Storms that form, intensify and track along the northeastern coast of the United States and eastern Canada are called nor'easters. A nor'easter is so named not because of where it is found, but because of the leading winds in the left forward quadrant of the storm that blow onto land from the northeast. Over time, British sailing captains shortened "northeast" to "nor'east" to save their pen hand.

NOAAThe collision of the remnants of Hurricane Grace with a strong Canadian cold front and arctic air created what was popularly termed "The Perfect Storm" based on the rarity of such a combination. While they can form any time of year, "nor'easter season" typically starts in October and lasts through March. Mariners, coastal residents and snow lovers as far inland as Pittsburgh, Buffalo and Albany recognize the approach of a nor'easter from the increasing winds out of the northeast, the rapid drop in surface pressure and the onset of heavy rain. Some of the greatest snowfall totals in the eastern half of the country have been caused by nor'easters. Tragically, many lives have also been lost to nor'easters, both on land and on water.

Although it has been a few years since a truly memorable nor'easter has struck, every year a number nor'easters form and make their characteristic trek up the coast. When conditions are right – when there is enough moisture, and the air entrenched over land is cold enough – incredible amounts of snow can fall. Wind gusts can equal a category 2 or 3 hurricane. Beach erosion can redefine shorelines and ocean swells can reach perilous heights.

Perhaps one of the most famous nor'easters was the Halloween storm of 1991. You know it as "The Perfect Storm," which was immortalized in Sebastian Junger's book and later in the movie. The storm was not a classical forming nor'easter. Rather, it was created by the collision of arctic air, a low pressure system and the remnants of Hurricane Grace, which combined into an intense "nor'easter" or "unnamed hurricane." The storm destroyed 1,000 homes as it thrashed the coast from Maine to the Carolinas, and it created ocean swells of 100 feet that doomed the fishing boat Andrea Gail and its crew of six far off the coast of Massachusetts, as told in Junger's book.

Nor'easters certainly do not get the media coverage that land-falling hurricanes receive, but in many ways, the danger to life and property can be just as great.

HOW IT HAPPENSNOAASurface analysis map with a satellite overlay on January 28, 2008. The nor’easter brushed Cape Cod and Nantucket with eight inches of snow.

A nor'easter develops just as every other mid-latitude storm develops, as two differing air masses in temperature and moisture collide. In this case, however, nor'easters form when cold Canadian air spills southward and collides with warm moist air supported by the Atlantic Gulf Stream. A favored location for this to happen is off the coast of Cape Hatteras in North Carolina.

If the atmospheric conditions are right, the warm moist air is lifted over the denser colder air and a low pressure center and cyclonic circulation is begun. The center of low pressure moves along the thermal boundary between the cold and warm air, and as a result, often hugs close to the coastline between North Carolina and New England.

As the low pressure system begins to move up the coast, air rushes counterclockwise around it, bringing winds that blow from the northeast. These winds pick up moisture from the ocean. Lift causes the moisture to rise and form clouds and form precipitation. If the colder, denser polar air trapped at lower levels of the atmosphere is below freezing, a snowstorm is born.

In fact, nor'easters are known to bring heavy snowfall or flooding rains–depending on the temperature–large waves, costal erosion and in some cases, hurricane-force winds.

SNOW, RAIN OR SOMETHING ELSE

As anyone who tracks the weather: Nor'easters are notoriously difficult to forecast. Since the storm develops from the collision of warm and cold air, there is usually a distinct boundary between snow and rain, and sometimes sleet and freezing rain occur in the transition zone. The location of the boundary is profoundly affected by the track of the low pressure center of the storm.

The warm, moist air from the ocean is in conflict with the cold polar air mass situated over land. As a general rule, snow falls to the north and west of the low-pressure center, while rain falls east and south. If a major metropolitan area straddles this boundary, it is anyone's guess whether it will be raining, snowing, or worse, a major icing event.

The Atlantic coast, from northern Georgia north, up the coast, can suffer high winds, pounding surf, and extremely heavy rains during these storms. However, swells have been known to cause damage through the Caribbean as well.

Surfers far removed from the immediate effects of the storm are eager to feel the swells that emanate from the storm. Nor'easters can cause severe beach erosion, as well as flooding in the associated low-lying areas.

Beach residents may actually fear the cumulative effects of nor'easters over those of hurricanes, because nor'easters happen more frequently and cause substantial damage to beach-front property and their dunes.

Farther inland, nor'easters are capable of dumping massive amounts of precipitation. Incredible amounts of snow and ice can accumulate, paralyzing a region, shutting down airports, highways and schools. Ice storms, caused by rain falling through sub-freezing surface temperatures, can devastate communities as power lines snap from tree limbs falling on power lines. Electricity can be out for days.

BOMBING

Nor'easters, fueled by the powerful collision of dry polar air from Canada with warm moist air from the tropics, can become explosive, quickly intensifying from a weak storm into a hurricane-force storm. Meteorologists refer to that atmospheric explosion as "bombing." The precise definition of a "bombing" is a pressure drop of at least 24 millibars of pressure within 24 hours.

In fact, people often mistake nor'easters for tropical cyclones, seldom differentiating between the two weather systems. Nor'easters differ from tropical cyclones in that they are cold-core, low-pressure systems, meaning that they thrive off the collision of cold and warm air.

Tropical cyclones are warm-core, low-pressure systems, which means they thrive off warm ocean temperatures. Their strongest winds are near the surface, and concentrated near the center of the low.

Keep in mind that the most intense nor'easters can form an "eye" and look quite similar to a hurricane in a satellite image.