User Zethos_VII asks:

Would be nice to have rain and thunder speeding up the water erosion, like 1,5x and 2,0x respectively.

I'm not sold on thunder doing anything, but I like the 2x speed change due to rain.

https://forums.minecraftforge.net/topic/80196-solved-1144-check-if-its-raining-on-a-block/

world.getBiome(position).getPrecipitation() == Biome.RainType.RAIN && world.isRaining() && world.canBlockSeeSky(pos)