Why do you need to complicate things?

The man has nightmares about a huge storm and thick yellow rain falling from the sky. He wonders if he's crazy or not the entire movie, then both his wife and his daughter see the incoming storm, yellow rain starts falling from the sky and the wife basically admits he was right. Why do you need to complicate this?

It's really NOT an open ending. Here's an open ending: they all see the storm, they look at each other, it starts raining. Not yellow stuff, just plain water. The movie ends. Maybe that's just a regular storm and he's crazy. It's open to interpretation.

Here's another open ending: he's inside, at the window, a few steps away from his wife, who doesn't see what's happening outside. He sees the incoming storm. Is he imagining it? The daughter is outside, playing in the sand, her back against the storm, so she hasn't seen it yet. He looks at his wife in horror, trying to find the words. She looks worried at him and says: "What?". The movie ends. Will they also see the storm, or is he imagining it? Or maybe it's just a regular storm. It's open to the interpretation.

These are open endings. The director could have chosen to go with either one, or something similar, yet he decided to end the movie by giving you all the clues that the guy was right. The most definite one is the thick yellow rain, which implies that something unusual, out of the ordinary is going to happen. What more do you need?