Predicting flows in a place like Colorado is generally relatively easy. Predicting flows in the northwest is a joke! I've tried it many years in the past, and I generally get burned. Obviously, the biggest factor in a runoff model is precipitation. The northwest is incredibly unpredictable in this regard. The best advise on flows is that you'll have a good idea of the flows 3 days out from your trip because that's generally what the weather forecast is good for. Refer to the northwest river forecast center.
Last year after being burned by high flows on the Middle Fork, we decided to run the Main at 55,000 cfs and dropping. It was a good run, and whiplash wasn't really a big issue (probably in the 40,000 cfs range). Since I was the TL on a run I'd never done before, I attached a GPS with all the waypoints of campsites and rapids. Elkhorn Big Mallard, Whiplash, and Chitam were the only ones that really stood out in my mind. You can download them off the internet. Like the Grand, the eddies are more significant than the rapids. I'd have a few safety kayakers along, at least.