is it possible when it snowst the map changes from a summer map to a winter map?
Not without using two maps.
Actually yes. I saw a script that allows you to change the map like in 2003. It would have to be the same exact map without snow though. I'll get it for you.
http://www.phylomortis.com/resource/script/scr041.html (http://www.phylomortis.com/resource/script/scr041.html)
I Will try this now