JMT Sleeping Bag

A 30 degree bag will be just about right, unless you're a cold sleeper. I have used anywhere from a 20 degree Marmot Hydrogen to a 35 degree WM Highlight. The Highlight was too cold, and the Marmot was a little warm. The 'sweet' spot seemed to be last year with a WM Summerlight 32 degree bag.

During those months you can probably be OK with a 30 F bag. Here is why I say probably.

A few years ago I took my brother on a backpacking trip in August to climb (me) Middle Palisade. We camped at 10,000 ft. The low must have only hit in the 60's F and I was hot in my BA Yampa (40 F). The next week Dave and I went to climb White and left the day before to let Dave acclimate. I told him that we were going to be right across from where Craig and I had been and to plan hot. Ha! We got there to see a storm blow in that dropped the temps to 22 F! We had to drive back down to get to the nearest store to buy warm gear. We stayed at only 8000 ft that night and froze.

If you are a side sleeper consider getting a 20 F quilt that can take you through a larger temp window and is more comfortable to boot.

Do you use a tent, an ultra light tarp, or something in between ?

a 30* bag with a tent should cover 95% of all situations.
I'm not a tarp person so can't speak to that.

I have a SMD Lunar Solo that I take on the majority of my trips, and would take on the JMT.

Also, 22 degree low's wouldn't worry me too much, especially for only a night or two. I've stretched bags lower than that before. I guess another main question is, I'm not going to see much lower than that on the JMT that time of year, and that would be rare even, correct?

Yep, 32* MH Phantom was perfect for our JMT hike back in August. I used a 15* Phantom in 2008 but it was in September and I was glad I brought the warmer bag.

I've never hiked the JMT but if it were me…. I'd take a 30deg (EN-tested for men) bag. You can extend the temp range downward with better ground insulation or clothing. You can unzip or just sleep under it warmer. I used a 30deg. bag for the southern California section of the PCT in mid-summer (July) and that worked for me. At the higher altitudes of the JMT and being a little further north I'd say it is going to average a good 10-20deg. cooler.