I have personally never seen the temp below ~30F on the JMT July/Aug. Not saying it can't happen, but I think it's unlikely. It also has a tendency to warm up fairly quickly. To get a sense of likely daily temps, you might want to look at the sensor data. I often use tioga pass as my stand-on for the high sierras, and then subtract 10f to make up for elevation differences.

so what to do about clothing? Depends on you camp life plan. I have some friends who are either hiking or they are asleep. A number of these folks have done the JMT with zipoff pants, a tech-tee, 100wt fleece, and rain shell. That's it. When it's cold they are either under their quilt, or hiking hard (early morning) where activity level removes the need for much insulation. On the few days they want to be up and inactive when it's the coolest, they use their quilt / bag as a cape, wrapping it around the body.

That time of year I bring a down vest (you should be able to find something decent in an end-of-season clearance someplace like landsend or Eddie Bauer, sierra trading post, etc for less than $40, sometimes as little as $25) because I don't always start so quickly in the morning and might want to watch the stars for a bit. Honestly though, the vest is used more consistently as a pillow than an insulation item. part of this is because my metabolism seems to shift when I am going 15+ mile days. So long as I have enough food I run pretty hot at the end of the day. Around town I am pulling a light jacket on when it's in the low 60F. After a day of hiking (even after I stop), I often find myself setting up camp in my hiking clothing when it';s in the upper 40F.

If you are going to be sitting around, especially if you have a leisurely start in the morning outside your sleeping bag, then you would want something that would keep you comfortable down 30F.

--mark