Winter storms forecast to hit U.S. as country gears up for Thanksgiving travel

With millions of people preparing to travel for the Thanksgiving holiday, two strong storm systems, including the first major winter storm of the season, are forecast to bring snow, wind and rain across a large swath of the United States this week.

More than 25 million people were under winter storm watches or warnings, the National Weather Service said Monday.

What to expect this week

According to the National Weather Service's Weather Prediction Center, the first system, which is expected to bring thunderstorms and possible tornadoes to the central and southern Plains on Monday, will collide with arctic air as it moves north, causing widespread snow to develop across the Dakotas, Minnesota and Wisconsin on Tuesday and Wednesday.

Up to a foot of snow is possible in parts of North Dakota, the weather service said, adding that the snowfall “may also be accompanied by gusty winds, leading to lower visibility on roadways.”

Snow in the northeast

Separately, an upper-level low pressure system was forecast to develop above the Great Lakes later this week, resulting in cooler temperatures, cold rain from the Ohio Valley to the East Coast and accumulating snow for the central Appalachians and parts of the Northeast, the weather service said.

Up to six inches of snow is possible on Thursday and Friday, especially in the higher elevations.

Read more from Fox Weather: Snow, rain to slam U.S. as winter storms ramp up ahead of Thanksgiving travel

‘Atmospheric river’ in the northwest

Meanwhile in the Pacific Northwest, a dayslong “atmospheric river event" was expected to bring heavy rain and snow to Oregon and Northern California beginning Monday, the weather service said.

“Widespread rainfall amounts of 4 to 7 inches are expected through Wednesday across this region, which could produce areas of river flooding and increase the risk of mudslides,” the weather service said. “Heavy rain and the associated weather hazards from this atmospheric river event are also expected to continue beyond midweek.”

What about Thanksgiving?

Weather service forecasters have yet to issue forecasts beyond seven days, but the U.S. Climate Prediction Center’s 6-to-10-day outlook suggests above-average chances for rain in the Pacific Northwest, upper Midwest and Great Lakes during the early part of next week.

Nearly 80 million people are expected to travel more than 50 miles for Thanksgiving next week, according to AAA, with Tuesday and Wednesday expected to be the busiest times to be on the roads.

Read more from NBC News: Almost 80 million expected to travel over Thanksgiving in record-breaking getaway