Nor'easters: What they are and how they form

The storm expected to bring as much as 60 to 90 centimetres of snow from northern New Jersey to southern Maine is a nor'easter, a weather system commonly seen on the East Coast and known for bringing heavy precipitation and powerful winds. 

The storms, so named because of the strong northeasterly winds that herald their arrival, can happen at any time of the year, but are more common and powerful between September and April. Along with the wind, rain and snow, nor'easters can lead to flooding and coastal erosion.  

They can also cause power outages and travel chaos. 

Thousands of flights were cancelled on Monday as the storm bore down on the East Coast.

Governors and mayors declared emergencies and ordered the shutdown of highways, streets and mass transit systems to prevent travellers from getting stranded and to enable plows and emergency vehicles to get through.

Nor'easters a 'nasty event'

The storms usually head in a north or northeasterly direction along the U.S. East Coast and into Atlantic Canada. They typically form within 160 kilometres of the coast in an area stretching from Georgia to New Jersey.

"During winter, the polar jet stream transports cold Arctic air southward across the plains of Canada and the U.S., and eastward toward the Atlantic Ocean, as warm air from the Gulf of Mexico and the Atlantic tries to move northward," according to a U.S. National Oceanic and Atmospheric Administration website detailing the dangers of nor'easters.

It is where these two systems meet that nor'easters are born, says David Phillips, senior climatologist with Environment Canada. 

"When that warm, moist air from the south bumps into the cold air to the north — a low-pressure area meets a high-pressure area — wow, you get some strong winds," said Phillips. 

The warm, moist air from the Gulf Stream is forced upward by the denser, colder air, he said. As it rises, it cools and becomes precipitation.

"It really is a nasty event, because of the fact of the copious amounts of precipitation, and most of it looks like it’s going to be, because the cold air is so dominant, will be snow and not rain," he said of the storm bearing down on the U.S. East Coast.

"So it was intensified, it got lots of moisture and energy from the ocean and then it carried on in its regular northeastern pattern," he said. 

'Going to pile up very quick'

Nor'easters can be slow-moving storms, providing lots of time for significant precipitation. 

"This could be five-centimetre-an-hour kind of snowfall, so it's going to pile up very quick," said Chris Murphy from The Weather Network. 

Nor'easters can also be quite large, with a radius of up to 1,600 kilometres, according to a 2013 hazard mitigation plan from Massachusetts. Sustained wind speeds of 30 to 65 km/h are common, with gusts reaching between 80 and 95 km/h or higher.

A number of powerful nor'easters have struck the East Coast, and the current one could become of the one worst to hit the region since record keeping began in 1872. 

The so-called "perfect storm" in 1991 caused almost $1 billion in damage, according to the NOAA.