We were on the Summit Denali discovery cruise/tour from 8/3-15. In Sakgway we did the White Pass Scenic Railway on 8/11. It was hot in the train car, so I spent a lot of time on the platforms on either end of our car--good place for photos. I really liked this excursion. The scenery is breathtaking. We were blessed with almost perfect weather the whole trip. A bit of rain as the glass-domed train pulled into Denali, but it was almost dinnertime, so it didn't interfere with anything. in Sitka it was cloudy, but warm enough that we didn't need our sweatshirts and started to sprinkle just as we were lining up for the tender back to the ship, so that didn't interfere either. On the land tour we had blue sky and sunshine every day and especially in Talkeetna where you get the best view of Mt. Mckinley. Juneau, Skagway and Ketchikan were sunny and warm.
Actually, we hardly ever needed our sweatshirts and never even touched our windbreakers. You will be amazed at the flowers in Alaska. They have a 73 day growing season, but the 16-24 hrs. of sunlight make up for that and the flowers are spectacular.
There was a freak snowstorm and resultant floods in Denali and other areas, including Skagway ,in July and the train had to be evacuated. In Sept. I would be worried about snow. Just make sure you check the weather report.