I am happy to hear that you are going.
Allow me to share just some experience, can?
My first trip was taken on 16th Sep 2007. 14 days.
Flew in from Kathmandu, Nepal right into Lhasa. Weather
Then Lhasa - Gyantse - Shigatse - Tinggri - Qomolangma (Everest) Base camp - Zhangmu (Nepal border) - Kathmandu.
All by Toyota Land Cruiser 4WD.
Two of us only + Guide + Driver.
Weather 0 degree early hours to 20 degree max day time. Tourist Crowd
Not rainy season for major parts. But Qomolangma Base camp was snowing, stormy. Couldn't see Mt Everest at all - misty.
Our very experienced Guide saw the clouds & told us we must leave one morning after two days at Base Camp cos storm coming. We took his advice & left. Another German group of 10+ tourists stayed on and were stranded there for another 3 more days when the storm came.
But because just autumn, it all can be cloudy & misty. I also missed quite a few far distance peaks and general landscape views.
Full of people, especially in Lhasa. In the major monasteries, we were rubbing shoulders with local Tibetan devotees and Chinese tourists.
My second trip on 20th Nov 2009 - 10 days
Flew into Chengdu. Flew to Xining next day. Then Qingzang Railway to Lhasa (23 hours).Weather
Then Lhasa - Gyantse - Shigatse. Did not do Qomolangma (Everest).
Toyota 12-seater minibus (solid & stable!).
My two old parents, both myself & SereneXMM and our three teenage children + Guide + Driver.
Weather minus 11 degree early hours to 10 degree max day time.Tourist Crowd
No snow. No rain. Just very cold. And very nice.
It's winter already. The sky is deep blue. Hardly any clouds. But most fields were bare. Not much green left. Only yellow.
More importantly, the air is crisp and crystal clear.
No mist whatsoever. Best time for photography.
USM, just to share with you, this time round I saw every single mountain and snow-capped peak that I missed two years ago, as we drove along the long roads. It was magical!
All the major cities are almost devoid of tourists as most are too frightened to go during winter. We virtually had every monastery to ourselves & took own sweet time.
Whole Potala Palace ours, with the occasional Tibetan families on prilgrimage.
I will go again in winter the next time round. Enjoyed too much already.
Unless I want to see the Shoton festival, then will have no choice but to go during August.