But it's the same meteorology as the commonly-heard "Lake Effect" snows in the Great Lake areas.
In this case, you have the strong northeast winds blowing out of the Fraser River valley southwest across the northern Puget Sound and San Juans and then into the northeastern side of the Olympic Peninsula.
As the winds cross the Sound, they pick up a little bit of moisture. As they slam into the Olympic Mountains, the air rises and condenses, dropping its newly-found moisture as snow.
We've heard Port Townsend once received 6 feet of snow over a four day period due to this Strait effect.