersey and New York all the way up to Maine.

When storms develop as this one has, says Seitter, they can intensify rapidly, with high winds sometimes at hurricane force and heavy, fast precipitation. A nor'easter can dump snow at a rate of a couple inches an hour, so that within a few hours the coast is buried under a foot of it, making it difficult for road crews to keep up. The winds and heavy snow also make power outages more likely, Seitter explains, especially along the coast, like on Long Island or Cape Cod, where the snow tends to be heavier.

"My message to New Yorkers is to prepare for something worse than we have seen before. Now is the time to get ready for this extreme weather," Mayor Bill de Blasio said Sunday.

"The city is doing everything we can to prepare and keep New Yorkers safe," de Blasio tweeted Monday morning. Hundreds of flights have already been canceled out of New York-area airports.

"Typically, these things kind of blow up into a major storm in about a day and take a day to move through," Seitter says. In this case, the storm is will develop around New York Monday night, move throughout New England Tuesday, and get into areas of Maine Wednesday morning. "The real issues are making sure that you're prepared to go without power for a few days or in some cases longer if things are really bad," he says.

"As long as [people] have taken some reasonable precautions and are ready to spend a day or two at home," he says, "[they're] not going to run into trouble."