I have lived in Km for the past 3 years... so I should be able to give you some insights....

Weather - pretty moderate year round. Can get a bit chilly in winter if the sun is not shining. Sometimes there are several days in a row where there is no sun and the temperature will get down around 40 F or maybe high 30's F. When this happens it can feel a bit chilly the majority of the time in the winter is pretty nice. We did have snow one morning last year. Summer is the rainy season, usually starting in June or July. This means rain almost everyday, but not raining all day. Some days will rain just for an hour or two then stop. There will be some days where it is a total washout. The temperature is cooler in the summer due to the rain. The hottest month is usually late April-May, before rain starts. Never gets unbearably hot/humid.

As for expat community, there actually are more expats there than I thought there would be, I have been told there are a few thousand but can't say for sure. We are able to get most everything we need, there are several shops selling imported groceries etc. There is an area of town with many western style cafe/bars some better than others.

Here is a link to the best expat website on Kunming that I am aware of -
http://gokunming.com/en/

People are very friendly to Westerners, can't speak for foreigners from other areas.