Dominant swell for Godrevy doesn't make sense

"I'm looking at today's forecast for my nearest break (Godrevy), and don't think it makes sense. You're showing 6.1 feet at 15 seconds as the dominant swell. But when I look at the components of that swell (primary, secondary & wind swells) you've got 2.3ft @ 15s, 1.6ft @ 10s & 5.4ft @ 7s. There's no way those three swells are going to combine to look like a 6ft 15s swell - surely that's a mistake. I've checked the sea and there's not much happening. The dominant swell seems to be a short period swell. Surely the hour by hour forecast should read 6ft @ 7s (which is actually what one of your competitors is showing)."

First lets have a look at what you're seeing:

Godrevy Multiswell Forecast© 2010 - Magicseaweed.com

Now very first thing for me to point out is that as I've said before we don't create this data, this is pretty much a raw view of the NWW3 model that we use for the forecasts and that most other forecast sites are also using, our business is interpreting it and displaying it in a way that's useful specifically for surfers. Secondly although we've updated the forecast logic recently to show more clearly surf making swells this forecast as it stands would have looked the same with the old logic - nothing there has changed. Doesn't make the question less valid, just means that it's an issue you might have had reading the models five years ago.

First thing to understand when the model creates the 'headline' swell figures that in this case we're picking for the main forecast (and that we list as dominant) it does two separate calculations in two slightly separate ways. The 'height' will be a combination of all swells running, in this case the 6ft. This is the average potential size of the combination of the 5ft, 2ft and 1ft swells that are present, although not specifically taking into account them hitting the beach as surf. The second calculation is the 'peak' period - this is the swell period where there's the most energy. You can see that sometime mid morning the forecast predicts the power of the new long period swell will be greater than the power of the local wind swell, and the period 'jumps' from 7 seconds to 15 seconds. In reality you can see that both swells have been present all along, just in different proportions - on the beach you won't see a sudden change.

This next image is taken from a wavebouy just a few hundred feet from the coast very close to the break (Perranporth so subtly different but exposed to the same incoming swell), it shows the amount of energy present right now (11am) at all periods:

Perranporth Buoy Spectral© 2010 - www.channelcoast.org

Just as the forecast predicts you can see there's energy at both the 15 second mark, and clustered around the 7 second mark.

So what does it all mean for the surf? Frustratingly the forecast is 'right' in that it's doing what it should do and trying to make a complex situation available in just two numbers AND you're right - the actual incoming swell is NOT the same as a pure 6ft@15 that the forecast appears to predict. It's one of those circumstances where I'm really not sure what the perfect answer would look like. I suspect that it'd be more useful in this specific instance to show just the pure 2.3ft@15 as the main forecast, however the wind swell IS heading towards the beach and they will combining, already the buoy is reading over 4ft and our eyeball reporters are calling it a junky 3ft (which is always under the face height and further up the coast).

Certainly as regards other forecast sites right now they'll be using the same data and they'll just be giving you the headline figures anyway - if they are saying 7 seconds they've either not updated or they are using the 6am data, or they're using a very very slightly different variant of the model, there's no one out there running a system right now that's doing any more complex or sophisticated work with the raw spectral data.

Bottom line? I suspect this is one of those situations where the human eye reading the 'full swell breakdown' is going to get the best interpretation. There will be waves, right now I'd take that 2.5ft@15 and expect it to throw the occasional fun chest high set wave given a break in the wind and some way of avoiding the confusion of the lumpy wind swell - alternatively keep the suit dry and watch it fill in to a solid 8ft@13secs tomorrow....

Hope this makes sense - happy to clarify any bits if you've got comments below.

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