As is indicated by the NOAA map below, lake-effect (feet of it in some locations!) will occur on the leeward side of the Great Lakes during the next couple of days–and it’s simply caused by cold air passing over the warm lakes, right? That’s certainly the main factor, but it’s not the only reason for the snow.
Cold Air/Warm Water
The contrast between the cold air and warm water is, indeed, the main reason for the production of lake-effect snow. As the bitterly passes over a large body of warm water, the air is warmed and moisture is added to the air; this warmer, more moist air rises–because that’s what warm, moist air does–and condenses into clouds and precipitation, producing snow.
Friction–well, actually the difference in friction between water and land–is the other part of the snow-making equation, and it’s glossed over in discussions of lake-effect snow. It shouldn’t be, though, since it’s an important contributing factor in snow production.
Just as you slide freely over ice (too freely, unless you’re an Emergency Room doctor in need of business) compared to sliding over gravel, air glides freely over water compared to land (and trees and building on the land). When this freely flowing air passing over the lake meets this additional friction associated with the land, the air begins to pile up (converge) as it’s forced to slow its speed. As the air piles up in one location, it begins to rise. (It can’t go anywhere else, such as down, and the friction is preventing its movement along its intended path.)
This upward motion significantly increases the intensity of the snow, and it’s the reason that the snow is often heaviest a few miles inland (as opposed to right at the lake shore).
The processes that produce lake-effect snow will continue as long as air cold enough to produce lake-effect snow is blowing over the lakes, so as long as the wind continues to blow from the same direction, any given location will continue to receive snow. It’s not like a normal storm that moves through the region. The result is often incredible amounts of snow, and I wouldn’t be surprised to hear of some reports of five, six, or seven feet of snow in some locations this week.