One thing I forgot to mention in the previous comments: your description of the storm is excellent, really building up the tense atmosphere, adding an air of dread. This is a town that should be used to storms, and at no point does it seem like that's not taken into account: they know storms and this is worse than anything they've seen. Libra continues to add to the intrigue by seemingly remembering the storm that chased the Arsenic.
The chapter serves as a perfect set piece between the second and fourth installment, and almost the entire chapter serves to build dread, anticipation. It speaks to elements of the supernatural but also presents physical danger. Tristitia giving in to fear and hiding is both the sensible thing to do and unexpected in a story like this, further establishing her as a person and not a vehicle to drive through the story.
When you describe Libra being untouched by the water, it leaves me wondering if this is a misunderstanding of the naga conduit's protection from lightning, or if this is something entirely new.