I guess I’m assuming.. Bare bones, everything out there has a scientific reason for acting the way it does. Wind, humidity, temperature, yadda yadda. What randomness is preventing us from entering all the values into a computer program, hitting fast foward, and seeing close to exactly how things will be far down the road?
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Chaos.
Weather is the classic example of something that’s VERY sensitive to both the fine details of initial conditions, and the small reinforcing changes that occur with time. It’s not just a hard mathematical problem, it’s one that puts limits on what we can precisely know.
As supercomputers have evolved along with much greater and higher resolution monitoring of the global weather system, it’s become possible to predict weather with startling accuracy up to about 3 days. From there you can predict with *decent* accuracy out to about 7 days, and with rapidly decreasing accuracy out to around 10 days. From there you really can’t make fine predictions at all, such as “it will rain in a given region at a specific time.”
If you go much further out, say the overall trends of weather for months in large regions, you can do that. You can sometimes predict that a given year will be hotter, or drier, or colder or more filled with hurricanes, but you can’t tell if it will rain in Kent a month from now.
All of this weather prediction is really just solving equations with lots of variables (partial differential equations) and those equations become harder and harder to solve in the case of chaotic systems. We’ve gotten to the point where you can only make marginal, decreasing gains even with newer and better supercomputers. The problem isn’t our model, the problem is that weather is so chaotic, small forces can be amplified over time and lead to radical changes in outcomes.
tl;dr All models have errors, but in the case of modeling chaotic systems those errors rapidly accumulate and inflate, rendering the model worthless. In the case of weather these effects on accuracy occur as described above.
If you want some less ELI5 reading: https://www.ecmwf.int/en/elibrary/79859-chaos-and-weather-prediction
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