I think from your site that this might be a polyphemus caterpillar, but I couldn’t really tell because the pictures of them weren’t quite sharp enough. So here is my picture. The kids found it in the back yard on September 9 near a maple tree, I believe. We live in Portland, Or. Thanks!
You are correct. This is a Polyphemus Moth Caterpillar. Here is BugGuide’s description of the caterpillar: “Larva: body large, bright green, with red and silvery spots below setae, and oblique yellow lines running through spiracles on abdomen; diagonal streak of black and silver on ninth abdominal segment; head and true legs brown; base of primary setae red, subdorsal and lateral setae have silver shading below; end of prolegs with yellow ring, and tipped in black.” Most of the identifying features cannot be viewed from your camera angle. A lateral view is best. While the Polyphemus Moth Caterpillar might be confused with the Luna Moth Caterpillar in the east, there is nothing similar looking in the western U.S.