If Mega Meganium uses weather ball (fire) in the rain will it be weakened or will it be full power due to its ability?

Mega Sol causes all moves used by the Pokémon to behave as if it were under the effects of harsh sunlight. So if its raining would Weather Ball (fire) be at full strength or would rain still weaken it? Thing of using Mega Meganium in my rain team for Pokemon Champions.