If possible, it is suggested that you take flights to and from Lhasa. The train tickets between Chengdu and Lhasa are needed urgently throughout a year. And the scenery is amazing along the railway, no matter which direction you choose, I'm sure you'll see the splendid view on the plateau. You'd better ask an agent to book a train ticket for you.
For the weather, it would be cold in the morning and evening but it would be warm in day time, anyway, warm clothes are necessary. Good Luck!