。
It looks like it'll rain before night falls.
Literal
Night [to-に] become before rain [to-に] become-[looks-like-そうだ].
Two uses of ~になる: 夜になる ('become night = night falls') and 雨になる ('become rain = start raining'). ~そうだ here is the appearance-based inference ('looks like, seems like'), distinct from the hearsay sense (which attaches differently to plain forms).