from howstuffworks.com
Nor'easters can occur in the eastern United States any time between October and April, when moisture and cold air are plentiful. They are known for dumping heavy amounts of rain and snow, producing hurricane-force winds, and creating high surfs that cause severe beach erosion and coastal flooding. A Nor'easter is named for the winds that blow in from the northeast and drive the storm up the east coast along the Gulf Stream, a band of warm water that lies off the Atlantic coast.
There are two main components to a Nor'easter:
Gulf Stream low-pressure system - (counter-clockwise winds) These systems generate off the coast of Florida. The air above the Gulf Stream warms and spawns a low-pressure system. This low circulates off the southeastern U.S. coast, gathering warm air and moisture from the Atlantic. Strong northeasterly winds at the leading edge of the storm pull it up the east coast.
Arctic high-pressure system - (clockwise winds) As the strong northeasterly winds pull the storm up the east coast, it meets with cold, Arctic air blowing down from Canada. When the two systems collide, the moisture and cold air produce a mix of precipitation.
Notorious Nor'easters
Winter conditions make Nor'easters a normal occurrence, but only a handful actually gather the force and power to cause problems inland. The resulting precipitation depends on how close you are to the converging point of the two storms.
A powerful Nor'easter can bring travel to a standstill, closing city streets and making it nearly impossible to get anywhere for days. The thing to remember with Nor'easters is that the storm systems occur frequently, but only a few of them are powerful enough to make it into the news. There are two types of Nor'easters:
