Download and place the Weather file in your StarCraft II / Mods folder
Open your map.
Click File and Dependencies...
Click Add Other...
Scroll down to Weather.SC2Mod and click OK.
How to add weather:
Open Trigger Editor.
Create a new trigger with the event of whatever you wish (Periodic Event is probably best).
The action is easily done by creating a + Weather.
Tips
The snow setting is actually rain; I don't know why but it is. Best way to make snow on the other hand is to place Weather Char - Ash and set their custom color to white.
I am making a third person shooter and I use this in my weather trigger which creates a new weather every 10 seconds around each player unit that lasts 10 seconds.
Download: Weather - Beta 3.1 made by DarlD.
Installation:
How to add weather:
Tips