Maybe the simplest answer to your question is that weather patterns are chaotic.

The concept of chaos is not difficult to understand. A system is chaotic if a very tiny change in its state can cause a major change to its state further down the road. There are a bunch of videos on Youtube of double pendulums - this is one of the simplest chaotic systems.

So, weather is difficult to predict. Weather forecasters can generally do a pretty good job of predicting the weather 3 days away, but more than that, and it gets a bit dicey. Having said that, there are macro patterns like El Nino/La Nina which can affect the weather for a long period of time.