Tibet is huge (greater Tibet covers almost 30% of the total area of China) so the weather is going to vary depending on where you go. In Zhangmu (ele. 2300m) it is going to be a lot warmer than Mado (ele. 4500m) which is 1350km north. The southern region of Tibet (Lhasa-Shigatse) is the warmest region of Tibet, while northern Kham, Amdo and Ngari are the coldest regions.
Most tourists to Tibet go only to the Friendship Highway area (Lhasa to the Nepal border). This region sees very little snowfall and the temperatures are milder than most of Tibet. It will still be cold, but not nearly as cold as some regions. It is rare for buses to not run or for 4WD's to not be able to drive due to snow in southern Tibet. Other areas of Tibet see snow often. The road to Golmud, Namtso and Jyekundo are often closed due to heavy snow.
Check out my website for more information of Tibet weather. I have an article that gives temperature ranges for winter and summer.
Life in Tibet