Not guessing, just sharing information. Last year, we had reservations in Tuolumne from July 4-9 and we received the following email from recreation.gov a few days before our stay: "This notice is to inform campers with reservations for the Yosemite Tuolumne Meadows Campground to expect early spring-time conditions for the first few weeks of July. Due to late season storms and cool temperatures, you should anticipate snow on the ground in some areas, excess water, and many mosquitoes in the campground.” We found availability in Hodgdon Meadows for a few nights and then a few more in the valley, so we canceled. Since we tent camp, I did not have a huge desire to camp in the above mentioned conditions (I have been eaten alive there before and didn't want a repeat).
This year they didn't start taking reservations until July 15 (unlike last year). So with the larger snow levels and recent storms, who knows. Last year I believe it partially opened July 1st. I am very much looking forward to my short visit there July 18-21 before I hit the valley though!
