stranded passengers
© AP/Rick BowmerTravelers surround the ticket windows at Portland International Airport Tuesday, Dec. 23, 2008, in Portland, Oregon.
Chicago - Hundreds of holiday travelers spent the night in the nation's second busiest airport and others faced delayed or canceled flights and highways choked by snow and ice as storms kept up their assault on northern states.

More snow fell Wednesday in the Midwest, where the National Weather Service said up to 4 inches was possible in Chicago. The Northwest faced more snow and sleet early Wednesday, with up to 20 inches possible in the Cascade range. And more snow and ice spread over the Northeast.

"We're seeing quite a bit of messiness out there," said weather service meteorologist Ed Shimon, who's been at work at the agency's Lincoln, Ill., office for six straight days, a period when the state has seen snow, ice and subzero temperatures. "It's something different every day - never a dull moment."

About 500 travelers had to spend the night at Chicago's O'Hare International, the nation's second busiest airport, after stormy weather canceled more than 500 flights Tuesday, said Chicago Aviation Department spokeswoman Karen Pride.

More than 100 flights were canceled at O'Hare on Wednesday. On Tuesday, cancellations totaled more than 500.

South of Portland, Ore., crews shut down a section of Interstate 205 at Oregon City because a buildup of ice and snowbanks, said Dave Thompson, public affairs manager of Oregon Department of Transportation. The department also was requiring tire chains on all highways in the Portland metro area.

The weather service posted winter storm warnings and advisories for large parts of the West, plus a swath of the Midwest and most of the Northeast.

Across Massachusetts, icy roads caused numerous accidents Wednesday morning, and state Trooper Thomas Murphy Interstate 495 in Middleborough and Wareham was closed because of wrecks.

Police in southeastern Pennsylvania reported a spate of ice-caused accidents and traffic delays, including one 22-vehicle crash. No serious injuries were reported. Trains and buses also were delayed by ice, according to the Southeastern Pennsylvania Transportation Authority.

Many roads across northern and central Indiana were ice-covered Wednesday morning, but State Police reopened a 30-mile stretch of Interstate 69 northeast of Indianapolis after closing it for about four hours due to ice.

The weather service warned of a significant flood threat this weekend in northern Indiana because of heavy rain expected later in the week and rapid snow melt.

Despite more snow falling Wednesday in the Seattle area, operations at Sea-Tac Airport were back to normal Wednesday, said spokeswoman Terri-Ann Betancourt. She said the last of thousands of passengers who were stranded by weekend cancellations were gone by Tuesday, and the only flight cancellations were caused by delays or cancellations at other airports.

"I've lived here 16 years and this is the first time I've thought 'I wish it would rain!'" Betancourt said.

At least 15 people died in car crashes on rain and ice-slickened roads Tuesday - four each in Indiana and Kentucky, two in Missouri, two in Kansas, one in Oklahoma, one in Ohio, and a state lawmaker was killed in West Virginia.