。
It looks like it's going to storm.
Literal
Storm [into-に] become-appears-to-is.
~そうだ attached to a verb masu-stem (なり → なりそう) expresses visual inference — 'looks like X will happen / seems about to X.' Critically distinct from ~そうだ attached to a dictionary form, which means reported hearsay ('I heard that X'). The difference hinges entirely on whether そう attaches to the stem or the full verb.