Formation Nor'easters develop in response to the sharp contrast in the warm
Gulf Stream ocean current coming up from the tropical Atlantic and the cold air masses coming down from Canada. Very cold and dry air rushing southward and meeting up with the warm Gulf stream current, which is typically near even mid-winter, often causes
low-pressure areas to develop and intensify. In the
upper atmosphere, the strong winds of the
jet stream remove and replace rising air from the Atlantic more rapidly than the Atlantic air is replaced at lower levels; this and the
Coriolis force help develop a strong storm. The storm tracks northeast along the East Coast, normally from North Carolina to Long Island, then tracks east toward the waters off
Cape Cod. Counterclockwise winds around the
low-pressure system blow the moist air over land. The relatively warm, moist air meets cold air coming southward from Canada. The low increases the surrounding pressure difference, which causes the very different air masses to collide at a faster speed. When the difference in temperature of the air masses is larger, so is the storm's instability, turbulence, and thus severity. The nor'easters taking the East Coast track usually indicates the presence of a
high-pressure area in the vicinity of
Nova Scotia. Sometimes a nor'easter will move slightly inland and bring rain to the cities on the coastal plain (New York City, Philadelphia, Baltimore, etc.) and snow in New England (Boston northward). On occasion, nor'easters can pull cold air as far south as Virginia or North Carolina, bringing wet snow inland in those areas for a brief time. The Miller classification classified storms into two categories: type A and type B. Type A storms form in the Gulf of Mexico or along the coast of
Georgia or
South Carolina, and cause heavy snow mainly to parts of the inland upper south, Mid-Atlantic, New England, and Atlantic Canada. Type B storms form from a parent low-pressure system over the Ohio Valley, which then undergoes a center reformation over
Gulf Stream off North Carolina or Virginia. These storms can bring a swath of wintry precipitation from the
Great Plains and the
Ohio River Valley to the Middle Atlantic and New England.
Characteristics Nor'easters are usually formed by an area of
vorticity associated with an upper-level disturbance or from a kink in a frontal surface that causes a surface low-pressure area to develop. Such storms are very often formed from the merging of several weaker storms, a "parent storm", and a polar jet stream mixing with the tropical jet stream. Temperatures usually fall significantly due to the presence of the cooler air from winds that typically come from a northeasterly direction. During a single storm, the precipitation can range from a torrential downpour to a fine mist. All precipitation types can occur in a nor'easter. High wind gusts, which can reach hurricane strength, are also associated with a nor'easter. On very rare occasions, such as in the
nor'easter in 1978,
North American blizzard of 2006,
Early February 2013 North American blizzard, and
January 2018 North American blizzard, the center of the storm can take on the circular shape more typical of a
hurricane and have a small "dry slot" near the center, which can be mistaken for an
eye, although it is not an eye.
Difference from tropical cyclones Often, people mistake nor'easters for
tropical cyclones and do not differentiate between the two weather systems. Nor'easters differ from tropical cyclones in that nor'easters are
cold-core low-pressure systems, meaning that they thrive on drastic changes in temperature of Canadian air and warm Atlantic waters. Tropical cyclones are warm-core low-pressure systems, which means they thrive on purely warm temperatures. However, in rare cases, such as the
1991 Perfect Storm, a small tropical cyclone can develop inside the
warm seclusion of an intense nor'easter if the
sea surface temperatures are sufficiently warm. Nor'easters can rarely also turn into tropical or subtropical cyclones, such as
Tropical Storm Wanda in 2021.
Difference from other extratropical storms A nor'easter is a strong
extratropical cyclone, often experiencing
explosive cyclogenesis. While this formation occurs in many places around the world, nor'easters are unique for their combination of northeast winds and moisture content of the swirling clouds. Similar conditions sometimes occur during winter in the Pacific Northeast (northern Japan and northwards) with winds from NNW. In Europe, similar weather systems with such severity are hardly possible; the moisture content of the clouds is usually not high enough to cause flooding or heavy snow, although northeasterly winds can be strong.
Geography The
eastern United States, from
North Carolina to
Maine, and
Eastern Canada can experience nor'easters, though most often they affect the areas in the
Atlantic Canada and
New England. The effects of a nor'easter sometimes bring high surf, strong winds and rain as far south as coastal
South Carolina. Nor'easters cause a significant amount of beach erosion in these areas, as well as flooding in the associated low-lying areas. Biologists at the
Woods Hole Oceanographic Institution on
Cape Cod have determined nor'easters are an
environmental factor for
red tides on the Atlantic coast. ==List of notable nor'easters==