"I would not expect the night time temps to drop below 50 in mid-Aug."
I also advise you prepare for possible temps in the upper 20s along the JMT. While I have hiked the JMT only in late July-early Aug., I have spent many weeks in the high Sierra in July/Aug., and on at least 70% of those trips temps dropped to the high 20s-low 30s. At the end of last July, SOBO JMT, I left the trail in Mammoth for warmer capeline layers due to freezing temps (literally) in upper Lyell Canyon and subsequent worries about hitting equally/colder temps at higher elevation further south.
I do not doubt Kevin's research, and his advice about prudent overnight site planning is sound. But sometimes it's unexpected and just not feasible to make significant adjustments to your schedule/day's destination. Also, what's 27* at 11k' is still going to be pretty darn cold at 9k' or within a few hour's hike. I guess for some it's worth the gamble... of course the "what ifs" are endless and you can't plan for them all - so the statistics weigh heavily. And yes, there are weeks that are warm and toasty all night long, but I absolutely *would* be prepared for hours of cold weather night/day.
I forgo rain pants and just bring long underwear. Sleep in an EE Revelation X 20* (w/30% ovrfl). Bringing an xtherm for this year's JMT. The tent may go up in a deluge.
I have to add that I ended up administering CPR for 2 hrs. in a cold wind/rain storm just below Piute Pass following a number of consecutive blue bird days in Aug. a few years back. Since then, it's really hard for me not to chime in when I see something that sounds potentially unsafe and implies something that is plain wrong (i.e. not to expect night temps to drop below 50*) - any time of year. Shock, injury, illness, etc. ain't pretty and sometimes help is not as close as one would think - even on the JMT.
And I agree, 2L water capacity is also a must. For the same reasons Kevin B. outlined.
I'll be going south again this summer... off the trail by Aug. 8, so I'll probably miss you! Have fun!