Clouds are translucent to the UV light needed for vitamin D synthesis. Even so the days get pretty short up north.
Also you haven’t mentioned your skin tone. Persons with darker skin will obviously have a harder time synthesizing vitamin D from sunlight. Fortunately the PNW is rich in salmon, one of the few good natural dietary sources of vitamin D, which certainly helped earlier peoples to maintain their health.
I've also started adding more salmon into my diet. A 100g serving seems to be about 500 IU of vitamin D. Apparently fat helps with absorption of vitamin D, so it seems like a good way to get it. Still, not sure it would be enough, and I'm not willing to forego the supplement this winter to see where I'd end up.
Also you haven’t mentioned your skin tone. Persons with darker skin will obviously have a harder time synthesizing vitamin D from sunlight. Fortunately the PNW is rich in salmon, one of the few good natural dietary sources of vitamin D, which certainly helped earlier peoples to maintain their health.