Found a couple of caterpillars on an oak which I think are Cyclophora dataria. Posting two photos of the larger - one taken with a flash and one in natural light. I’m pretty sure on the larger one and think the second, smaller one is likely a younger version. Basing the ID on Miller & Hammond’s “Lepidoptera of the Pacific Northwest: Caterpillars & Adults”. Link below is to the online version. Cylcophora dataria is on Page 99.

http://www.fs.fed.us/foresthealth/technology/pdfs/FHTET_03_11.pdf

The photo looks like a pretty good match and they say: “Colours range from tan, to brown, to gray, but always with the darker middorsal semicircle countered by the anterior lateral light markings, particularly on A2 through A5.”