OMAHA, Nebraska (AP) – Holiday travelers battled slick, icy roads and flight cancellations and delays on Wednesday as a major winter storm began to spread across much of the nation's midsection — and the worst of the weather was still expected to come.

The slow-moving storm was likely to intensify Thursday as it continued its trek north and east, bringing heavy snow, sleet and rain to a large swath of the Plains and the Midwest. A foot or two of snow was possible in some areas by Christmas Day.

"It's an unusually large storm, even for the Plains," said Scott Whitmore, a National Weather Service meteorologist in Topeka, Kan.

In northwest Kansas, snow started falling before sunrise Wednesday, after freezing rain had already iced up roads.

A stretch of Interstate 70 in western Kansas was snowpacked by mid-afternoon. The state Department of Transportation warned that travel would be almost impossible in northeast Kansas by Thursday afternoon.

"It's kind of hard to stay on the roads. You've got to go slow," said Jason Juhan, a clerk at the Love's truck stop in Goodland, Kan. "People are just trying to get through and get to where they need to as fast as they can."

Still, he saw an upside: "It's been a few years since we've actually had a white Christmas out this way."

The storm began in the southwest — where blizzard-like conditions shut down roads and caused a pileup involving 20 vehicles in Arizona on Tuesday — and spread east and north, prompting weather advisories from the Rocky Mountains to Lake Michigan and part of the Four Corners region.

In Colorado, numerous minor accidents prompted state transportation officials to close a section of Interstate 25 from Wellington, Colorado, to Cheyenne, Wyoming, for several hours. One accident on I-70 seriously injured a state snowplow driver.

Parts of Nebraska were coated with ice that was up to 1/4-inch thick and a number of churches were already canceling Christmas Eve services in anticipation of more ice and snow.

Slippery roads were blamed for at least six deaths — three in accidents on Interstate 80 in Nebraska, two in a crash on Interstate 70 in Kansas and one near Albuquerque, New Mexico. South of Phoenix, a dust storm set off a series of collisions that killed at least three people Tuesday.

In Chicago, more than 200 flights at O'Hare International Airport were canceled, along with about 60 flights out of Midway International Airport, the city's Aviation Department said.

The storm forced the closure of the Mount Rushmore National Memorial in South Dakota. The National Weather Service in Sioux Falls warned of treacherous travel conditions from Wednesday through Friday night, calling the storm "life threatening."

A tropical jet stream pumping in moisture from the storm's south was likely to cause plenty of snow throughout the Plains and the Midwest, with the biggest accumulations expected from eastern Nebraska to the Upper Mississippi Valley. Freezing rain was possible across parts of Kansas, Nebraska, Iowa, Illinois and Indiana.