best time to visit southwestern china and Tibet (possibly everest base camo)

Polly, the best time to visit Tibet is from June, July and August. It's a bit late for you go there in October. The weather conditions may not be good. For visiting southwest China, you can go there during summer time (June to August).

And you should be aware that foreigners are not allowed to enter Tibet from Feb. 15, 2015. And there is no further notice on when the Tibet tours will be resumed.

We are planning a similar trip in October 2015 travelling from Australia. Sure it's not going to be warm, average temps between 4 and 16 degrees C in Lhasa. Check out this website for detailed information on expected weather patterns in October. The one that excited us was the low chance of cloud to see Everest.

There are still plenty of tours operating in October and November. Have a read of some of the forums on Trip Advisor, ask travellers who have been that time of year, there are still lots of people going then.