We travelled in the second week of September.
We flew into Seattle for one night and the weather was nice although wearing a thin jacket was necessary. Same thing in Ankorage but a slightly warmer jacket was needed in the morning. We were lucky in that for a week about half thedays were very nice, sunny and warm.
Be prepared for rain and cool weather if you go inland to Denali (which is a recommended trip).
It was rainy and cold at Juno and Sitka.
Ketchikan and the rest of the days on ship were sunny and pretty warm. We lucked out I think. We could have worn shorts ion Ketchican.
You have to hope for nice weather but if you go to the mountain it will be wet and cold. It was in mid-september.
Hope that helps.