Nice picture of what appears to be a Catocala hermia to me. This species is wide spread in the midwest/west, so is found in WA. Not knowing how true the colors are in your picture, it's also possible its a Catacala luciana, also widespread in the west, as they are very similar to C hermia. The dorsal forwings of C. hernia tends to be grayer, those of C. luciana browner, and what color the basal area is of the dordal hindwing is another key. In C. hermina its a deep pink, in C. lucina orange. Both of their larva feed on Populus (cottonwood) and Salix (willow), so your proximity to a raparian habitat supports them being in your area.
HTH,
Larry

Thanks, Larry. From your description, it may be hermia as the underwings were bright pink. I did some more searching on the internet and thought it may be C. aholibah. However, looking at pics of hermia, that looks more like it as the underwings were bright pink with a wide white trailing edge. Also, aholibah from what I read prefer oaks which are rarer here accept for ornamentals (red oaks). Thank you so much for your help! Also, the color was uniform gray, not as brown as in the picture...when it landed on the side of our house, which is gray, it blended in quite well.

BN