Answer:
Snow, known as "lake-effect snow," is generated from the temperature contrast between the cold arctic air moving over the relatively warm waters of the Great Lakes (or other large body of water). Unlike most winter storms, lake effect snows do not build their foundation upon strong areas of low pressure. Instead, they are fueled by the same dry arctic air that is responsible for clearing skies over land in other parts of the country. Specifically, cold arctic air passing over the Great Lakes picks up moisture and deposits it as snow inland from the downwind shore. So while other parts of the northeastern United States are clearing up after a recent cold frontal passage, communities near the Great Lakes wait for the lake effect snow machine to fire up!
Lake Effect snow is bands of narrow, but at times very heavy snow that is generated by cold air blowing over large areas of fresh water; such as the Great Lakes. The air moving over the warmer waters of the lake picks up the moisture from the water and deposits it as snow on the down wind side of the lake. Areas famous for Lake Effect snow are the eastern portions of Lake Ontario in New York such as Oswego County. In February 2007 some portions of Oswego County, New York received up to 11 feet of snow in a 5 day timeframe.