Reading Your Forecast – Waves

Heading to the forecast page for your local beach and scrolling down the page for the ‘Long Range Surf Forecast’ section you get the to most important tool set MSW has to offer, we’re going to walk you, step by step, through using this to forecast the surf at your favourite spot.

Swell Rating

The first thing to check is the ‘swell rating’. This isn’t a final and perfect guide to the waves you’ll see on the beach but it does give you a rough guide to the sort of power you can expect in the swell and should at least roughly correlate to your own reading of the swell data. It’s a simple double check for your own interpretations – or a way to spot the swells that might be worth the time to look at more closely. The number of solid stars reflects the power and size of the swell, the greyed out stars indicate that the wind is less than perfect (onshore) and the overall rating is decreased.

Swell Period

The next thing to check is definitely the swell period. While you can expect the height of the swell to vary the height of the waves on the beach the swell period is really your best clue to the kind of waves that might be created. As a very simple overview to give you an idea:

1 – 4 Seconds
Forget about it – this is swell so small and weak in the very early stages of being generated from strong wind that it can almost never be surfed. Sea in this state will look lumpy and bumpy but you’ll struggle to see individual waves. Because this sort of swell is unable to travel far from the storms that create it you can guarantee that there will be strong local winds when you see these kind of waves forecast.

5 – 6 Seconds
Much of what’s written above still holds for swells in this range. But you will start to see the odd weak ridable wave face if you’re very desperate. Again it’ll be almost impossible to find this sort of swell without the strong onshore winds that create it so expect really sub par conditions for surfing.

7-9 Seconds
This would be typical wind swell considered surfable by many surfers, especially those in areas that don’t get great waves. Typically this sort of swell will still be in the path of the strong winds that created it, but it will hang around for a short time if the wind direction changes so offshore conditions are possible. The waves will generally be weak and jumbled up without clear sets. However a good sand bank or bit of reef can generate decent waves and large storm swells at the upper end of this range can produce decent waves at the right spot with the right local winds. The surf produced by these swells will normally be smaller in height than the swell that creates it, losing power as it enters shallow water.

10-12 Seconds
Swells in this range will often be starting to head away from the storms that create them. It’s possible for them to travel in open ocean for some distance. They can often create good quality surf. These swelsl won’t bend or ‘refract’ into hard to reach surf spots and are less likely to barrel when smaller than longer period swells. On average sandy beaches these mid period swells can create some of the best conditions, a reef or point break needing the swell to refract can prefer a longer period of swell. The waves produced by these swells will often be about the height of the swell itself given the right direction on the average beach.

13-15 Seconds
These swells are definitely ‘groundswell’ – normally created some considerable distance from the beach by powerful storms. They most often arrive without the storm that created them, making for a good possibility of calm local conditions. They’re powerful, they will bend or refract around headlands or into more sheltered coves and create, especially when smaller, hollow barrelling waves on even average sand bottomed beaches. These swells will have regular sets and look a lot more ‘lined up’ than lower period swells. Typically this sort of swell is great for creating surf and the waves created will often be bigger than the swell height as the swell refracts to focus it’s energy in shallow water and the wave shape changes.

16+ Seconds
Extremely powerful swells generated by distant storms and often travelling the breadth of the largest oceans to reach the beach. These swells can refract considerably as they approach shallower water, bending into sheltered coves, around significant headlands and focusing their energy on beach to create, with the right local conditions, ridable waves considerably larger than the swell height at the right spots.

Swell Direction

The next thing to check is the swell direction. Not all surf breaks are created equal. A long sandy beach exposed to deep water might be open to almost any ocean swell, a rocky cove might need a swell to be heading in exactly the right direction to start to make surfable waves. The first step, even before you head to a new beach for the very first time, is to get an idea of what sort of swell direction might be best for your beach and the easiest way to do this is with a tool like google maps. Here are a couple of examples:

Moliets – South West France

Moliets Swell Window,-1.390629&spn=0.052981,0.132093&t=h&z=14

The long sandy beaches here run unobstructed for hundreds of miles. The water near the beach is generally deep, there aren’t shallow sandbanks away from the shore at most spots. There aren’t offshore islands. The beach generally faces just north of west. We could reasonably expect swells from the WNW  to make the biggest waves here (the large arrow on the map), but we could expect waves on the beach from swells from NNW all the way to SW. We call this the ‘swell window’ and we’ve shaded it on the map.

Bantham – South West England

Bantham Swell Window,-3.888559&spn=0.023477,0.066047&t=h&z=15

If you zoom right in on the map you’ll see that the beach itself faces SW. Immediately on the left of the beach is an island which blocks swell from the west, but if you zoom further out you’ll see that in fact the whole of the area is blocked from westerly swells from the protruding southern most tip of England. You’ll also see heading south from the beach that the coast blocks any swell coming in from the East. So the best direction is SW and the swell window is WSW through to S, much narrower than for France.

So you’ll have an idea of the perfect swell direction for your local break and a larger swell window that’ll allow for surfable waves. First thing to determine is whether or not the swell is heading towards your beach. The MSW forecast normally excludes swells that are heading in completely the wrong direction, but it doesn’t necessarily understand the finer points of your local spot, if the swell is heading in the perfect direction you can start to estimate the sort of surf you’ll see on the beach by looking at the swell height in conjunction with the notes we’ve made about swell period above.

If the swell is within the broader swell window but perhaps not heading directly towards the beach you can normally expect the waves to be smaller than they’d otherwise be. How much so depends on two things, the precise local shape of the seabed – something you’ll get a feel for by simply correlating your own experiences there with the forecasts and the period of the swell. We make a brief mention of it when talking about period that longer period swell bend or ‘refract’ better when entering shallow water. This can allow a swell to make quite considerable changes in direction and means that longer period swells at less than perfect directions will generally make bigger waves than shorter period swells from the same direction.

Swell Height

As we’ve mentioned already the ‘swell height’ ISN’T the same as the breaking wave height on the beach but clearly there’s a correlation. Using the rough guide to swell period above you can see that, on the average beach, all other factors being equal a swell of about 10 seconds in period will break at about the same size as the swell height assuming the direction is optimal for the beach.  If you can work out how period and direction affect your local break you can simply scale your forecast for breaking wave height based on the swell height. We’re constantly working at MSW to try to factor as much of this science as possible into a forecast, but ultimately you need some local knowledge and experience to take the data we give you and calculate your own surf forecast.

Bear in mind the swell height we give is the average height of larger waves – generally the largest wave will be about 1.5x as big as this. So if we’re forecasting 6ft of swell the largest wave in the swell will be around 9ft.