I am going to talk on averages. There is a BIG difference between the begining of Sept. mentioned above and mid Sept. NO comparism.
Averages- you are going during the worse potential weather of the Alaska season- mid Sept. You can expect, on AVERAGE 3 days of rain, temps 40-60, dusk WILL by by 7pm. So if you have an afternoon arrival, it will be significant. A few tours will NOT be running- glacier dog sledding, bear tours, likely.
This is based on my FIRST hand Alaska travels since 1994- 26 trips. I used to pick up these mid Sept trips- but always as my second and one time third trip of the season. Always, the earlier trips were better. Reason, I went- the discounts were signficant- $350/400 range. The discounts are not signficant enough for me to now consider them, so I do not go past the first week.
You do need to be WELL prepared in your attire, if you continue with this timeframe, necessary is good rain gear (I take rain pants and waterproof foot wear). you need 2 pair of walking shoes, layers, water resistant jacket, hat, gloves. And the acceptance of the worse potential.