This image shows only the swells directed at Black Rocks that coincided with light winds or offshore conditions over a normal August. It is based on 1736 predictions, one every 3 hours. The direction of the spokes show where quality surf generating swell comes from. Five colours show increasing wave sizes. The smallest swells, less than 0.5m (1.5 feet), high are coloured blue. Green and yellow show increasing swell sizes and red shows the biggest swells, greater than >3m (>10ft). In each graph, the area of any colour is proportional to how frequently that size swell happens.
The diagram suggests that the dominant swell direction, shown by the largest spokes, was SSW, whereas the the prevailing wind blows from the ESE. The chart at the bottom shows the same thing but without direction information. For example, swells larger than 1.5 feet (0.5m) coincided with good wind conditions 9% of the time, equivalent to 3 days. Open sea swells exceeding >3m (>10ft) are unlikely to arise in a normal August but 7% of the time we expect swell in the range 2-3m (6.5-10ft) 7%, equivalent to (2 days). Taking into account the proportion of these swells that coincided with predicted offshore winds, and given the fact that Black Rocks is slightly protected from open water swells, we estimate that clean surf can be found at Black Rocks about 9% of the time and that surf is spoilt by onshore wind 51% of the time. This is means that we expect 19 days with waves in a typical August, of which 3 days should be clean enough to surf.
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.