No one can predict. But historically there is more rain as Sept. marches on and cooler temps. Dress in layers, with a removable lined nylon jacket, rain gear- ponchos good, 2 pair of walking shoes, I no longer take jeans- they are just too bulky, take forever to dry and get and tend to stay damp. Last few years I have taken my lined wind pants with excellent results for me. But I do spend all my time out doors. Knit hat and gloves.
Unfortunately, you can't predict the weather - so it's best to be prepared for any eventuality. We cruised last Sept during the same dates and ended up being mainly in "shirt-sleeves", except around the glaciers. It actually got sunny and into the high 60's while we were in Ketchikan!! (this is very rare). As Karen said, all you can do is plan on layers and clothes that dry easily. Have a wonderful cruise!