I have had plenty of sub-freezing nights in the high Sierra in August. If you plan on camping high (say 11,000 feet) you need to be prepared for temperaturs into the mid-20's (F). There have been seasons where I got rained on only one night. There have been other seasons where I got rained on several days in a row on several different trips. What I have found is that in the Sierra, you almost always have sunshine after a rain, so you can dry out. Rain storms can be intense and severe, but you will not likely get a week of solid rain and drizzle. I sleep cold and have never regretted having a 10-15 degreeF sleeping bag. However, I do not see a need for tent, other than the luxury of it. I have done many 10+ day trips with my bivy sack.