If it’s raining, fish your level 50 Sliggoo out of your box
At this point, there are a ton of different wacky evolutionary tracks for creatures: and Pokemon Scarlet & Violet has most of the classic ones, while adding some new wrinkles in. Here’s an explanation of how rain works in the new generation, as well as a quick rundown/reminder that you should fish your Sliggoo out of your box at a moment’s notice.
Rain is random in the new generation
Although some weather effects can be forced out through location-based triggers (like snowing while on Glaseado Mountain), rain will randomly occur in most zones in Paldea at intervals tied to the in-game clock (not your Switch itself).
There is one proviso though: try not to hang around the Asado Desert if you want to trigger a rainstorm.
Sliggoo evolves into Goodra at level 50 while it’s raining
Although rain is a fairly innocuous weather effect, there is one thing that you need to keep in mind when it’s happening: if you have a Sliggoo that’s level 50, you can take advantage of the rain. At level 50 while it’s raining, your Sliggoo will evolve into a Goodra.
You can trigger this in battle: though it’s safer to just use an EXP candy or Rare Candy to instantly force it, rather than risk the battle taking too long, and it not actually providing enough XP to push you over the edge. If you still need Goodra’s predecessors, you can find the locations of Goomy and Sliggoo in the gallery above.
As a tip, I like to add Pokemon that can evolve to a separate box (usually the last one), that way I’m always aware of party members that I can swap in and out to complete my Pokedex naturally. When it was raining, I opened the box menu quickly, press the L button to swap to the last box, put Sliggoo in my party, and didn’t miss out.