Winterlike storm to unleash travel-disrupting snow in northwestern US for days

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

The storm will help boost snowfall for ski country and bring needed moisture to areas that have been experiencing abnormally dry to drought conditions.

Mostly rain will fall at sea level along the coast and over much of the Interstate 5 corridor in Washington and Oregon, but the air will turn cold enough for some snow in some areas east of the Cascades and west of the Rockies.

Travel delays on the coast will be relatively minor compared to most storms during the winter. Rain, vehicle spray and large puddles will slow some people down. Episodes of rain, fog and a low cloud ceiling may lead to minor airline delays at Seattle-Tacoma International Airport.

However, for motorists heading over the passes and in areas east of the Rockies, winter driving skills and equipment will be a must.

Storm to deliver soaking rain in coastal, low elevations

Up to a few inches of rain is forecast to fall along the lower west-facing slopes of the Olympics, Coast Range and Cascades into this weekend.

Episodes of rain are in store for Seattle and Portland, Pendleton, The Dalles and Medford, Oregon.

A bit of snow can mix in at times during the weekend in Pendleton and The Dalles. Some road surfaces over the hilly areas may become slippery at night and during the early morning, where temperatures dip to near the freezing mark.

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 and northern Rockies, as well as the ridges and peaks in the Olympics, will likely receive well over a foot of snow from the long-duration storm that will bring several rounds of wintry precipitation.

Motorists should be prepared for wintry conditions over Snoqualmie Pass along I-90 in Washington. Initially, the snow will melt as it falls on the road surface into 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 into early 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.

Provided there is enough moisture lingering, the best chance of accumulating snow in the city of Spokane will be from Saturday to Sunday.

Farther east, over Lookout Pass along I-90 in Idaho and along most highways in western Montana, periods of snow will make roads slippery into Saturday.

Farther south, it will take some time for it to get cold enough for accumulating snow on the passes in the Cascades of southern Oregon. However, rounds of slippery conditions are likely to develop on I-5 over Siskiyou Summit in Oregon from Friday night through this weekend with the worst conditions possible on Sunday.