Weather in general
Generally speaking we can/will experience all kinds of weather - From hot and humid conditions in the valleys and cold and windy conditions in the mountains.
January-February = winter with some snow and many clear days.
March-May = spring with less clear days, but higher temperatures.
June-September = summer and rainy season. Some rain and pleasantly warm.
October-December = autumn with almost no rain and many clear days.
Chilly at night all year round.
Check this site for a statistic about the Lijiang weather: