I would expect lows in the thirties, or lower, and highs in the fifties and sixties, but it's hard to say what you'll actually get. It could be warmer, or it could snow.
The best way to get an idea of what is actually going on is to regularly check out the raw data from the Tuolumne Meadows Weather Station. Do it often and you won't be surprised (or frozen) in September...
The forum won't let me post any links yet (even though I technically qualify according to the rules it showed me when I tried to post one) so if you google: tuolumne tmec1 weather data
http://raws.wrh.noaa.gov/cgi-bin/roman/meso_base.cgi
The second link shown is a mesowest utah edu site
http://mesowest.utah.edu/
Try the "change to Graphical" link on the left column, then play with the pull-down menus above the graphs until you get a display that works for you, then bookmark the page and check it often.
TMEC1 is the name of the Tuolumne Meadows Weather Station...