This image shows only the swells directed at Matosinhos that coincided with light winds or offshore conditions through a typical March. It is based on 1724 predictions, one every 3 hours. The direction of the spokes show where quality surf generating swell comes from. Five colours illustrate increasing wave sizes. Blue shows the smallest swells, less that 0.5m (1.5 feet) high. Green and yellow represent increasing swell sizes and largest swells greater than >3m (>10ft) are shown in red. In both graphs, the area of any colour is proportional to how frequently that size swell was forecast.
The diagram suggests that the most common swell direction, shown by the biggest spokes, was WNW, whereas the the most common wind blows from the N. 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 40% of the time, equivalent to 12 days. Open sea swells exceeding >3m (>10ft) are unlikely to occur in a normal March but 15% of the time we expect swell in the range 2-3m (6.5-10ft) 15%, equivalent to (5 days). Taking into account the proportion of these swells that coincided with expected offshore winds, and given the fact that Matosinhos is exposed to open water swells, we calculate that clean surf can be found at Matosinhos about 40% of the time and that surf is messed up by onshore wind 56% of the time. This is means that we expect 30 days with waves in a typical March, of which 12 days should be surfable.
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.