How the rating system works?
After much deliberation and research we think a traffic light system is the most sensible way to translate an overall picture of the conditions to be expected from the forecast. So lets break down what those colours mean and how they are determined by the model.
We started testing the models forecast data at well known surfing locations across the UK & Ireland alongside checking the webcams, plotting the data against live reporting from wave buoys we are able to calibrate the forecast locations to the best of our abilities.
The chart below represents a months worth of observed swell height data (blue line) from The Porthleven Wave Buoy and how that data has performed against the forecasted data (red line). You can see the accuracy of the data even at a raw level before we process it further to factor in any local headland shadowing or refractions is extremely accurate. This is the first step carried out at developing a location forecast to ensure we get an accurate view of the expected wave heights.
Our next important variable as we know is wind and how that will effect the surface conditions of the swell when it reaches the location – the wind variable is being fed by the GFS and ECMWF models updating 4 times a day. The wind direction arrows will turn green when the wind is offshore or glassy (light winds under 10mph from any direction) indicating good conditions for surfing.
Every variable in the forecast data is going to feed into the overall rating colour given by the model. This includes wave height, swell direction & swell period, these individual variables will also turn green with positive values for that location eg: swell period over 10 seconds or optimum swell direction – in the below forecast we can see that a positive value of the wind being from an offshore direction with a combination of rideable wave heights gives us the green square on the left. (Note the swell direction arrow turning green at 3pm as the swell shifts into a southerly direction optimum for that location.)
Now that we understand how the overall colour ratings work lets look into what the different colours mean. We have five colours. The definition of them are taking from the comments on the source code of the website
GREEN = Offshore winds within a 20 degree window so if Easterly 90 degree winds are offshore the algorithm will look for winds with a due course of 70 to 110 degrees. The other influencing factor will be calm or very light winds from any direction. Swell height influence will be any swell that generates ridable surf of 1ft (waist high) or bigger at that location. Check the actual heights next to the rating.
Lets look at the below – this translates to something like – “Good clean conditions in the morning at high tide, we know the winds are good and the swell direction is optimum its going to be 2-3ft clean and dropping off in height through the morning with the wind changing direction”, by 6pm in the evening the wind has swung right around to be onshore but look at the wind speed its turned green so its just light but as the swell direction has shifted too by now its just not got all the right data to turn the overall indicator green.
ORANGE = Moderate winds from any direction under 15mph or cross offshore winds (between 15mphn and 45mph) & ridable swell over 1ft. Overall average conditions but not enough positive variables to turn green. Swell height influence will be any swell that generates ridable surf of 1ft (waist high) or bigger at that location. Check the actual heights next to the rating.
RED = Moderate to strong onshore winds (wind speeds between 10 and 45mph) & swell – there’s ridable swell over 1ft with onshore winds but check the actual swell heights and wind speeds in the table next to the rating., surf if your keen but overall expect poor conditions.
Lets translate the forecast above. – From dawn we have heaps of swell being driven by strong onshore winds at high tide but by the end of the day on the evening high tide the wind has swung around to be cross shore and we can see the swell period has risen causing the overall conditions to improve
BLACK = Storm force swell (over 20ft open ocean swell) and or winds (over 45mph). Often dangerous coastal conditions, recommended to seek sheltered coves and bays or enjoy watching the energy from a safe point on land.
Lets break down the above, on the evening high spring tide (notice the high tide is at 6pm and 5.39m) a very high tide for this location. We can see that the swell has built through the night and continues to rise into a peak of 20 feet by the low tide at lunchtime and then drops in size towards the end of the day.
BLUE = This was a colour i found that was needed to display a condition i often found appearing in the data and thought might help non surfers, such as stand-up paddleboarders, swimmers and kayakers – it indicates very calm coastal conditions consisting of low to zero swell and very calm winds (under 10mph from any direction ) often seen at some of the traditional swell pullers in the British summer – a good indicator if the sun is out too for a good beach day.
Lets look at what the above forecast means – We can see a small swell with favourable winds is dropping through the night from a orange condition as the wind moves slightly away from offshore and a decreasing swell until we have a very small or finally ‘Flat’ swell reading with a 5mph South Easterly wind and good sunny spells.
Hope this helps shed some logic on what the colours indicate – we cant get it perfect but it should give you a good idea of what to expect. Please let us know of any ways we can improve it for your benefit.
Obtaining accurate results from wave models in coastal regions is typically more difficult. This is due to the complex interactions between waves and the local environment
As of 2021, the ECMWF's weather model is generally considered to be the most accurate weather forecasting mode and is arguably the best weather model on the planet.
It is thought that the distinct gradients visible in some of the wave parameters are function of the tides' phase and the mean wave direction.
Creating a forecasting tool with clear user experience by processing the right data you need.
10 Day Swell Charts
16 Day forecasts