Trek Chick:
I grew up in Washington State and lived there until I was bout 35. DH and I did a portion of the Wonderland Trail several years ago, were going to go all the way around, but I hurt my knee and couldn't continue.
This is an amazing hike. You are going to have a blast. It is outstandingly beautiful.
Mid August will most likely be gorgeous. I don't really think you will run into too much rain that time of year. (but you should be prepared). It is probably pretty close to the driest time of the year there.
Have fun!