You can check flows on American Whitewater - AW Homepage
Robe is probably the best thing that is close to Seattle in the IV/V range. There is also Canyon Creek right next to Robe (on the S. Fork Stillaguamish) if Robe is more than you want.
If it's raining, you'll have tons of possibilities between Seattle and Bellingham and even Southern BC: Clearwater near Bellingham, as someone mentioned N. Fork Nooksack, Norrish Creek near Abbotsford, BC, Fall in the Wall up on Snoqualamie Pass, and lots more! There are tons of options.
Have fun out there!