A swell model works by representing the whole globe as a grid of squares. It’d be impossible to ever calculate how every drop of water in the ocean will move and react and the whole purpose of a ‘model’ is to create a simplified view of things, the grid is part of that. Typically a swell model will split the globe up into squares between about 30-60 miles wide. This has a huge benefit, we can now calculate what every swell on every ocean will do with just over 80,000 squares in our grid – manageable for a modern computer. It’s worth understanding some of the implications of this for your forecast. Firstly if the model is dividing the world up into 60×60 mile squares it’s safe to assume that two breaks 5 miles apart are going to have the same forecast. This isn’t a big drawback – open ocean swells will have travelled hundreds of miles to reach the coast, while there might be significant differences between the two beaches there should be little difference in the swell they can expect to arrive – but it does highlight the importance of relying on your understanding of your local beach and the specific conditions it requires to work. So, for example, the model will allow for the obstruction of major offshore islands, but might miss anything smaller than the grid square size. It will calculate for swell refracting as it enters shallower continental shelf but not as it moves the last few miles onto the beach.
With a good understanding of the processes that occur local to your beach the swell model is an extremely accurate tool for forecasting the incoming swell, however it ISN’T a precise indication of exactly what will happen to the waves on the beach.