Pokemon Weather Mayhem ROM

Pokemon Weather Mayhem is a GBA ROM Hack by CannonsCreative based on Pokemon Emerald. It was last updated on March 31, 2025.

Description

Features

• Modifications to all weathers to make them more “dangerous”, plus 12 new weathers.
• All other weather mechanics set in the game already have been left unchanged (I.E. Swift Swim still works in Monsoon/Rain, Blizzard is a sure hit in Hailstorm, Sandstorm boosting special defense of rock-types, etc.)
• A new weather is set at the beginning of a battle and at the end of each turn.
• An expanded Hoenn Regional dex featuring Pokemon up to gen IX
• Mechanics and features from pokeemerald-expansion, notable ones listed here
• Updated battle mechanics to Gen IX, including ability, move, type, and stat updates.
• Pokemon have movesets updated to the most recent game featuring them.
• Multiple debug menus available for cheating in items, altering stats mid battle, and a battle sprite viewer for looking at the new Castform Forms.

Weather Details

• Modifications to all weathers to make them more “dangerous”, plus 11 new weathers.
  • Harsh Sun:
    • Fire-Type move damage is boosted 2.5x
    • Water-Type and Ice-Type moves have no effect
    • Non Fire-Type Pokemon take 1/16 damage at end of turn
  • Monsoon (Rain):
    • Water-Type move damage is boosted 2.5x
    • Fire-Type moves have no effect
    • Electric-Type Pokemon speed is boosted 2x
    • Flying-Type Pokemon accuracy is boosted 2x
  • Sandstorm:
    • Non Rock-Type, Ground-Type, and Steel-Type move accuracy is cut 50%
    • Non Rock-Type, Ground-Type, and Steel-Type Pokemon take 1/16 damage at end of turn
  • Hailstorm:
    • Non Ice-Type Pokemon take 1/4 damage at end of turn
  • Lightning Storm:
    • Non Electric-Type Pokemon are Paralyzed at end of turn
  • Earthquake:
    • Non Ground-Type and Flying-Type Pokemon take 1/8 damage at end of turn
  • Ether Storm:
    • Psychic-Type move damage is boosted 2.5x
    • Fighting-Type and Poison-Type moves have no effect
    • Normal-Type move damage is cut 50%
  • Pollen Storm:
    • Grass-Type move damage is boosted 2.5x
    • Bug-Type Pokemon speed is cut 50%
    • Non Grass-Type Pokemon take 1/16 damage at end of turn
  • Tornado:
    • Flying-Type moves have double accuracy
    • Non Flying-Type Pokemon accuracy is cut 50%
  • Regular Day
    • Normal-Type moves damage is boosted 10x
  • Humid:
    • Bug-Type move damage is boosted 2.5x
    • Non Bug-Type Pokemon take 1/16 damage at end of turn
  • Fog:
    • Psychic-Type move damage is cut 50%
    • Ghost-Type Pokemon speed is boosted 2x
    • Non Ghost-Type Pokemon accuracy is cut 50%
  • Lunar Eclipse:
    • Dark-Type move damage is boosted 2.5x
  • Acid Rain:
    • Grass-Type and Steel-Type Pokemon take 2x more damage
    • Non Poison-Type Pokemon are Poisoned at end of turn
  • Rainbow:
    • Fairy-Type move damage is boosted 2.5x
    • Dark-Type and Dragon-Type moves have no effect
  • Solar Flare:
    • Dragon-Type move damage is boosted 2.5x
    • Normal-Type move damage is cut 50%
• All other weather mechanics set in the game already have been left unchanged (I.E. Swift Swim still works in Monsoon/Rain, Blizzard is a sure hit in Hailstorm, Sandstorm boosting special defense of rock-types, etc

Known Bugs

• Castform Specific
  • Castform changes to the next weather form before the weather states to be changing. (Has no mechanical implication)
  • Castform does not change to new form on switch-in if it’s a new weather added
  • New Castform forms all use the normal castform as Box/Party sprite
  • New Castform form shinies are not properly implemented
• Weather Ball in new weathers do not have a graphical difference. They call the corresponding type for the weather, so it mechanically works as intended
• Enemy AI does not take new weather effects into action selection calculations. (This will probably stay unchanged because it’s really funny)

Useful Links

Discord

Documentation

Github