Posties Swell Statistics, Autumn: All Swell – Any Wind
This chart illustrates the variation of swells directed at Posties through a typical southern hemisphere autumn and is based upon 6580 NWW3 model predictions since 2007 (values every 3 hours). The wave model does not forecast wind or surf right at the coastline so we have chosen the most applicable grid node based on what we know about Posties. In this particular case the best grid node is 24 km away (15 miles).
The rose diagram describes the distribution of swell directions and swell sizes, while the graph at the bottom shows the same thing but lacks 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 only 0.2% of the time. Green and yellow represent increasing swell sizes and red shows the largest swells, greater than >3m (>10ft). In both graphs, 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 largest spokes, was SW (which was the same as the most common wind direction). Because the wave model grid is offshore, sometimes a strong offshore wind blows largest waves away from Posties and away from the coast. We lump these in with the no surf category of the bar chart. To keep it simple we don't show these in the rose graph. Because wind determines whether or not waves are clean enough to surf at Posties, you can select a similar diagram that shows only the swells that were predicted to coincide with glassy or offshore wind conditions. Over an average southern hemisphere autumn, swells large enough to cause surfable waves at Posties run for about 100% 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.