You don't get how these predictions work. Short-term, very exact weather-pattern predictions are hard to get right because tiny mistakes and inaccuracies evolve into larger and larger deviations from the actual patterns. This does not mean we don't understand the weather, because we have a very good grasp of how the weather works and interacts. However, those exact weather-patterns are irrelevant for the long-term global warming everyone is talking about. Because those aren't dependent on what happens exactly in weather patterns, they're dependent on large-scale tendencies. You only need to know general cause-and-effect rules to predict long-term tendencies.