"It's really setting us up for black ice and ice conditions overnight," said Dave McCormick of the Washington State Transportation Department.
He cautioned drivers to stay off the road, if at all possible.
"Basically, if people don't have to go out, don't go out, because it just hinders us," McCormick said.
In Washington state, state transportation crews were out in full force, plowing and sanding roads. But even their best efforts couldn't compete with Mother Nature.
The potentially treacherous road conditions are expected to play havic with some school schedules Monday and possibly beyond.
Dozens of school delays and activity cancellations already were being reported by Sunday evening, and many more were expected.
Conditions on hills and secondary roads were especially dicey, transportation offficials said.
S.R. 410, Chinook Pass, is temporarily closed due to the forecast of extreme weather conditions. The pass will be evaluated for possible reopening on Monday.
In the Puget Sound region, the West Seattle Bridge was closed for about three hours earlier Sunday when about 20 vehicles spun out along the slippery stretch of roadway.
Highway 512 also was closed for a time between 104th Street East and Meridian Street when two semis slid into each other on the icy pavement.
Both of those highways have now reopened.
The state Transportation Department also postponed its monthly maintenance work on the Evergreen Point Floating Bridge, originally scheduled for early Monday, because of the cold weather. So the bridge will be open to drivers throughout the night.
In Kitsap County, where up to five inches of snow fell Saturday night and Sunday morning, numerous spinouts and fender-bender accidents were reported.
And in Bellingham, where the wind chill plunged to 2 degrees above zero on Saturday night, cars slid on icy roads.
More information about road conditions will be posted as it becomes available.