I did an August trip and I doubt you will need a bivy at anytime, I used either a sheet or a light summer bag because it it still hot at night. A lot of great advice on how to sleep comfortably in the Grand threads on here. We had temps over 100 for about 4 or 5 days, 3 days in the 80s when it rained, and the rest of the time it was in the 90s. It was hot even later and I never wore a fleece or sweatshirt.
Have a great time and if you have space open up I live here in Eugene, just throwing it out there.