maybe this will help?
http://www.moddb.com/games/half-life...-precipitation
didnt really read it, but its step by step, maybe help you catch something you may have missed
edit: after further skimming
"...They will now become boxes in which the precipitation will fall. It’s important to note that the precipitation will not fall FROM the boxes, it will fall IN the boxes. Select which kind of precipitation you want under the “Precipitation Type” box."