Bug Guide's descriptions and photos left me uncertain whether this was P. occidentalis or P. modesta, but Pacific Northwest Moths has well-populated range maps (from university collections) for each, which show that P. modesta historically didn't get south of central Washington. That tends to confirm my impression from both sites that my moth's forewing is too pale and the dividing line through the middle too distinct for at least a typical P. modesta.

For more resolution: Flickr.

I'll leave this at the genus level pending expert review; if not contradicted, I'll move it to the species. But I wonder how long to wait?