What good is a prediction if it's subject to change during the time period it is supposed to be useful? A prediction that keeps changing by large amounts before a predicted event happens (or doesn't happen) is practically useless for long-term planning and is an indication of either of 2 things:
1. The "predictions" are actually wild guesses or made-up numbers to satisfy those who are expecting a prediction.
2. The phenomenon that is being predicted is the outcome of the complex interactions of many factors that may be subject to random fluctuations and is, therefore, inherently unpredictable. Consequently, the "predictions" are actually wild guesses or made-up numbers to satisfy those who are expecting a prediction.
A typical example of changing predictions was provided yesterday by the National Weather Service. Early in the morning, when it was quite cloudy, they were predicting a 100% chance of thunderstorms for where I live.
Early in the afternoon, it was starting to get sunny and the chance of thunderstorms was now 60%.
The initial "100% Tstms" meant that we were absolutely, definitely going to have thunderstorms: if it had been possible to observe a large number of weather conditions identical to the one we had yesterday morning, on the average, each and everyone of those conditions (100%) would have produced thunderstorms. But when the "prediction" later became "60% Tstms Likely", it became clear that the earlier predicted absolute certainty was baloney, because it had been subject to change. If you say to me "The chances are 100%, but that may change later", then the chances are not 100% and the only thing that is certain is that you don't know what you are talking about.
If a prediction keeps changing, then it's not a prediction. The NWS's lowering of its prediction from 100% to 60% was quite substantial and clearly illustrates the unpredictability of complex events like localized thunderstorms.
General weather patterns are predictable, but are not of much use. For example, predicting that there will be some thunderstorms somewhere in the state of Maryland sometime during the next 24 hours is neither impressive nor useful.
It was important for us to have an accurate weather prediction for yesterday, because we had an outdoor party planned for the late afternoon. After I learned about the 100% chance of thunderstorms, we briefly considered postponing the party, but then decided to go ahead with it anyway.
At the end, we never had thunderstorms; it didn't even rain. We did feel a few drops for a few seconds at one point, but nobody even bothered to get up and go inside. So much for predicting the weather.
The party was good with plenty of food and wine.