"Storm" sign proves the ending isn't a hallucination/dream.
I don't think the ending can be a dream that Curtis is having. The reality of it is laid out pretty clearly, with details that don't need to be there unless you're meant to figure it out. To wit:
1. Early in the film, we see Sam teaching little Hannah the sign for "storm."
2. It's also made clear early on that Curtis is far behind Sam and Hannah in his acquisition of sign language. His wife even chides him for it. He has rudimentary sign language, but a concept like "storm" (not "rain," but "storm") is relatively advanced in any language. He wouldn't struggle with basic signs and yet know that one.
3. In the final scene, Hannah (looking out to sea over her father's shoulder) signs "storm." Curtis reacts with confusion; he's not sure what she's trying to convey. Then he looks behind him and sees what she is seeing. If he didn't know the sign for "storm," how could he dream that his daughter was signing "storm" to him and then see the storm?
I suppose in theory Sam or Hannah could have taught Curtis the sign for "storm" in an unseen event during the course of the movie. But I don't see why the scriptwriter would have left this trail of breadcrumbs, so to speak, if not for us to trace them back and realize that the final scene is really happening - some sort of apocalypse the family will have to face together. (To those who are saying it's ironic because the event is a tsunami and so they would have been safe in Ohio - there's much more than just the tsunami. There are multiple cyclones, a weird cloud, and that thick yellow "motor oil" rain. Nothing ordinary and Earthly would cause that. Whatever's going down, it's much bigger than just a tidal wave that would affect only the coast.)