The event
It was played on October 17 and 18, with about 900 players across all divisions (about 860 in Masters). Fontvieille finished with a 15-2 record and beat 2022 World Champion Eduardo Cunha in the final.
How the rain works
Pelipper has Drizzle: it sets rain as soon as it comes in. In rain, Water moves deal 50% more damage and Fire moves half.
Basculegion, with Swift Swim, doubles its Speed in rain and hits with boosted Water attacks. Archaludon makes use of Electro Shot, which in rain fires on the same turn instead of charging first and also raises its Special Attack.
Two Megas on one team
Fontvieille brought two Megas, Golisopod and Raichu X, but only one can Mega Evolve in each battle. Bringing two gives flexibility: depending on what the opponent brings, he chooses which one to lead with and Mega Evolve.
Mega Golisopod is one of the six new Megas in Reg M-C. Sneasler, the other Pokémon it shares with the Baltimore champion's team, is a fast physical attacker that punishes Tyranitar, Archaludon and Steel types.
Sun versus rain
Only one weather can be active, and the last one in wins. Against the Charizard Y sun that won Recife and Louisville, bringing Pelipper in afterwards wipes the sun and leaves Venusaur without its doubled Speed. The same works the other way, so these matchups are decided by switches and the order each one comes in.
You can see how rain teams' win rate changes depending on what the opponent brings on the matchups page.