Im from CA..Central valley area(Fresno-Bakersfield) and Sept months can be quite hot(90's to mid 100's) It all depends the area your going to be visiting. La is always nice, Hottest it gets there is like in the 90's, on a really hot day.
The weather this time of year is usually the same for the most part for 3-4 months straight... Sunny. The temperature changes from week to week though and August-September is usually the hottest months. It will probably be around 90-95 degrees, but could be hotter.