Not PNW, but Appalachia to mid Atlantic
DD has been a 95 miata set up for auto-x (coilovers, exhaust, etc)
Summer I am on 15x7's with (iirc dunlop direzza ZII)
Winter, E30 bottlecap wheels with general artic artimaxes (or however its spelled, find them on tire rack, they are cheap and good)
Furthermore, I did time in the mountains of western MD with it and had to go up dirt/gravel hills to get to moms house regularly. It was always snow covered. I never failed to make it to the top. Also, I have an open diff and no abs. Only concerns would be if snow was deep enough to beach it. So you track depth and progress of local snow removal and dont be an idiot in a really bad storm (which locally would be a foot or two overnight)
I am seeing some rocker rust now after 4-5 years of this treatment ( and western md is VERY HEAVY on salt roads stay white till 2-3 good rains after season)
Aside from the wheel swap, softening up the koni's, and putting on the hardtop not much to tell. Oh yeah, spraying under carriage with Fluid Film. https://www.amazon.com/Fluid-Film-11-75-Spray-3-pak/dp/B00B93ZXGS/ref=sr_1_3?ie=UTF8&qid=1513673924&sr=8-3&keywords=fluid+film just to slow down the rusty march.
Look into local laws on studs vs no studs. Blizzacks are wonderful I hear, but pricier. I didnt go studs because I didnt want the limitations and have only had a few ice situations where I think they would have been any help. Think about it, read what tire rack has to say, talk to locals about ice storms vs snow showers. I would say if laws are good on them and you see more ice than snow consider it. I see/saw more snow than ice.
Just get a cheap second set of wheels, throw snows, and learn to countersteer. Snowy parking lot doughnut practice actually has purpose
Any curiosities not covered?
*edit, dont know what PNW area you are going to be/what conditions would be. setup advised above might work for canadian level winter while you might just have the cold and wets.