I don't know if the director intended the ending to be ambiguous, but many of the shots and filming techniques suggest the storm is real. There are no dreamlike qualities to the final scene. If it were indeed a dream, the shot of the wife in the kitchen is completely pointless considering a dream would only be seen from Curtis' perspective. The daughter is also the first to notice the storm and the camera focuses on the wife's hand as the rain falls, both of which make no sense in a dream.
It's not a dream, though. If you take "the storm" as a metaphor for the schizophrenia he's been holding at bay his whole life-- then throughout the movie he has episodes where it behind overwhelming him and in these scenes "the storm" becomes real. However, no one around him KNOWS about his illness, so they can't see it, all they see is the effects of it.
In the
finalscene, after he sees doctors and the family is educated on the diseases, he's having another episode where the schizophrenia is overwhelming him; except THIS time, the wife can see it because she understands it. The daughter could be pointing out something odd her father is doing; but you can see on the wife's face, she is the one who understands what the "storm" is. That's why they show the oily drop on HER hand. Because she's stepped out of the allegorical cave and can now see his disease, his "storm", for what it is; a real, honest threat to him and not just some abstract, make believe, thing. That's why the last shot used didn't make it seem like a dream; because realism was a metaphor for her understanding. The disease (The storm) was now "as real" to her as it was to him.
Or you could look at it as the world's ending. And yeah, that's valid because because his main intent with the last shot was only to say "they all can see
ITnow"---but it's up to you, the audience member, to decide what "it" is. If you thought he really was a prophet than "it", is something like the Armageddon. If you thought he was experiencing schizophrenia; then "it" is him having an episode, and now the wife can translate the signs of it (Even if it's only the daughter saying "dad did X strange thing" IE "it's raining")