You: Why is the weatherman always wrong!
Me, an intellectual: It's because weather is governed by non-linear differential equations, which are inherently chaotic.
Me, carrying on as you roll your eyes: That means that the evolution of this system is prone to change greatly with even minor variations of the initial conditions.
Me, following you out of the shop: This is best exemplified by the infamous "butterfly effect," whereby the wind variation caused by a butterfly flapping its wings in the amazon can supposedly lead to hurricanes across the ocean.
Me, following you down the street: Thus, while they can collect very detailed data with satellites, weather balloons, on-the-ground measurements, and the like, there's always a certain degree of uncertainty in any attempt to model global weather patterns.
Me, shouting at you after you've broken into a run: REALLY, UNDER THE CIRCUMSTANCES, I WOULD SAY THAT METEOROLOGISTS ACTUALLY HAVE A FAIRLY IMPRESSIVE TRACK RECORD!
