In the real world we don't have perfect waves, of equal height and period heading in the same direction at the same time. Standing on the cliffs above the beach you can see this for yourself. The ocean varies from day to day, sometimes a regular consistent line of waves will approach the beach, on other occasions the ocean may appear a confused jumble of waves heading in slightly different directions and of different sizes. The state of the sea is always more complex than groups of identical waves, especially nearer to the storms that create them, and so although we often describe the state of the sea as though we were talking about individual waves ('the waves were 4ft yesterday') what we're actually doing is talking about a general summary of the state of the sea. This is hugely important to understanding your surf forecast, it's just about the single biggest factor creating misinterpretations of the model data available on MSW and elsewhere.
Think about the last time you had a surf and the simple question 'how big was it?'. Firstly as you sat waiting for waves you probably discarded most of the smallest ones that came through. As surfers we're waiting for the sets, the groups of bigger waves that make for the best surf. Now imagine you caught ten waves over your session. We can guarantee they won't have been identical. So how do you answer? Do you give your judgement of the height of the biggest wave you caught? Or the smallest? What you probably do, unconsciously, if you're trying to make your answer as useful as possible is give some idea of the average wave you surfed. What you've done is taken a whole big set of data (all the waves you saw) and created a summary that you think is most useful to answer that simple question. This is exactly how every forecast model works.
So if the 'height', 'period' and 'direction' you see on your forecast are an interpretation of the overall state of the sea how do they work specifically?
When we talk about wave height we're almost always talking about Significant Height. Statistically this is the average height of the largest 1/3rd of the waves. This tends to correlate to the size a trained observer gives when asked to describe a swell and is the mathematical approach swell models and wave buoys use to calculate the height – it's also quite likely to be close to the 'average size of the sets' logic you used to answer the question above. For us as surfers it means that, on a typical day, we could reasonable expect to see waves smaller than the given height and for the better waves to be larger. In fact given the average mix of waves in a swell it's reasonable to expect that the significant height is something like the average set wave height, but that the maximum height will be about one and a half times bigger.
When we talk about period we're normally talking about peak period. This means that, if you can imagine the sea at a location having waves of different periods, the period of the waves that have the most energy. Put more simply still it means something like the period of waves in the swell in which we can expect the biggest waves to arrive. It's really important to understand the strengths and weaknesses of this approach and it's something we'll come back to time and time again when we dig deeper into surf forecasting. A swell could arrive with a peak period of 14 seconds, it means you could reasonably expect the larger waves to be 14 seconds apart, this would generally make them relatively powerful and good for producing surf. However this number tells us nothing about the mix of waves in the swell, they might all be 14 seconds apart, of they might vary from 10 all the way to 16 seconds apart. The overall character of the swell can vary, even when this statistically derived number stays the same.
Dominant Direction tells us the direction in which most waves of the swell are heading. Again it tells us nothing that will allow us to determine whether all waves are heading the same way, or if they are heading in a range of different directions.
We'll keep coming back to these interpretations and explaining the issues they can create. Even if this hasn't made perfect sense the most important bit to start to understand is when we're talking about the state of the sea we're generally using the language of statistics – these allow us to simplify complex information, but in this simplification some information can be lost or hidden. Imagine the following:
'How big was the surf today?'
'There were lots of waves between 1 and 2 feet high that I left, I caught several waves between 3-5ft and I saw at least one 6ft wave while I was in the water.'
'It was 4ft.'
The second answer is the one you'll generally be getting from your computer forecast model. You can see how much information is lost in this simplification. It doesn't mean it's not useful, but with a better understanding of how swell models work you can start to fill in some of the blanks yourself.