The figure illustrates the combination of swells directed at Prestons through a typical March and is based upon 1723 NWW3 model predictions since 2007 (values every 3 hours). The wave model does not forecast wind or surf right at the shore so we have chosen the most applicable grid node based on what we know about Prestons. In the case of Prestons, the best grid node is 30 km away (19 miles).
The rose diagram describes the distribution of swell sizes and directions, while the graph at the bottom shows the same thing without direction information. Five colours represent increasing wave sizes. Very small swells of less than 0.5m (1.5 feet) high are shown in blue. These happened 46% of the time. Green and yellow illustrate increasing swell sizes and red shows largest swells greater than >3m (>10ft). In either graph, the area of any colour is proportional to how frequently that size swell happens.
The diagram suggests that the prevailing swell direction, shown by the longest spokes, was ENE, whereas the the most common wind blows from the NW. Because the wave model grid is away from the coast, sometimes a strong offshore wind blows largest waves away from Prestons and out to sea. We combine these with the no surf category of the bar chart. To keep it simple we don't show these in the rose plot. Because wind determines whether or not waves are clean enough to surf at Prestons, you can view an alternative image that shows only the swells that were forecast to coincide with glassy or offshore wind conditions. Over an average March, swells large enough to cause surfable waves at Prestons run for about 11% of the time.
IMPORTANT: Beta version feature! Swell heights are open water values from NWW3. There is no attempt to model near-shore effects. Coastal wave heights will generally be less, especially if the break does not have unobstructed exposure to the open ocean.