Ah well..... I'm going to sound enormously controversial by saying this but.............. the climate change
Thing is no matter how powerful the computer or how complex the programme the result is dependent on so many variables both at the outset and all the way through the procees that when the models are run they never, ever end up with exactly the same result... they might on occasion be close (although the majority are often so very far apart they defy all logic) but never exactly the same.
Also trying to predict the way crowds behave in an emergency is a difficult one to second guess. But generally anything to do with the weather is really difficult to foretell.