Arctic or polar continental air masses  cross the warm lake.  The cold airmass is usually only a few thousand feet thick, and is "capped" by a layer of warmer air, called a temperature inversion.  This inversion limits the extent to which the snow clouds can grow.  The addition of heat and moisture from the warm lake modifies the airmass and modified air rises and cools allowing moisture to condense into snow clouds.  It also causes the inversion to raise, allowing snow clouds to grow in height.  After moving easily across the "smooth" lake surface, the modified air slows down and "piles up" as it approaches the downwind shore.  The convergence produces additional lift, which is further enhanced by orographic features (hills, mountains) downwind of the body of water.  The result is lake effect snow, though during early Fall, lake effect rain can occur.