Formal wear is a difficult choice. On all the Millenium-class ships we have been on, formal wear is long cocktail dresses for the ladies & dinner suits & bow ties for the men, but sit at a table of Americans & you will look overdressed! They just don't like dressing up. Celebrity, for some unknown reason, don't insist, once on board, that formal means formal. A middle ground for men is a suit & tie.
The ambient temperature for the more southerly parts of Alaska and up on the passes in September is very warm but seldom hot however take rainwear with you because they certainly get a lot of that. Ketchikan for instance has 13 feet of rain a year and yet when I was there it was beautifully sunny. You will find a more accurate answer if you Google “Alaska weather” .
Formal wear is becoming easier but you won't feel out of place on those nights if you wear a D.J. or a tuxedo but ladies seem to be happier nowadays in cocktail dresses. Many Americans do dress up but you will often find the oddest garb being worn. The best of flying to the US is that in general the luggage allowance is better than most other routes so you can take more to accommodate your moods and preferences. By the way if you have an overnight stay at the Airport Fairmont in Vancouver don't change it as it's the best airport hotel I've stayed in, regards Neil Down