United Airlines, hit hard by weekend storms in the Midwest, canceled dozens more flights Thursday as the second storm since Christmas threatened to pile 20 inches of new snow on Colorado.

Up to 8 inches of new snow were expected in Denver, which set a record for its snowiest Christmas with the nearly 8 inches that fell Tuesday.

United canceled 168 flights nationwide Thursday mostly because of the weather in Denver, its second-largest hub, to help prevent planes from being stranded there. That's about 5 percent of the airline's daily schedule.

"There will be impacts in other parts of the system and we're doing our best to accommodate everyone," United spokeswoman Megan McCarthy said.

It marked the fourth straight day that United had canceled flights. The Chicago-based carrier grounded fewer than 5 percent of its flights Wednesday as it moved crews to deal with schedules disrupted days earlier when bad weather hit O'Hare International Airport. McCarthy said the airline had recovered from those delays before the storm moved into Colorado.

Meanwhile, discount carrier Skybus Airlines said Thursday it was operating normally again after two days of cancellations due to maintenance issues with a pair of its seven planes. The Columbus, Ohio-based company canceled 18 flights over Tuesday and Wednesday, affecting an estimated 1,000 holiday travelers.