FYI, between May and August, the interior desert southwest (i.e., not near the California coast, from about Las Vegas southward) is going to be 85-95F in the mountain areas like Flagstaff, and 95-115F in the low lying areas like Vegas and Phoenix. I opted not to go to my own high school commencement in the Phoenix Metro area, because they were holding it outside in mid-May and it was already 100F during daylight hours. It does not cool down very much at night, and the locals’ idea of uncomfortably humid starts at 15-20%. ALWAYS have water with you, especially if you’re not used to this, and especially especially at altitude. And take engine overtemp warnings VERY seriously.
Also, if you’re planning on doing a lot of camping down near the southern end of things, be prepared to make alternate plans in case the park is on fire. I am not kidding. I worked conferencing at Northern Arizona University one summer, and we had a bunch of European students absolutely freak out in July because there was a huge dark cloud of smoke, visible for miles, somewhere to the west of the campus, and the entire town smelled like pine smoke. Those of us who lived there ignored it, because it happened every year.