Llangennith Swell Statistics, Autumn: All Swell – Any Wind
This image shows the range of swells directed at Llangennith over a normal northern hemisphere autumn, based on 6516 NWW3 model predictions since 2006 (values every 3 hours). The wave model does not forecast wind or surf right at the shore so we have chosen the best grid node based on what we know about Llangennith. In this particular case the best grid node is 17 km away (11 miles).
The rose diagram describes the distribution of swell sizes and directions, while the graph at the bottom shows the same thing but without direction information. Five colours show increasing wave sizes. The smallest swells, less than 0.5m (1.5 feet), high are coloured blue. These happened 16% of the time. Green and yellow represent increasing swell sizes and red illustrates biggest swells greater than >3m (>10ft). In each graph, the area of any colour is proportional to how commonly that size swell happens.
The diagram implies that the prevailing swell direction, shown by the biggest spokes, was WSW, whereas the the prevailing wind blows from the SW. Because the wave model grid is offshore, sometimes a strong offshore wind blows largest waves away from Llangennith and away from the coast. We combine these with the no surf category of the bar chart. To simplify things we don't show these in the rose plot. Because wind determines whether or not waves are clean enough to surf at Llangennith, you can select a similar diagram that shows only the swells that were forecast to coincide with glassy or offshore wind conditions. During a typical northern hemisphere autumn, swells large enough to cause surfable waves at Llangennith run for about 84% 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.