If the southern boundary of the Pacific NW was to be estimated by how the climate feels, and a tad bit based on culture. I'd put the line through Redwood National Park, and at an angle so it excluded Orick, next to it.
I drive down to the redwood parks often. Several times per year. And Jedediah Smith Redwoods (photo) "feels like" the Pacific NW to me. Likewise, Prairie Creek Redwoods, and Redwood National Park.
But when I pass Redwood National Park and get down to Trinidad, Arcata, Fortuna, or Redcrest, the area doesn't feel the same.
So I'd say draw a line right outside Orick, CA.
What I'd find even harder to answer, is where the north boundary is. I suppose we could say the border with Canada. But there is so much in British Columbia that feels Pacific NW.