Weather Explainers

What Is A Nor'easter?

At a Glance

A nor'easter typically features winds from the northeast off the Atlantic Ocean.
These storms are most often associated with Northeast winter storms, but snow is not a requirement.
Nor'easters can happen any time of the year, but are most frequent and strongest September through April.

A nor'easter is a strong area of low pressure along the East Coast of the United States that typically features winds from the northeast off the Atlantic Ocean – hence the term "nor'easter."

Nor'easters are most often associated with strong winter storms crawling up the Northeast coast, but snow isn't a requirement for such a storm.

These storms are most frequent and strongest between September and April, but can occur any time of the year.

Nor'easters usually develop between Georgia and New Jersey within 100 miles east or west of the East Coast. In general, they move northeastward and reach maximum intensity near New England or Canada's Maritime Provinces.

A nor'easter may bring heavy rain or snow, strong winds, coastal flooding and rough seas to the affected areas.

The heavily populated Interstate 95 Northeast Megalopolis, including Boston, New York City, Philadelphia and Washington D.C., is especially vulnerable to this type of storm due to the proximity to the coast.

(EXAMPLES: Winter Storm Stella (March 2017) | Winter Storm Grayson (January 2018))

The U.S. East Coast is an ideal spot for nor'easters, especially during the winter months. This is because the polar jet stream transports cold air southward out of Canada into the U.S., then eastward toward the Atlantic Ocean where warm air from the Gulf of Mexico and Atlantic tries to move northward.

The warm waters of the Gulf Stream – a warm, swift Atlantic Ocean current that originates in the Gulf of Mexico, stretches to the tip of Florida and follows the United States' eastern coastline northward before crossing the Atlantic – help keep the coastal waters relatively mild through the winter, which acts to warm the cold air above the ocean water.

This difference in temperature between the warm air over the water and cold air over land provides the instability and energy needed to develop and fuel nor'easters.

Above is an infrared satellite image from the March 1993 "Superstorm."
(NOAA/NASA)

There are a couple of ways nor'easters can develop.

The first type develops primarily along the Gulf Coast or East Coast and tracks northeastward up the coast. This is considered a "classic" nor'easter and referred to as a "Miller Type-A" storm.

The second type originates in the Midwest and slides eastward toward the Appalachians. Once it hits the mountains, the storm loses its coherent low-pressure center, but the low redevelops along the East Coast and rides up the coast as a classic nor'easter would do. This is referred to as a "Miller Type-B" storm.

(MORE: 2 of the 3 Largest, Heaviest Northeast Snowstorms Since 1956 Have Happened in March)

If a nor'easter threatens your town, be sure to take the proper steps to prepare. This includes stocking up with at least three days of food, water and other necessary provisions, as you may be left without power for multiple days due to strong winds potentially knocking down trees and power lines.

If roads become snow-covered or flooded, stay off them until they are plowed or flooding subsides. It is best to stay home until the storm passes. Conditions can change quickly and you don't want to be stranded on an unsafe road unable to get home.

Brian Donegan is a digital meteorologist at weather.com. Follow him on Facebook and Twitter.