Likely, based on average climate data typical for August, you're reasonably unlikely to meet up with more than a few showers here and there. Except for the southern Washington and Oregon coastlines, you can generally expect less than 1" of rainfall in the coastal towns through August.

Of course, August to early September is when things can begin turning ... and occasionally some gully-washers begin hitting the northern coastline about this time.

Myself, I'd probably ride in the region with fenders, for a trip as far as Seattle->Cabo.

Up to you. Probably won't need them that often.