Latest Atlantic wave forecast from the Scripps Institute wave height predictor.

Met Office surface pressure chart.

Key to pressure chart symbols on Met Office website
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Surf Forecasting

Wave prediction and surf forecasting is based on a complex combination of factors originating from the science of Oceanography. To cover the whole subject of surf forecasting in detail would fill volumes, so what we’ll be attempting to do in this guide is not provide a lengthy oceanographic thesis, but a basic understanding of the processes which go into creating the waves we ride. This will include details on how waves form, how swells develop, swell direction, wave height, wave speed, wave duration (period), the influence of bottom contours, tides and what information is available to us all to help with our own wave forecasting.

How do waves form?

In a word wind. Wind is the source of all of our waves, but its not a simple case of the stronger the wind the better the waves. Strong local onshore winds (wind blowing inland off the sea) will create waves but they will be messy, close together and lack form, so a nightmare to surf. The ideal conditions for classic waves are produced when strong storms created by low pressure weather systems blow miles from our shores combining the factors below:
Wind velocity – The deeper the low pressure system the faster the wind blows imparting more energy which creates friction over a flat sea. The more energy the bigger and more powerful the waves.
Duration – The longer the wind blows over the same area the bigger the swells.
Fetch – This is the area of ocean the wind is blowing over. The greater the area of wind blowing in the same direction the more energy is released. Large storm systems can cover hundreds or thousands of miles of open ocean.

Wave speed
The speed of a wave has a close relationship its wavelength-simply enough small waves tend to move slowly at a few knots. Medium-size waves may move at tens of knots; large waves move 30 – 50 knots and more.

Predicting swell height at the shore.
As we mentioned before the swell height of an open ocean wave measured by a wave buoy isnt goint to be the height that the wave breaks at the shore. Again there are a number of factors which effect the eventual height of the waves we ride:

Bathymetry
Bathymetry is the terminology for the shape of the sea bed, which has a significant effect on a swell. As waves enter shallower water they slow, lose energy, power and wave size. For example waves hitting the SW coast of the UK are slowed by the offshore continental shelf so they will lack the power of say Hawaii which receives waves directly out of deep ocean waters which have lost none of their power.

Beach shape
Beach shape is part of bathymetry. A swell hitting a lava reef from deep water will create a larger wave than the same sized swell moving up a gently sloping sandy beach (Teahupoo vs Saunton North Devon!). Other obstacles will have an effect like sandbars, artificial reefs, sunken ships, point breaks.

Swell Direction
Each different break has its own optimum swell direction which will always affect the way a surf spot works and wave height. Some swells may miss a break entirely others may combine with the other factors mentioned here and hit the reef or beach in just such a way to create perfect waves.

Local Winds
Onshore winds are a surfers worst enemy. A strong wind blowing off the sea will blow waves flat and make them mishaped and messy. Offshore winds however are what we are looking for as they hold up a wave and provide a clean surfable face and maintain or grow the waves height.

Refraction
As I mentioned above swell moving into shallow water will slow down. With the right seabed shape (eg. a pointbreak) the wave nearer the beach will slow more than the swell still in deeper water. This effect will ‘bend’ the swell into the break and can increase the size of the breaking waves (in the case of concave refraction) or reduce it (in the case of convex). This effect is also dependant on the period of the swell and increases with higher periods.

Starting to predict your local waves
The amount of information available may seem a little daunting, but by regularly checking wave buoy heights, pressure systems, wind direction etc it’ll soon become second nature to fairly accurately predict waves at you local beach. Happy surfing!

Useful Resources
UK Met office – Surface Pressure charts

Surfline – Online surf forecasting