More than 4 million people across five states were under winter storm alerts on Sunday as heavy lake-effect snow continued to bury the Great Lakes region, prompting emergency declarations and the deployment of National Guard troops to rescue stranded motorists.

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.

The lake-effect snowstorm has disrupted holiday travel across the region. Some areas could get up to six feet of snow by Tuesday.