Well its to easy please download these programs first (If you already have the program, do not download):
http://hce.halomaps.org/index.cfm?fid=1542 = HEK+
http://hce.halomaps.org/index.cfm?fid=1358 = weather tags of Kornman00
Well here we go.
In your modeling part you need to add portals, portals are plane to divide the map and add effects on the BSP. (if you don't know create a portal, here is a tutorial:
http://hce.halomaps.org/hek/. On the Level Creation Part 2)
After that compile it with Tool (Aduh!). Now open Sapien or A Hobo (of Kornman00), close all windows except : Hierarchy view, Game view , Properties Palette view and Tool window.
Now go to:
Mission > Cluster properties > Palettes > Fog palettes. Create a new palette clicking the button "New Instance". Rename it if you want, now in "Particle system", browse the next tag:
tags > levels > c40 and select the tag "c40_snow".
Go back in "Cluster properties" and select "Weather". Now pass through the portals and change "NONE" to the named Weather properties. If you dot not understand send me a PM.
See ya