I was under the impression that weather models are at least partially chaotic. Is this not so?
In such systems, even if we have a perfect model we cannot know the initial conditions with sufficient accuracy to make reliable long term predictions. This is not just a limit of our current technology, but a property of chaotic systems. If weather is a chaotic system, then we will never make serious progress beyond short term forecasts.
... you can get 'attractors', basins in the dynamics. If either several different models, or slightly tweaked starts with the same model give you the same or similar-enough answers, then you know you've stumbled into a basin by luck. Small errors aren't going to change much. If your outcomes are different though, then you know that the weather from that starting state is very sensitive and unpredictable.
A change in the initial conditions might make a change in the result, but it isn't guaranteed to. At least not any noticeable change in the next few days, weeks, even months. So if you find that, for most of the possible current conditions, the end result is mostly the same, you can have a good probability of being right.
In such systems, even if we have a perfect model we cannot know the initial conditions with sufficient accuracy to make reliable long term predictions. This is not just a limit of our current technology, but a property of chaotic systems. If weather is a chaotic system, then we will never make serious progress beyond short term forecasts.