Lake effect snow forms when cold, below-freezing air passes over a lake's warmer waters. This causes some lake water to evaporate and warm the air. Then, the moist air moves away from the lake. After cooling, the air dumps its moisture on the ground, potentially becoming snow.Cold winds in the winter typically prevail from the northwest in the Great Lakes region, producing the most dramatic lake-effect snowfalls on the southern and eastern shores of the Great Lakes.