Eyonix revealed that Weather System is in development, but there is no official release date to implement the feature in any near patch schedule.  When more details are available, it will be announced by Blizzard:

Eyonix: Weather is still being worked on, and you’ll be happy to know that the progress of effects looks phenomenal. Once I have additional details to provide, or am able to show off a preview of such, I will do my best to share with the entire community.