Compare the trends in the info to the real situation at your spot to build a profile that works for you. Each day's info represents a snapshot - a fixed point - on a moving curve, going up or coming down. Small swell on Day 1 and big swell on Day 2 obviously indicates an upward trend between 2pm on Day 1 and 2pm on Day 2.
Period is key
You can make an informed call if you appreciate the key factor: wave period (the interval in seconds between swells in a set). Period is the average seconds it takes for the most dominant swell to pass a fixed point, not including other smaller swells around at the time.
Power to the period
The longer the period, the more powerful the swell, exponentially. A slight increase in period often means a big increase in swell power. Even if the height doesn't increase in the deep ocean, it's gonna increase BIG time when it encounters the sea floor at the coast.
Why?
Longer period swells move much faster than shorter period swells. They have longer wavelengths, and carry more energy that extends deeper into the ocean. Each energy pulse therefore moves more volume of water, which compresses higher (height) once the sea floor at the coast is encountered.
Example
6' swell at 10 secs travels at HALF the speed of 6' swell at 20 secs
6' swell at 10 secs has 16 times less energy than a 6' swell at 20 secs
The 10 second swell arrives on the shore as a weak 2-3' wave.
The 20 second swell arrives on the shore as a potent 6-8' wave.
 HOW LONGER PERIOD BOOSTS LENGTH, SPEED AND WAVE SIZE * | ||||
| BUOY HEIGHT | PEAKPERIOD | WAVELENGTH | SPEED (DEEP SEA) | SURF AT COAST |
| WSW 10 ft | 6 secs | 56 metres | 16.7 km/h | Sloppy 2-3' |
| WSW 10 ft | 7 secs | 76 metres | 19.5 km/h | Weak 2-4' |
| WSW 10 ft | 8 secs | 99 metres | 22.4 km/h | Weak 3-4' |
| WSW 10 ft | 9 secs | 126 metres | 25.3 km/h | Soft 3-4' |
| WSW 10 ft | 10 secs | 156 metres | 28 km/h | Soft 3-5' |
| WSW 10 ft | 11 secs | 188 metres | 30.8 km/h | Med soft 4-5' |
| WSW 10 ft | 12 secs | 224 metres | 33.7 km/h | Medium 4-6' |
| WSW 10 ft | 13 secs | 264 metres | 36.5 km/h | Med solid 4-6' |
| WSW 10 ft | 14 secs | 306 metres | 39.3 km/h | Solid 5-7' |
| WSW 10 ft | 15 secs | 351 metres | 42 km/h | Solid 6-8' |
| WSW 10 ft | 16 secs | 400 metres | 45 km/h | Big 8-10' |
| WSW 10 ft | 17 secs | 451 metres | 48 km/h | Grinding 8-10' |
| WSW 10 ft | 18 secs | 505 metres | 51 km/h | Dik 10-12' |
| WSW 10 ft | 19 secs | 563 metres | 53.5 km/h | Dik 12'+ |
| WSW 10 ft | 20 secs | 624 metres | 57 km/h | Dik 15' |
* NB
This table pertains to the West Coast of SA. The very westerly direction of the swell means it's coming straight on at your average reef or point. The table is a very simplistic and loose interpretation to generally indicate the exponential ratios between open ocean swell data and the potential wave when it hits the coast. In South Africa, we have many swell directions and coastal aspects facing in all directions.
Don't take the above too literally. Compare your break with the buoy data and BUILD YOUR OWN PROFILE. Different swells mean different things for breaks on SA's West, South or East Coast. This table is looking at a SW Cape area, assuming a swell direction of more or less WSW (call it 230 degrees, which is 5 degrees West of true SW at 225 degrees). In other words, the swell is quite West, coming quite straight onto an exposed break, but having undergone slight refraction. The surf at the coast is the POTENTIAL surf - a guesstimate based on rough calculations for above swells breaking on to a shallow reef or sandbar.
WHERE DOES THE INFO COME FROM?
The data comes from Wavescape Ocean Watch via the NOAA Wave Watch III model. It is updated three times a day. You get swell, wind and tide information for each of the above areas, including primary swell height & swell direction, as well as potential surf height and direction at the coast (in degrees). We have developed our own algorithms to pull out the data needed for the individual beach areas.
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