I have sailed twice on "O" in AFT cabin 6090 with it's 180 degree view. It was wonderful. Out of 24 sailing days in June we had 3 days rain. The rest were absolutely beautiful, we wore shorts. That's not to say it will be cold. One never knows.
I am going on my third Alaska on "O" in May, and yes, I am in the same AFT cabin. I do anticipate much colder weather this time around but will be prepared. Living in New England we do 4 beautiful seasons and have the clothing for all. I don't melt in the rain.