Lake effect snow happens when cold Arctic air sweeps across the Great Lakes while they are still unfrozen. Cold air is very dry air, which sucks up the moisture from the lakes like a sponge. Usually cold dry air doesn't bring snow, but when that moisture-laden air reaches land in the lee of the lakes, all that moisture turns into snow squalls.
There are three parts to lake effect snow: unfrozen lakes, cold air, and a stiff wind. Without the unfrozen lakes, there is nothing from which to pick up moisture. Without the cold air, that moisture doesn't turn into snow. Without the stiff wind, that moisture can't get carried over land. But when all three come together, the Great Lakes snow machine turns on and the snow just keeps on coming.
The Great Lakes snow machine turns on the first time that the upper air temperature falls below freezing. The surface air temperature doesn't have to be below freezing for snowsqualls to start. Lake effect snow will continue as long as the windward Great Lake remains unfrozen. After the lake freezes, the moisture source is cut off and the snow squalls stop.
Lake effect snow squalls only hit regions which are downwind from the unfrozen lake. All other sides of the lake are completely unaffected. Because prevailing winds in the area are from the northwest, areas which are to the east and south of the lake are usually the hardest hit. Inside this snow belt, snow squalls will hit whenever there is a steady wind off the lake.
The Southwestern Ontario peninsula gets it worst. Not only is Southwestern Ontario surrounded by water on three sides, but the upper Great Lakes also line up so that cold air can pick up moisture across the widest parts of both Lake Superior and Lake Huron before releasing the snow to the lee of Lake Huron and Georgian Bay. Unlike the lower Great Lakes, Lake Superior and Lake Huron don't completely freeze over., so that part of the Great Lakes snow machine never completely shuts off. The Niagara Peninsula gets it when the winds shift to the east. The Bruce Peninsula, at the northwest end of Southwestern Ontario, gets it especially hard because it is almost completely surrounded by water.
Northwest winds are usually good weather winds in Southwestern Ontario. Storms bring east winds. This means that every passing storm in Southwestern Ontario has a one-two punch: the storm itself, and the squalls which set up behind it when the northwest winds kick in. These snow squalls often leave much more snow than the original storm!
The snow comes off the lake as one or more parallel streamers which line up with the wind. Each streamer is between fifty and 200 kilometres long, but is also so narrow that one side of the city can be in whiteout conditions while the other side has bright sunshine. Multiple streamers can have spaces between them which are completely clear of snow. At the same time, places inside the streamer can get 2 centimetres of snow or more per hour. Very heavy streamers can bring as much as 10 cm of snow per hour.
This can be extremely dangerous for drivers, who can go from clear skies to whiteout conditions in seconds. Both the 401 and 402, Canada's two largest highways, go right through the snowbelt and are prone to snow squalls.
Ontario is used to it and can usually keep up with the snow without having to close those highways, although smaller highways closer to the lake do get closed a lot. However, in mid-December 2010, a one-two storm punch forced both the 401 and 402 to shut down between London, Sarnia, and Chatham when snowplows could not keep up with the falling snow. Over three hundred cars and trucks were stranded, and some drivers had to be rescued the next day by snowmobile and helicopter.
Streamers usually wobble back and forth so that no place gets hit for longer than a day or so. that kind of streamer usually dumps about 20 or 30 cm of snow before it leaves.
However, sometimes a streamer stays in one spot for days at a time. That can dump unbelievable amounts of snow in that narrow area. In early December 2010, London, Ontario, got a metre of snow in three days, but nothing further away than 20 km east, west, or south of London saw a single flake of snow. Just north of London, during the same squall, Lucan got stuck under the worst of it for a day or so longer, and got nearly two metres of snow!
Although the Great lakes give lake effect snow its name, other bodies of water can also cause lake effect snow. Upstate New York gets lake effect streamers from Lake Champlain and the Finger Lakes. Oceans, seas, and even bays can all cause lake effect snow, provided the land is to the lee of the prevailing wind.