A storm associated with colder air will spread rain and mountain snow over part the western United States late this week and this weekend.

The storm will help boost snowfall for ski country in the Northwest and bring some rain to areas still at risk for wildfires.

Rain will spread through low elevations of the interior Northwest and southward along the Pacific coast through California.

Storm to deliver soaking rain in coastal, low elevations

A few inches of rain are forecast to fall along the west-facing slopes of the Cascades, Coast Ranges and northern Sierra Nevada with the storm.

"While less rain will fall along the east-facing slopes, enough rain may still occur to cause flash flooding and mudslides, especially in recent burn scar areas," according to AccuWeather Senior Meteorologist Mike Doll.

Episodes of rain are in store for Seattle from Thursday through Sunday. In Portland, Oregon, aside from a shower on Wednesday, the real wet weather will set in on Thursday.

Farther south, in San Francisco and Sacramento, California, the bulk of the rain will fall Friday night and Saturday, when airline delays are possible.

Some rain is also likely to reach Los Angeles and San Diego this weekend. Even a small amount of rain mixed with oil and other residue on area roads can lead to slippery streets, highways and intersections.

Storm to deliver snow to mountains, passes

As the storm progresses, colder air will invade. Freezing levels will lower. Enough snow will fall to lead to slippery travel at most passes from Washington to Idaho, Oregon and Northern California.

The storm will last for several days in some areas, which will lead to wintry travel over the passes.

The high country of the Cascades, northern Rockies and northern Sierra Nevada, as well as the ridges and peaks in the Olympics, are likely to receive well over a foot of snow from the long-duration storm that bring several rounds of wintry precipitation.

Motorists should be prepared for wintry conditions over Snoqualmie Pass along Interstate 90 in Washington. Initially the snow will melt as it falls on the road surface on Thursday. However, as temperatures trend lower, slushy and slippery conditions are likely to develop as periods of snow occur from Thursday night into Sunday.

"For cities such as Spokane, Washington, it will be a close call between cold rain, a rain and snow mix and accumulating wet snow from Wednesday night to Saturday," according to AccuWeather Senior Meteorologist Frank Strait.

"Several inches of snow will fall on the mountains to the north and east of the city," Strait said.

Farther to the east, over Lookout Pass along I-90 in Idaho and along most highways in western Montana, periods of snow will make roads slippery from mid- to late-week.

Farther to the south, it will take some time for the atmosphere to get cold enough for accumulating snow on the passes in the southern Cascades and the northern Sierra Nevada. However, rounds of slippery conditions are likely to develop on Siskiyou Summit in Oregon from Friday night through this weekend.

"It may not be until late Friday before it is cold enough for accumulating snow at Donner Pass along I-80 in California," according to AccuWeather Lead Storm Warning Meteorologist Eddie Walker.

"Because of marginal temperatures during the day, the most likely time for slippery travel over Donner Pass will be at night on Saturday and Sunday," Walker said.

Plain rain is likely to fall over the Cajon and Tejon passes in Southern California.