Our first cruise to Alaska was July 4 (2 yrs. ago), and we are sent again for next year July, 12. We had great weather the whole trip. We didn't have any of the rain you hear about. It got up to high 60's during the day, and a lot cooler at night.
We cruised on Infinity the last week of Aug, first week of Sept. a few years ago and had amazing weather for that time of year, 2 afternoons the temps were in the 70's, so the shorts I packed got some use. We did see rain, only once in the afternoon in Juneau after we did the whale watching excursion, so that was OK... Our time in Hubbard Glacier was absolutely beautiful, guess the prior weeks it wasn't half as nice, according to the captain that day. I think its hit or miss, no way to tell what the weather would be like and I was very apprehensive about going so late, but it was wonderful.