The difference is knowing the size, speed and composition of a hurtling rock tells you where it's going to go, where as knowing the size, speed, and composition

The difference is knowing the size, speed and composition of a hurtling rock tells you where it's going to go, where as knowing the size, speed, and composition of a hurricane does not tell you where the hurricane will go. Complexity isn't a problem, the issue with weather prediction is that it's chaotic - extremely small changes to starting conditions cause large changes to the end result.