rain/snow could be done with particles I think so, but you have to do it your self. not sure what is the most efficient way of doing this. but no it's not built-in yet, and carving tool, vertex editing/ vegetation tool , etc is suppose to come this year. I don't think this is planned for this year, but josh did say depending on time after getting those planned he may be able to do a few other things like this.
Edit: best way of this is a shader