Hi Steve, I did this tour in March 2011 so I imagine the weather might be similar to Sept, with Sept probably being a bit warmer. You should expect it to be warm to hot in San Fr, with the possibility of mist and rain (SF is notorious for unpredictable and often bad weather), but elsewhere very warm to very hot in Palm Springs and Las Vegas. It was mid 30s in Vegas in March, so could be hotter still in Sept! So I'd pack t-shirts and shorts with a couple of long sleeves/trousers for the evenings.
Hope this helps!
Bob
