SR20, AKA, North Cascades Highway, is the northernmost route in WA that crosses the Cascades. It's western terminus on the Olympic Penninsula at Discovery Bay (a little SE of Port Townsend) and it's eastern end is at the town of Newport on the border with Idaho (~50 miles N of Spokane). Due to the heavy snowfall the area receives, they don't keep Washington & Rainy Passes (the two westernmost passes, just east of Ross Lake) open during the snow season. Exact open/close dates determined by Mother Nature, but generally closed from November to April.
The "Destination Highways - Washington" book rates the section subject to closure as the best motorcycle road in the state, with numerous other sections to the east also ranking well.
http://washington.destinationhighways.com/http://en.wikipedia.org/wiki/Washington_State_Route_20