(Reuters) - Fresh snowfall was expected to bring a white Christmas to parts of the U.S. West, Midwest and the Appalachian Mountains, while other regions of the country would slog through rain and fog, the National Weather Service said on Wednesday.

The forecast comes amid one of the busiest travel times of the year. The AAA travelers' association estimates that 98.6 million people, up 4 percent from last year, will travel more than 50 miles during the period from Tuesday to Jan. 4.

The NWS said a storm system moving east from the Pacific would bring snow showers from Washington state to the western High Plains on Thursday.

Accumulations from 4 to 7 inches, are expected in the higher elevations of the central and northern Rocky Mountains.

A winter storm watch is in effect from Thursday afternoon through Friday morning in Colorado's Teller and northern El Paso counties with 5 to 10 inches of snow expected.

The Colorado Department of Transportation said emergency crews would work 12-hour shifts in the Denver metropolitan area on Thursday. It warned drivers on Interstate 70 to expect significant delays.

Light to moderate snow was expected between Wednesday and Thursday in the Great Basin in the west, the Upper Great Lakes, the Northern Plains, parts of the Ohio Valley, areas of the Northern Appalachian Mountains, the Upper Mississippi Valley, higher elevations in California and the Rockies, according to a NWS advisory.

Snow could pile up from Chicago to Sault Ste. Marie, Michigan, according to forecasting company AccuWeather. Parts of Wyoming, Montana, Idaho and Nevada could also see snow, the company reported.

On Hawaii's Big Island, an unusually strong blizzard is bringing up to 8 inches of snow to the summits of Mauna Kea and Mauna Loa. The accumulation could give residents below on the Big Island a sight of snow on Christmas Day, said NWS forecaster Norman Hui.

Meanwhile, severe thunderstorms menaced the U.S. Southeast and emergency workers in Mississippi surveyed damage from tornadoes that killed at least four people and injured 20.

The NWS canceled a winter weather advisory in Chicago as higher temperatures turned Wednesday’s forecast of snowfall to rain.

The flight tracking website FlightAware reported 2,323 U.S. flight delays on Wednesday. Cancellations totaled 316, with the largest number at O'Hare International Airport in Chicago.

Heavy fog delayed some flights at airports in San Francisco, Detroit, Philadelphia and New York's John F. Kennedy International.