Fluffy is correct (of course he is),
Create a brush with the trigger texture somewhere in your map. Press Ctrl + T to open up the properties editor and select func_precipitation. There should also be an option to change the precipitation type.
NOTE: If you want the snow to look like snow change the precipitation type to 'Snowfall', not 'Snow'. 'Snow' looks like rain and 'Snowfall' looks like snow.
Also, here is a video if you don't get what i said: YouTube
.Fluffy is correct (of course he is)