They have models that perform a lot of math to simulate future weather and they slightly modify parameters which aren’t precisely known as well as to account for the chaos that is weather which can be affected by just about anything. Once they run enough simulations to get the probabilities of things happening they can then base the guess on the highest probability event.
7 days out? They get 80% correct so still pretty good. 10 days out? It goes down to about half. Past that? Don’t even bother.
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