Surf Forecasting for Change: Part One

Ben Freeston

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Updated 1891d ago

There are forecasts you can plan a long haul trip on. Swells you can travel half the globe to chase that'll arrive as reliably as your flight. Other times the conditions can bounce around until the day of action and never quite seem nailed on. The good news is you CAN tell the difference. Here's how.

Winds make waves. In a nutshell that's how we get surf. If we know where the wind will be blowing and its speed and direction, we can predict the resulting swell as it travels across the ocean. This splits our job into two parts:

Forecasting a travelling swell

This bit is (relatively) easy. Once the storm has happened we can nail on swell arrival to a few hours at worst and get the size and period mostly as right as you'll need to make your call. You can find out how we do it here. It involves super computers and lots of maths and physics but it's a solvable problem. Every year science gets closer and closer to the perfect solution and we implement those improvements in our own, surfer specific, swell model.

Predicting the wind

This bit is hard. This is the one that consumes the computing power, the complex satellite data and the best and brightest scientists and meteorologists. Progress here has been made at a steady pace since satellite data met modern computer power (about 35 years ago). Prior to this, weather forecasters were little more accurate than simply assuming tomorrow will be much like today! Even now, despite considerable advance, after about 10 days wind and weather forecasts are no more reliable than assuming the weather will be similar to the long term average seasonal conditions. There is a very real chance that this problem is, by nature, unsolvable in the longer term.

Read on to find out why Indo not only gets perfect long range swell with great winds - It's also a lot easier to forecast for long range.

Read on to find out why Indo not only gets perfect long range swell with great winds - It's also a lot easier to forecast for long range.

So if weather forecasting is prone to error over time and forecasting a travelling swell is largely reliable for as long as there's ocean in which it can travel, you'll start to understand that the key to understanding the likely change in conditions over time isn't precisely the number of days in advance, but how many of those days are prior to the storm's formation and the swell beginning to travel. The good news is we give you the tools to figure this out. If you've not been using our swell charts now's the time to get familiar.

Running the above chart through a few days we can clearly see this storm of interest start to form around Nova Scotia. The storm forms fairly quickly and delivers most of its swell energy in a relatively short time frame. Tracking forward we can follow the swell until it hits the breaks we're interested in in France. This can be easier with the 'period' or 'surf' charts for smaller swells.

For Magicseaweed Pro users this is even easier. You can click the Quad Charts icon and view swell and period charts side by side

What do you take away from this? Firstly the time frames for reliably calling this swell are radically different if you're in Hossegor or New York. In France you have four full days of almost complete confidence. If you look at this forecast eight days in advance you're actually only waiting four of those days with much potential for change. The same eight day forecast in New York is a full eight days of weather forecasting, meaning you're pushing towards the dice rolling range at 10 days out.

This leaves us with one of the greatest inequities in surf. If you've followed us so far you'll appreciate that the forecast is typically most reliable in areas a long way from a storm's formation. Think the equatorial latitudes: Indo a prime example. That's right, the further you are from these storms the more lined up, long period swell you're likely to see AND the more reliably it can be forecast.

  • The most reliable forecasts are for swells from storms that have already happened.
  • Because swell travels at a steady speed this means distant storms are easier to forecast into the future than nearby storms.
  • Because the start of a distant swell will normally have a longer period there's a link between swell period and how reliable the forecast is likely to be, with longer period swells generally more accurately forecast into the future.
  • Therefore coastlines that see regular long distant swell are generally forecast more reliably than those that rely on local wind swells.
  • By knowing where and when a storm occurred you can estimate how confident you should be in a forecast.

We'll build on this concept in a couple more articles dealing with the practical applications of this for your making the call - how you can better understand exactly how changes might manifest and the range of possibilities to consider when packing your board bag once you've decided it's worth a look.

Follow me on Instagram for my latest forecast musings and the occasional surf photo. Any questions on this subject hit me there or in the comments below.

Cover image: Boby Dimitrov