The full spectral forecast is the most comprehensive, useful and complex service on the site at the moment. You'll really need to read this carefully to get the most out of it.
Most of the forecast data on this site, including the 'Summary' and 'Detailed' tabs on the local pages and the swell charts are created from the NWW3 forecast model. This mathematical computer software uses a forecast of global wind speeds and directions to forecast swell for the next week or so. The model calculates every swell created and tracks it as it spreads and dissapates. Just like what actually occurs in the ocean the model knows that there can be more than one swell passing any given point at any given time, a strong west swell overlapping with a weak southerly swell for example. The problem, for surfers, is that the main output of this software is simply to tell us the 'dominant swell' at any given point and the 'significant wave height' (something like the combined wave created by all swells at a location). Normally this is pretty good. A decent surf making swell is normally the dominant swell and you can spot it easily on the forecasts and charts. This isn't, however, always the case. Long distance, long period swells sometimes sneak through without showing up and if you use the forecasts regularly you will have seen the reports sometimes flicking between a longer period swell heading towards the beach and a short period swell heading away from the beach (often with strong cross shore winds). In reality the swell isn't switching direction of course, both swells are there running in opposite directions.
To counter this the spectral forecast gives the full output for selected locations. Showing, in this case, the five dominant swells at any given time, hour by hour, for over a week in advance.
Our Spectral Data is always for the location of a wave buoy. It's really important to understand why this is. It
isn't because the wave buoy has any direct bearing on the spectral data, the data doesn't come from the buoy. It's worth repeating that - the spectral forecasts
are not created by the wave buoys or based on their readings. It's simply that these locations are useful for monitoring incoming swells and, of course, useful to calibrate the forecast against the observed data. So the people who run the mathematical forecasting model thought it'd be useful to use these same locations. We could still get the spectral data if the wavebuoys didn't exist.
The spectral page gives the distance from the forecast to the beach and an idea of the time it'd take a swell to travel between the two. The lower time is based on a swell of 15seconds period, the longer time on a 9second period wave (the speed of a wave is directly related to it's period). Assuming the swell travels from the forecast location to the beach you need to add this time to the forecast time. In some locations, like Hawaii, where the swell can come from any direction the swell may hit the break
before it reaches the forecast location. Check the direction and you may need to subtract the time from the forecast.
Swells tend to reduce gradually in height as they travel, particularly if the water is shallow away from the coast - so bear in mind that the heights given in the spectral forecast are not directly comparable to the heights in the normal forecast (which already factor in some decay).
All in all, like all our data, use coupled with experience should answer specfic local questions on what to look out for. Next time there's a sneaky south swell on the beach when the main forecast said a weak westerly check the spectral and you'll probably see where and how it came about...
