December is usually the peak return time for hatchery winter steelhead in western WA. Some years January is. The most productive rivers in WA are on the Olympic Peninsula coast. Smolt to adult survival has dropped to record low levels among Puget Sound tributary rivers, so I would avoid them. The downside to traveling to western WA for winter fishing is the unpredictable nature of the weather. A river that is perfect for fishing on Friday may be running up in the trees when I get there Saturday morning. (Always check the USGS stream gages on the internet before leaving home!) Good choices for December steelhead include the Bogachiel, Hoh, and lower Queets Rivers. Some early winter steelhead have already shown up and been caught this week.
You can increase your chances significantly by booking trips with a guide. If you decide to venture this way, send me a PM or email, and I'll answer any questions that I can.