Two years ago it snowed twice in August in the Wind Rivers. If you are doing the Highline Trail, you need to be good to 30 degrees, even in August. If you go in September you need to be good to 20 degrees. I have always taken an 10-degree bag in the Wind Rivers and never regretted it (I am a cold sleeper). It is now thunder storm season- hard to say when it ends, but plan on one to three short duration but intense storms a day. Overall, there is about a 1 in 3 chance that you will get precipitated on in the Wind Rivers. That is not to say that you will get one day storm, two days clear- the weather comes in waves. Week long rainy streches are possible. Week long clear streches are possible.

The Sierra has less severe weather, thus is a better place to try out fast packing for the fist time.