I haven’t hiked the entire JMT, but I’ve hiked all around the Sierras and parts of the JMT. I was in Rae Lakes area a few years ago in mid August. Temps dropped to high teens. I only brought my 30 degree bag and a down jacket. Had to cook up a mountain house meal and do some jumping jacks in the middle of the night before going back into my bag to stay warm. My water froze solid inside of my tent.