Oh, snowy but not too cold, thus I think you can go there in Feb. or Nov. Then average low temperature at that time may be about -4. Wear warm winter downcoat and you may bear the cold.
Generally speaking, good time to visit Lhasa is from March to October when the climate becomes mild and humid, especially May.
The temperature in May is about -2 to 13C. It is also cold. You need to take winter clothes when go there in this month.