A strong winter storm is hitting the Pacific Northwest Wednesday, Jan. 18, dumping heavy snowfall amounts and slowing traffic on Interstate 5 and mountain passes.

Traction devices are required on I-90 Snoqualmie Pass and Stevens and White passes, the Washington State Patrol said.

Reduced traffic and collisions were reported on I-5 at State Route 520, I-405 and SR 167, all major commercial routes, the WSP said.

Snow accumulation of 3 inches to 6 inches is expected in the Seattle area and more than that in the Cascade Mountains east of Seattle before the storm moves toward Idaho and Montana later Wednesday. Snowfall of up to 20 inches is forecast for the South Puget Sound area in Olympia and south.

Temperatures are forecast to hover around freezing through Wednesday before warming to the mid-30s Thursday, Weather.com reported.

On the other hand, it’s raining in Portland, where temperatures are in the 40s, Weather.com reported. Rainfall is forecast to be heavy at times through Thursday.