Lake-effect snow is produced in the winter when cold winds sweep across large bodies of warmer lake water. The air mass moves swiftly across the lake, picking up water vapors which freeze and drop on the shore as lake-effect snow.
If the air mass is uplifted during this process the snow fall is intensified and produces very heavy snow fall in a smaller, targeted area. This can produce snow falls of many inches per hour. These areas are often referred to as snow belts.
Lake-effect snow forms all over the world but is best known for its impact on communities around the Great Lakes of North America.
In order to produce lake-effect snow, the air crossing the lake must be much cooler than the surface air. When the air is very cold, thunder snow can be produced by the storm. The lightning and thunder are caused by the increased instability and energy produced by the storm.
There are a number of things that must happen before lake-effect snow can occur. The instability caused by the differences between the lake surface temperature and the temperature at about 4,900 feet above sea level is significant. It provides conditions that are necessary to produce tall, thick clouds that provide heavy precipitation.
Also the further the wind travels across a lake, the heavier the precipitation may be. This is referred to as the “fetch” factor in lake-effect snow production. Other factors that impact the creation of lake-effect snow include: wind shear, upstream moisture, upwind lakes, synoptic forcing, topography and orography.
Accumulation of snow and ice has an important affect on the formation of lake-effect snow. A lake loses its ability to produce lake-effect precipitation as it freezes. The ice free area shrinks, reducing the distance or “fetch” for wind to pass over.
Communities lying on the south/east side of the Great Lakes are most affected with heavy lake-effect snow. This is because the prevailing winter winds are from the northwest in that region. Snowfalls of over 300 inches per season are recorded in many of these towns.
www.weather.com sums it up simply “Whether an area gets a large amount of snow from lake-effect snow is dependent on the direction of the winds, the duration they blow from a particular direction and the magnitude of the temperature difference between the water and the air.” Their site also mentions that the snow produced by lake-effect is lighter in weight.
www.wekipedia.com has lots of good information about lake-effect snow.