Niantic announced a couple of new updates for Pokémon Go, which includes a weather system and several Pokémon from the Hoenn Region. For those who don’t know, Hoenn is from Pokémon Sapphire and Pokémon Ruby versions.
Starting later this week, Pokémon Go players will be seeing Treecko, Torchic, Mudkip, along with several others from the Sapphire and Ruby games. Niantic also stated there will be “even more to be discovered over the next several weeks.”

Pokémon Go is also updating with a dynamic weather system, which will affect several mechanics in the game. When players discover, catch, and interact with Pokémon, the weather will play a hand in all of it. Niantic urged Trainers to “pay attention to your local weather patterns”, as it will impact the Pokémon.
For example, if you are trying to find the new Mudkip, your chances will be increased if it is raining out. Meanwhile, certain types of Pokémon will be more common to come across during various weather conditions. Also, attacks of certain types of Pokémon will be affected by the weather. Niantic’s example is Charizard’s Fire Spin, which will be more effective during sunny days.
Source: Niantic