CHICAGO - A wintry mix of Gulf Coast thunderstorms expected to travel north, predicted snowfall in the Great Lakes, and blustery conditions in the nation's midsection threatened Tuesday to snag holiday travel plans nationwide.

As snow fell in some states Tuesday - nearly two feet in South Dakota's Black Hills - a strong storm system expected to drop rain along the East Coast and snow from Missouri to Michigan developed in Mississippi, Georgia, and Louisiana, among other Southern states. The storms in the South generated likely tornadoes, damaged some buildings, and left thousands without power.

Meteorologists predicted rain into snow for parts of the Great Lakes region, with several inches of Christmas Eve snow expected in parts of Illinois. Officials at both airport hubs in Chicago readied for the potential of holiday delays and cancellations, particularly with more people expected to fly this year.

"I'd be nervous about the possibility of not being able to get out," Chicago area meteorologist Charles Mott said. "I would definitely make plans about possibly staying put or doing something else."

About 4.2 million passengers are expected through O'Hare and Midway international airports during an 18-day holiday travel period ending Jan. 6, said Chicago Aviation Department spokeswoman Karen Pride. That would be a 3 percent increase at O'Hare and a 9 percent jump at Midway compared with last year. Pride urged travelers to allow plenty of time and monitor airlines closely.

Elsewhere, a blast of cold and snowy conditions affected travel Tuesday.

Just west of Green Bay, Wis., a school bus was involved in a three-vehicle crash on a snow-covered road. No serious injuries were reported.

In eastern Colorado, Interstate 70 was shut down into Kansas because of strong winds and blowing snow.