How to Read a Swell Forecast for Shore Diving in Australia

Whether an Australian shore dive happens is decided by the swell far more than by the weather. Sydney can be sunny and windless under a 3 m southerly swell that makes every east-facing site undiveable, and grey and drizzly on a day of 0.6 m swell when the whole coast is open. Reading the forecast takes ten minutes to learn and saves a great many wasted drives.

The three numbers

Swell height is the significant wave height of the swell component, roughly the average of the largest third of waves. As a rule of thumb for exposed shore entries on the east and south coasts: under 1 m is good, 1–1.5 m is diveable with care at protected sites, 1.5–2 m closes most rock entries, and above 2 m you should be looking inside a harbour or bay. West-facing and protected sites can take more. Swell period is the time between waves in seconds. Long-period swell (12–16 s) carries far more energy than its height suggests and wraps around headlands into places short-period swell (6–8 s) never reaches; a 1.2 m swell at 14 s can be worse at an entry than 1.8 m at 7 s. Swell direction is where it is coming from, and it is the number that lets you pick a site: a south swell leaves north-facing and west-facing sites flat, an east swell hits everything on the NSW coast, a south-west swell is what the Victorian back beaches and the WA south coast face directly.

Wind on top of it

Wind creates short, steep local chop (the 'seas' figure in the Bureau's coastal waters forecast) and stirs up visibility in the shallows. Offshore winds flatten the surface and clear the water: westerlies for the NSW coast, northerlies for the Mornington Peninsula bay side, easterlies for Perth and Geographe Bay, south-westerlies for the Queensland coast. Onshore winds above about 15 knots make a mediocre swell forecast worse. The sea breeze in summer is the afternoon killer in every capital: dive at 7 am, not 1 pm.

Where to look

The Bureau of Meteorology's coastal waters forecast gives wind, seas and swell (height, direction and sometimes period) for each stretch of coast, updated twice daily. Swell models such as Swellnet, Willyweather, Windy and Surf-Forecast give hourly height, period and direction several days out. The Manly Hydraulics Laboratory (NSW) and comparable state wave-buoy networks publish live buoy data, which tells you what is actually arriving rather than what was predicted. Surf cams pointed at the beach next to your site are the final check.

Region cheat sheet