The first substantial snow and ice storm of the season will hit travel hard along the Interstate 5 corridor, as well as the interior northwestern U.S. by Thursday.
While the storm will begin as rain along the immediate coast of Oregon during Wednesday night, enough cold air will be in place for an icy mix to develop east of the Coast Range.
"Along I-5 in Oregon, especially from just north of Eugene to Salem and Portland, travel will be slippery during the Thursday morning commute," according to AccuWeather Senior Meteorologist Brett Anderson.
The swath from Tacoma to Seattle and Bellingham, Washington, will likely receive its first inch or more of snow and sleet of the season, before a change to rain occurs during the afternoon and evening. Part of this area received its first coating of snow on Monday.
"For the I-5 corridor around Puget Sound, the evening commute Thursday will be slippery," Anderson said.
The hilly areas in the I-5 region, as well as bridges and overpasses, are the most likely to become slippery.
The combination of snow and ice, followed by rain and a low cloud ceiling, will lead to airline delays at Seattle-Tacoma International Airport.
Farther inland, cold air will hold on. Areas east of the Cascades can expect a prolonged period of snow and/or ice as a result.
"Icy travel is likely to persist through the Columbia River Gorge into Thursday evening," Anderson said.
Enough snow to shovel and plow will fall on Spokane and Yakima, Washington, as well as Pendleton and The Dalles, Oregon.
Travel along the I-82, I-84 and I-90 corridors will be hazardous, prior to temperatures approaching the freezing mark, Anderson stated.
As a second storm rolls in from the Pacific, rain will fall along the coast, and either rain or a rain, ice and snow mix will occur inland as far to the east as Idaho on Friday.
Where a few inches of snow is followed by drenching rain, urban and poor drainage area flooding will occur.
Meanwhile, the air will remain cold enough for heavy snow to fall over the Cascades and Bitterroots. From 1 to 2 feet of snow will fall over the high country of the Cascades with a foot of snow likely over the ridges in the Blue Mountains and Bitterroots.
It is possible the passes in the Cascades and Bitterroots may close for a time, due to the heavy snowfall rate.
Farther south, at Donner Pass, California, and Siskiyou Summit, Oregon, icy conditions to start will be followed by plain rain later Thursday. Both of these passes should remain above freezing during Thursday night and Friday.