Nor’easter

Nor’easter, extratropical cyclone in the western Atlantic Ocean that forms along the eastern coast of North America. The storm is named for the winds primarily blowing from the northeast that buffet the east coast of the United States from the Mid-Atlantic states and New England to Canada’s Maritime Provinces. Nor’easters tend to bring heavy precipitation that can result in flooding and significant snow accumulations. Although nor’easters can occur throughout the year, they tend to form and reach their greatest intensity between September and April.

Formation

Nor’easters stem from the confluence of several weather elements. Like tropical cyclones, they require warm water to produce the rising parcels of air necessary to form. During the winter months, cold, dry air spilling southward out of the Arctic mixes with warm, humid air moving northward from the Gulf of Mexico, each of which is drawn eastward by the polar front jet stream—a region of long, narrow, high-speed winds that flow generally eastward in the middle and upper troposphere or lower stratosphere.

The path of the jet stream in the winter dips far south of the Great Lakes and over the U.S. South before exiting the continent high above the Mid-Atlantic states. Once over the Atlantic Ocean, the jet stream tends to turn northeast, and its path can be influenced by the relatively warm waters of the Gulf Stream current, which also flows northeast along the east coast (see also ocean current).

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In this way, the jet stream aloft sometimes steers cool coastal air at the surface over the relatively warm waters of the Gulf Stream about 160 km (100 miles) off the coast. The warm water adds heat and moisture to the air and lifts it, creating a cyclone (an area of low atmospheric pressure) that results in the storm’s development. Nor’easters then undergo a phase of rapid intensification as they travel north parallel to the North American coast and press into regions of colder air at the surface. As the difference in temperature between the air and the water increases, warming air at the surface rises more rapidly, accentuating the difference in atmospheric pressure between the center of the storm and the surrounding environment. (Should this fall in atmospheric pressure surpass 24 millibars or more at 60° latitude or 17.8 millibars or more at 40° latitude in a 24-hour period, the storm would also qualify as a bomb cyclone.) This process produces stronger winds, with the nor’easter often reaching its peak intensity over the ocean near New England and the Maritime Provinces.

Weather effects

Nor’easters are known for their high winds, which generate rough seas, heavy rains, blizzards, and flooding, as well as erosion caused by waves pounding the coast. Moderate-to-strong gale-force winds (that is, those 50–88 km [31–55 miles] per hour; see Beaufort scale) are common; however, they sometimes reach the level of hurricane-force winds (119 km [74 miles] per hour or above; see Saffir-Simpson hurricane scale). For example, peak wind speeds associated with the “Perfect Storm,” a powerful nor’easter that struck New England in late October and early November 1991, were measured at up to 129 km (80 miles) per hour; these winds pummeled the coastline between North Carolina and Nova Scotia with waves reaching some 9 meters (30 feet) and produced rogue waves of about 27 meters (90 feet) at sea. The nor’easter also generated waves that caused flooding in Bermuda and parts of the Caribbean region.