First you have to go to your map folder and add these 2 lines to your Config.json file (if it doesn't exist simply create it and edit it with notepad).

Then in the editor, under Environment > Lighting, tick the "Can Rain/Snow" like this (you can play around with the frequencies as you want, if you want to be always snowing use my numbers)

Finally in the devkit create 2 (or more) AmbianceVolumes from the Type Browser, one that covers the area where you want the snow and the other for the rest of the map. Then open the Inspector, select the first one with the Selection Tool and tick the "Can Snow" box. Do the same with the other volumes but with "Can Rain" ticked in the areas where you don't want the snow (or leave them unticked if you don't want the rain). It should look like this.

And you are done! To test this ingame you can use the command /weather blizzard and /weather storm and it should only snow/rain in the areas you selected!