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More specifically it describes storms whose center of rotation is just off the coast and whose leading winds rotate on-land from the northeast. The resulting warm (relative to over-land temperatures) humid air moves onshore where the moisture precipitates out due to the drop in temperature. This produces heavy, cold coastal rains in the warmer months, and winter blizzards that affect the northeastern United States. They also can cause coastal flooding , coastal erosion and gale force winds.
Nor'easters are usually formed by Canadian (or Arctic) cold fronts mixing with warm air (and Low Pressure) fronts from the Gulf Coast that are pulled up the east coast by the northeast winds.
Such storms often move slowly and are frequently of significant intensity.
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