With a south to southwest flow aloft, there is a pronounced rain shadow effect over the south shore of Juan de Fuca strait (Port Angeles and Sequim) as well as southern Vancouver Island. Have a look at the video below that demonstrates the effect on the RADAR this morning. The flow is southerly bring moisture northward from an upper low centre off the BC Coast. Notice how the echoes disappear at the summit of the Olympics. This is the main reason that the area to the lee of the mountains gets considerably less rain than other coastal locations.

Olympic rain shadow