。
Lightning seems about to strike at any moment.
Literal
Thunder/lightning [subject-が] right-now [adverbial-にも] fall-come-seem-to.
今にも ('right now / at any moment') is a fixed adverbial — the にも ('even at') stacks the temporal に with concessive も. 落ちてくる is the directional auxiliary ~てくる on 落ちる: 'falling toward [the speaker].' そうだ on the masu-stem (落ちてき → 落ちてきそう) is the visual inference 'looks like.'