Update: I got a response to my email request, the trail will be opening May 1st!
The new bike path running along Mercer under 99, between 5th and Dexter, has been complete for a while now. It's been roped off and there are inconveniently placed light poles that may need to be moved first. Whenever it does open, it's going to be a major upgrade, and the rough surface should work great in the rain. Hopefully it will open soon.