Farmers Almanac: “Thunderstorms occur when an air mass becomes so unstable that it overturns violently. This usually happens when drastically different temperatures meet, such as when the air closer to the ground is unusually warm and humid and the air above it is unusually cool. Because the lower layers of air are colder — and have a lower dew point — in the wintertime, these kinds of atmospheric clashes are very unusual during colder months. Still, thundersnow does happen. Thundersnows are most common in the Great Lakes region, when cold air blows across the relatively mild water of the Great Lakes, forcing the air upwards rapidly enough that the instability causes lightning and thunder in conjunction with heavy snow.”