Why do weather forecasts sometimes fail to predict accurately?
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One reason is that predictions about what will happen to the system as a whole can be pretty accurate but predictions about what will happen in a given spot on the earth’s surface (which is what we generally want) are harder. Consider a big storm system with a sharp edge: we might say that it’s moving westward at XX km/h so we expect that it will be in this area in 24 hours. But there is a margin of error in exactly what its path will be. If the spot you are in is A, in the middle of the path, it’s almost certain that the storm will hit you. But if the spot is B, just on the edge of the area it will hit, a 1-degree difference in the path will make the difference between a sunny or rainy day. When meteorologists say that for A there’s a 90% chance of rain, for B there’s a 50% chance of rain, it’s a simplified expression of these different levels of confidence. But they might be very confident that the storm will move in a certain way, just not know exactly where its edges will be. Of course that confidence decreases sharply over time.
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