It's also possible to achieve this without the use of angelscript, provided you're making a linear level and don't want the rain to turn on and off again freely as you go back and forth between the two spots.

The "JCS Events" section of the JJ2+ documentation describes (under "Feb 14 2015 Additions") some parameters you can add to the Snow event, which lets you cause rain instead of snow using the "Type" parameter, and which lets you use the Snow event to turn rain off using the "Off" parameter. (Or if you're using MLLE, those parameters are already available for you.) The snow event activates when you get near it, not when you cross over the exact tile it's on.