You can set weather particle systems in sapein and edit them in guerilla. There should already be some of them in original single player maps.

to add snow etc you need portals in your map, after that
run the tool structure command to build the .scenario of your map in tool.
next use sapien or ahobo
in sapien and in the hierarchy view go to, mission\cluster properties\palettes\fog palettes.
create a new instance, give it a name.
now in particle system browse to: tags\levels\c40 and select the tag c40_snow.

then back to cluster properties and select weather.
next pass through the portals and change none to the named weather properties.