A north wind in Puget Sound usually signals that we have our winds coming from the offshore direction, instead of our usual marine-born winds. In the summer, that's a double-whammy of pulling air from the hotter interior of Eastern Washington and southern B.C., plus holding our cooler ocean breezes at bay.
In the winter, the north winds can bring the arctic air usually entrenched in Eastern Washington and southern B.C., while keeping the relatively warmer ocean breezes away.
The pattern is caused when we have a large area of high pressure further inland that pushes the air out to sea. It'd be more of a northeast wind, but the Cascades and Olympics funnel them so it's nearly due north through the Seattle area.