A nor’easter is a low-pressure system that forms a storm and travels along the eastern coast of the United States. While the storms often affect the Northeast, the term nor’easter is derived from the fact that the winds around the low-pressure system blow from the northeast. These storms are more common from September to April and may bring snow and high winds to the regions they affect. However, the storms can occur during any time of the year.

Nor’easters generally develop in the latitudes along the East Coast within 161 kilometers (100 miles) east or west of the coastline between New Jersey and Georgia. This region provides the perfect breeding ground for such storms. The cold, polar air plunging from Canada meets the warm, moist air of the Gulf Stream (a current of warm water flowing from the Gulf of Mexico) near the East Coast. The difference in temperature between the two air masses helps create instability, fueling storms. Although snowfall typically begins on the East Coast, the origins of these storms might come from the Gulf of Mexico or from the central United States, depending on the type of nor’easter.

Nor’easters are classified into two major categories, which were developed by researcher J. E. Miller in 1946. The first type of nor’easter, and the most common, is the Miller Type A nor’easter. These classic nor’easters form in the Gulf of Mexico and develop into full-fledged storms that moves along the East Coast. Miller Type B nor’easters originate as low-pressure systems in the United States' Midwest. These less-common systems diminish after crossing the Appalachian Mountains and reform into nor’easters on the East Coast.

Once fully formed, the storms normally move northeast up the coast. They intensify as they progress, reaching maximum strength between New England and Canada's Maritime Provinces. The storms typically bring heavy precipitation, often snow, in the winter. Nor’easters can also bring strong winds, coastal flooding, rough ocean conditions, and blizzards. Major cities, such as Boston, Massachusetts; New York City, New York; Philadelphia, Pennsylvania; and Washington, D.C., lie in the path of such storms and are often impacted by these storm events.

Some of the most famous storms in history have been nor’easters. These have included the Blizzard of 1888, the Ash Wednesday Storm of 1962, the Blizzard of 1978, and the Storm of the Century (also known as the Blizzard of 1993).