Just a heads up - The last couple weeks have been rather odd weather-wise, very dry and a lot of rather chilly nights all over the state (not below freezing, but a lot of nights in the 40s). While it's always tough to predict when the foliage will peak, I'm getting this gut feeling it might be on the early side this year. If that pans out, your time frame might actually be past peak in most of the state.
If you'll have the capability to go south of Portland (possibly FAR south), I might advise having that as a backup plan just in case.