Question

Why are the different weather forms of castform illegal on Pokemon showdown?

Comments

What format are you playing? Is it Gen 8 OU, or National Dex, etc?

Answer 1

One possibility is that you're playing it in wrong format. Castform isn't in non Nat Dex formats of current Gen 8 so it won't allow.

Or in case you're playing Nat Dex or a old gen format, you need to setup weather manually and enter normal Castform to change its forme. Basically it can't start in its weather formes from beginning, just like Zygarde Complete.

Source: Searching around the Teambuilder

Hope this helps!

Comments

Yeah thanks. I'm using Nat Dex AG. I thought that using this format you could basically use any pokemon from the start. Is there a format that you can use any Pokemon you want?Also is there a way to change the weather before a battle or do you have to have a pokemon change it during a battle?

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You can't change the weather before a battle. Castform has to use Sunny Day / Rain Dance / Hail to activate the respective weathers.

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Nat Dex AG still follows the rule of in-game obtainable mechanic so you couldn't use anything that's not legal in-game. You can try out Custom Game (challenge-only), Gen 7 Balanced Hackmons or Gen 6 Pure Hackmons which don't have this rule so you can start with any Castform weather forme or hacked stuff.

As for the weather, it's just like what TastyMushroom said, you have to use the respective weather-causing moves or a Pokemon with Drought / Drizzle / Snow Warning ability respectively.   .

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Also, if you're trying to win, stop using Castform in NDAG.