Officials in upstate New York counties have declared a state of emergency as they prepare for a major snowstorm that has already dumped over two feet of snow in the Midwest and is forecast to accumulate more this weekend.

The National Weather Service’s Buffalo office has issued a lake-effect snow warning for much of Western New York.

Lake-effect snow occurs when cold, dry air moves over a body of water, in this case, the Great Lakes, and picks up moisture from the lake's surface. When the air moves over land, it cools, and the moisture condenses, leading to snowfall.