Why nobody knows the weather 30 days out.
In 1961 Edward Lorenz typed 0.506 instead of 0.506127 into his weather simulation. Shortly afterwards the weather looked completely different. Some systems amplify tiny differences exponentially: they obey fixed rules yet stay practically unpredictable, because no instrument measures with infinite precision. That is why every weather forecast fades out after roughly two weeks.
Useful: For any forecast ask: does this system amplify errors or damp them? Only in the second case is long-range planning worth it.
Source: en.wikipedia.org