Hi Peter,
Greeting from Peggy, Top China Travel.
An information about overland trip from Chengdu to Lhasa:
Not recommended to have the trip during the time becuase of the weather.
Generally, the route is open. While in rainy or snow days, it wll be blocked. The blocking news may be timely news without infor in advance.
And also, rainy days may meet landslide which is dangerous.
Feel free to contact with us in any moment by mail if you have any question on China tour service.
Have a nice day!
Best Regards
Peggy