Well, there is two way (AFAIK), 1st, you can load the mod in the editor, then load the freeplay map. Press F9 (or is it Shitf + F9 can't recall) in that menu you should be able to toggle snow on/off. 2nd way is to edit the map texture, adding a white layer ontop of streets etc, although this requires you to have the map texture and is quite more complicated than the 1st way.