(Reuters) - Almost all of North and South Dakota were under blizzard, ice storm or winter storm warnings on Sunday as wintry weather was forecast to hit the Central U.S., meteorologists said.

The National Weather Service said it expected a strong winter storm to drop heavy snow - as many as 15 inches (38 cm) in parts of the Dakotas - across the northern plains on Christmas day, as well as rain or even freezing rain as far east as Wisconsin.

The service warned that the freezing weather would make ground travel near impossible in spots and could hamper the plans of some of the 94 million Americans who, according to American Automobile Association, were expected to travel by vehicle over the holiday season.

“Holiday travel will likely be poor across much of the northern-tier states,” the weather service’s Weather Prediction Center said. The weather service also warned of dense fog for large swaths of the country’s southern and central regions, bringing further difficulty to drivers.

For travelers making their holiday treks by air, flight tracking website flightaware.com was reporting only a few dozen flight cancellations or airport delays across the country as of early on Christmas morning.