Hi Molly, and welcome to cruise-chat. And, happy 40th anniversary!
We were in Alaska in the second-to-last week of August last year. Friends of ours were on another ship, same itinerary, that left three days before ours. We had sunshine in nearly every port, and they had rain in those same ports two to three days earlier. So you never know. Even so, they had a fabulous time (as did we). Once you're in the Inside Passage, you are so close to land that you'll still be able to see its beauty.