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
March 1993 ("Blizzard of '93") Snow, tornadoes and flooding from Alabama to Maine; damages in excess of $1 billion
Halloween 1991 More than 1,000 homes damaged from the Carolinas to Maine
President's Day 1979 Shut down Washington, D.C.
Ash Wednesday 1962 Northeast coastline hit for five days
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:

  • Offshore forming - These are the storms you hear about in the news. It is a news-worthy storm that moves east of east-coast cities, dumping lots of heavy snow. In an offshore-forming storm, the hardest hit metropolitan areas are likely to be Washington D.C, Philadelphia, New York City and Boston. Eventually, the system moves far enough north that the Canadian jet stream pushes it off the coast.
  • Onshore forming - These storms are less exciting than the offshore-forming storms. They move west of east-coast cities, with gusting winds and mostly rain.