Why stop at the Washington/Oregon border? Who would be stopping them? Especially considering geography with the Columbia River which would basically make whoever controls it control most all of western Oregon including the very productive Willamette Valley.
Sure, there's a limit to how much Japan could take and actually colonise, but that wouldn't stop them from "painting the map" with their claims. Look at Spain/Mexico, claiming all the American West with barely anyone on the ground enforcing it outside of a few clusters of settlements in California, New Mexico, and Texas (the latter two frequently besieged and looted by Indians).
Maybe a cluster of settlements in California (around San Francisco and San Diego certainly), some in coastal Oregon (Coos Bay, Tillamook, around the Columbia River mouth), and then around the Puget Sound and Vancouver Island. The ones in the north would come first, and everything north of California would fuse into one, with the two main regions being in the Willamette Valley/Columbia River and around the Puget Sound.
North of that is difficult, since it's so rugged, but maybe around Prince Rupert, and then from there something in the Inside Passage, and some settlements around the Gulf of Alaska. Probably something in a sheltered fjord, near where the major seaports are OTL. The main settlement would probably before a long shift to the Pacific Northwest since I believe you can grow rice (very highly prized) there and thus can grow more than buckwheat and whatever else you can manage to grow in Alaska.