I'd skip it. Middle Fork confluence down will likely be a muddy torrent, adorned with arboreal debris, for most of the next week (if the forecast is even close to right about the amount of rainfall coming to the upper drainage). Safety may be a serious question, to say nothing of the fishing prospects.

Maybe drop the toon in the Chehalis around Elma and float down to Fuller? If it's running reasonably clear, that would be safer, and might be better fishing, too.