Don’t forget to check the local forecast before you hit the road or check out Black Friday deals: Snowfall could snarl post-Thanksgiving travel plans across the northern Plains and Midwest, while roughly 9 million people are under freeze watches in the South.

In the Great Lakes region, a vigorous lake effect snow event is expected to last through the weekend before tapering off early next week. Six million people were under winter alerts Friday from northern Minnesota into upstate New York, where snowfall totals downwind of lakes Erie and Ontario were expected to reach 3 feet or more.

At times, snowfall rates will be blinding, at 3 to 4 inches an hour, and could be accompanied by thundersnow, a rare weather event that combines a snowstorm with thunder and lightning. The highest snow accumulations will be east of Lake Ontario, where some isolated areas could get hit by up to 60 inches of snow around the Watertown, New York, area.

Road travel could be especially difficult on Interstate 90, between Cleveland and Buffalo, and Interstate 81, north of Syracuse, New York. The “Sunday Night Football” faceoff between the San Francisco 49ers and the Buffalo Bills could be buried under snow because Highmark Stadium is in Orchard Park, a town that is forecast to pick up between 12 and 18 inches, with higher amounts possible.

Meanwhile, some 9 million people on Friday were under freeze watches and warnings across the South, stretching from Texas to the Carolinas. The cities under alerts include Abilene, Texas; Shreveport, Louisiana; Jackson, Mississippi; and Charleston, South Carolina.

Weather forecasters expect the coldest air in the South is yet to come, with an Arctic air mass spilling south out of Canada.