Padang Swell Monday 16th – 19th July 2012

Padang Forecast Blog: Great swell all week with a reinforcing pulse on Wednesday, which despite the headline numbers we expect to fall a little short of the mark at Padang Padang.

As we move towards the opening of the 2012 Rip Curl Cup at Padang Padang waiting period we’re in for a really good start in terms of swell. Monday sees a blast of long period swell with a reinforcing pulse on Wednesday bumping numbers almost into competition range. However there’s a warning in these figures for aspiring surf forecasters which we want to share with you.

Bruno Santos at perfect Padang Padang during the 2009 Rip Curl Cup

Here are some basic principles before I go into the detail. Firstly a ‘swell’ is a collection of waves, themselves of different heights and periods that are coming from a similar direction and grouped around a central peak period. We can describe this swell with its significant height (average of third largest waves), peak period (period at which there’s the most energy) and direction (normally the direction of the peak period). You’ve never had a surf where every wave was the same size and same period and you never will, even on the most ‘perfect’ swell.

A swell model doesn’t attempt to forecast each wave in a swell (it’s beyond any computer to attempt this) but instead tracks the energy in the sea at each period and direction. It calculates the height of likely waves from this using some robust and reliable maths, these waves themselves will be the combination of individual waves moving at different speeds as they combine passing over each other. Again this is always the case, without multiple waves combining we wouldn’t have sets.

Now if you have two swells passing over each other you’ll see further evidence of this combination. Just as waves in a swell will combine to create larger and smaller waves, so waves in separate swells will create some waves that are larger than each swell could on its own. After all the ocean itself has no real concept of a ‘swell’, that’s our term for a collection of waves, the surface of the ocean just rises and falls in response to the combined energy of waves passing by any given point.

This can be a real issue for surf forecasting. If you check out the image below you’ll see that, for example, in Bali there’s a fairly consistent SE wind swell running at this time of year. Now imagine sitting 20 miles out to sea in a boat with those incoming 7.5ft@16 and that 3ft@5 combining and passing over each other and underneath your boat. What you’ll actually see and feel are slightly larger overall conditions (certainly not 7.5ft+3ft on average, but larger than the 7.5ft alone). However if you’re surfing on the Bukit this combination is absolutely irrelevant to you, that wind swell (3ft@5) is heading away from the coast and that combination will not be affecting the waves you surf. It’s for this reason that MSW separates out the swells. It’s well worth noting that many competing surf forecasting sites don’t. If you’re using a surf forecasting site that only shows one swell the chances are that it is permanently factoring in swells which can’t possibly make waves on the beach, it doesn’t make it ‘wrong’ in the mathematical sense, it just makes it a lot less useful for surf forecasting. I.e. overcalling the swell.

So how does this relate to Padang? Well check the forecast for Wednesday above, you’ll see at 3am the model is separating out three distinct swells, that irrelevant short period wind swell, Monday’s nice looking 7ft@16 and a new long period blast at 2.5ft@20 (the top red circle).

Now scan down to the second red circle, as if by magic one of the two swells seems to have disappeared. In reality what has happened is the waves from these two ‘separate swells’ are now so similar in direction and period that the forecast model can’t separate them.

The maths tells us that that combination of 4.5ft@19 and 7ft@16 DOES make waves averaging 9ft at a peak period of 17 seconds. We know this is heading in the right direction for the Bukit. Yet our experience as surfers tells us that this won’t look quite the same as a ‘pure’ 9ft@17second swell originating from a single source. We’re always nervous of this sort of statement, particularly because to be absolutely honest we’re not entirely clear on why it should be the case. This data is for deep water, as the swell enters shallow water separate periods will transform differently, additionally there’s research that shows that ‘spectral shape’ (something likely to be altered by this sort of scenario) is an indicator of the size of wave groups (sets) but neither offer a clear explanation to our satisfaction. If you’re ahead of us in your understanding of this do let us know.

The bottom line for the 2012 Rip Curl Cup is that there are a number of issues with this swell. The direction is a little further south than perfect, the swell is forecast to peak overnight, the tides on Thursday morning are going to rule out a large chunk of time needed to complete the competition and this swell combination issue is also, we believe, a factor.

Don’t get us wrong – there will be awesome waves all week up and down the Bukit, there will almost certainly be some waves at Padang Padang meeting the contest criteria, but our current thinking is this is going to be a warm-up and hopefully a hint of the season to come, rather than a must run date for the event.