Below is a comprehensive explanation of the example WindGuru forecast to the right, looking at the primary prevailing winds for Three Anchor Bay, as well as swell considerations. Expand each section below for more information.
Three Anchor Bay has a very specific micro-climate, based on the protection that Table Mountain offers from the prevailing South Easterly wind. In a pure moderate to strong South Easter, the wind shadow extends from the Mouille Point lighthouse to Clifton Ridge. However, as the wind speed increases, the wind shadow shrinks towards Three Anchor Bay.
Looking at the example forecast, once the wind gusts indicated on WindGuru reach about 17 knots as on Tuesday 8:00, the South Easter starts catching Three Anchor Bay. Obviously this represents a safety risk, as the wind direction is off-shore and already pretty strong.
In this particular example, the average of 13 knots is quite low, and therefore conditions might be okay. If the average were 16 knots with gusts of 17 knots, it would almost certainly be too strong.
Westerly winds are onshore, have a long fetch over the ocean, and can make the sea very choppy, but there is less risk of surprises. Looking at the example forecast, the 10 knot SW on Wednesday at 14:00 will result in mild whitecaps, which are an indication of choppy conditions.
Wednesday afternoon is a good example of a worsening trend in the conditions, so if it looks marginal for you at 14:00, don't go out because it's going to get worse. Wednesday morning from 5:00 to 11:00 is a good example of improving conditions, so if it looks okay at 5:00 you know it's only going to get better for at least 6 hours.
Three Anchor Bay also provides some protection from the prevailing South Westerly swell. The swell will break on either side of the bay, leaving a calm passage in between. As the swell size increases, the passage becomes narrower, and when the swell height reaches about 5m (deep sea swell) on WindGuru, waves will break right across the bay. The swell period (time in seconds between wave peaks) also has a significant effect on the size of the surf. The longer the period, the more water the swell contains and the bigger the surf. As an example a 3 m swell with a 16 second period will break across the bay. A typical swell period is 11 seconds.
When conditions are marginal, it is important to watch the swell for some time to ensure you have seen the big sets, as there may be as much as 20 minutes between big sets. When the swell is big, paddle far from shore, especially rounding the point at Mouille Point lighthouse! Also, be sure to take the right line when returning to the beach. The correct line is shown in the satellite image below. Note that you need to line up the middle of the beach with the left hand carriage way of Helen Suzman boulevard or a prominent, pale gray block of flats in the distance.
There is always a risk of fog with a light onshore wind. Fog combined with big swell presents a significant risk - take a GPS or paddle another day!
Example WindGuru weather forecast for Three Anchor Bay:
Taking the right line into Three Anchor Bay in the event of a big swell: