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.

Heavy snow slammed the Great Lakes region during Thanksgiving weekend, clogging communities with several feet of snow.

Lake-effect snow typically happens in the late fall and early winter when the lakes are ice-free and the water is still relatively warm.