Lots of folks posting here are planning trips this year from September onwards (as am I). There are planned road closures in Yellowstone at that time.
Specifically, the road between Old Faithful and West Thumb will be closed from 2 September onwards.
And the road between Mammoth and Norris will be closed from 15 September to 29 September (as well as overnight closures at other times throughout 2014).
More information here: