Question

Take a Genesect with Download and a Gyarados with Intimidate for example. How does the game determine which ability happens first? Or is it calculated randomly?

Answer 1

It is the faster Pokemon has its ability activate first if I recall correctly. This is why some people prefer to have slower weather starters so as to make it so if they switch in at the same time as another weather starter their weather is kept.

Source Looking for more.

Comments

Yes that is correct. Slow weather pokemon override a faster one`s weather with their own. I just tried tyranitar and abomasnow. First abomasnow snow warning takes effect. Then tyranitar`s sand stream overrode it. SOURCE:EXPERIENCE

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Speed freak described it well enough. You don't need to go and repeat what he said. Thanks Speed freak, btw.