Here's my take on Sierra weather/conditions for a typical summer/fall (the hiking months):
June - buggy as all get out, warm days(70's), cool nights(30's), a few afternoon thundershowers, slight chance of snow showers early in the month, usually plenty of snow patches still on the ground if you go high, swimming pretty arctic, creeks running high making for sometimes tricky/dangerous fording, trails often pretty wet from melting snow.
July - still moderately buggy, wildflowers going strong, warm to hot days, cool nights, regular afternoon thundershowers, very slight chance of snow, creeks easier to cross, swimming still very brisk.
August - bugs thinning out or gone, wildflowers tapering off, warm to hot days, cool nights, some afternoon thundershowers, snow patches disappearing, creeks easy to ford, swimming at its best, especially last half of the month.
September - bugs gone, most of the people gone, wildflowers gone (usually, but not in a wet year), warm to hot days , cooler nights, usually dry but chance of rain, chance of snow, swimming still good the first half of the month. Second half of September gets into the anything can happen weather period - fabulous, or 40 degree rain, or two feet of snow overnight.
October - more of the anything can happen thing, with higher chance of big weather and generally colder, and you almost have your own private mountain range, since very few folks are out.
To me the last half of August and the first half of September are the best time to backpack in the Sierra. Bugs are pretty much gone, the weather is at it's best, the lakes are as warm as they will get.