Hello Chris
I went to China with the Yangtze Add On in September. Our Tour Manager told us that we had come at the best time for weather. We had rain one day, more like drizzle really ,and it didn't last for long. The temperature was in the high 30's most of the time although it did creep into the 40's once or twice. The months to avoid for rain are from mid April to mid August. I went on the 12th September trip.
However, the weather has a mind of its own wherever you go.
You will have a great time whenever you decide to go.
Cheers
Peter