Act 1 scene 3

The witches predicted that Macbeth would be king and he will be thane of cawdor.

The witches predicted for banquo that his sons will be kings and he will be lesser than Macbeth but greater, and he will be not so happy and yet happier.

When Macbeth learns that the witches predictions came true he believes that all of their predictions will come true. And the role of the weather sets the scene and foreshadows and unnatural event.