The code for autumnal woods biomes does not explicitly define a precipitation value. But it appears to take a default value of "rain." Low priority bug.

Eh, upon reviewing some other biomes I see there's others that leave it at default. I'm guessing this is intended.

Yeah, precipitation is defined here: TraverseBiomes.BIOME_TEMPLATE

Yep, I eventually found that. Sorry for opening the issue too early =P