How to Read the Charts

June 5, 2023
PURPLE PEOPLE EATER: This is the largest storm we have EVER seen. Photo:

So, so you think you can tell; Heaven from hell? Blue skies from pain?

With apologies to Pink Floyd, so you’re a savvy 21st century human, all hipstered up with trimmed facial hair and blithe coolness, but does your 100mb/s fibre line, Netflix account and a plethora of surf forecasting apps prove you worthy?

Your main Whats App group is called Barrel Brus or the Surfragettes, but you’re on a bunch of others, such as South Coast Fish Heads, Dungeons Keepers, the Hot Curls, Pier Pressure, Grannies Who Shred, and the Wave Pool Wannabes.

However, as a keen surfer (apologies for the hipster remark) obsessed with good waves, you find your hit rate lower than desired, despite the digital candy store at your fingertips. The skunk factor - exacerbated by the randomness spewed out by climate change - can be high. Remember when Rocky Road, for want of a random generic surf spot, was rated five stars by a forecast website but when you got there, it wasn’t even one?

There was wailing. You drove from the 2ft onshore muck, ears burning at the vloeking from your mates. What a waste of fossil fuel (and time escaped from school / varsity / jail / psychiatric ward), they wailed!

Does a surf forecast of 4m @ 16 seconds @ good swell direction x light offshore deserve the Full Monty of expectation? Maybe, but make sure you interrogate the data properly. There is context to everything.

Technical gremlins lurk at every turn. Hidden in the numbers, away from the naked eye, they snort and chuckle as they tear byte-sized chunks from wave model algorithms. They are the devil in the detail.

FORECAST MAGE: Internet models make life much easier, but you still knowledge. Photo:

But the ruler-edged lines you had hoped for? Nooit. Not there. Why? You were not looking at the full picture. Did you know your swell had to negotiate a huge area of stiff cross wind as it propagated silently through the ocean? That is why the surf was wonky and broken up.

Did you wonder why the surf the other day seemed all mixed up like there were different swells in the ocean? Well there were. Four to be precise. A 15 second 5ft west swell, a 12 second 3-5ft SW swell, a long range south swell AND a 1-2ft NW wind chop, all on top of each other, all jumbled up in a miserable mishmash of miffness.

The latter wind came from the stiff onshore that blew yesterday when the cold front arrived (in Cape Town). Did you factor that in? Peaky 2-3’ wind bumps can sully what looks to be epic surf.

It's not always easy to know about swell that you don't see in the forecast. A storm might have formed right in the middle of a perfect eight foot swell busy travelling from much further away. A new 4-6ft swell is now generated from the new storm. The result? At the end of the conveyor belt at your spot: a mixed bag of motley miffness, and some seriously pissed off mates.  

How to Read the Charts

So here are a few pointers.

Rule #1 Don’t believe in the ratings of others. It was back in April 2006 when I realised this. Granted, it was the prehistoric days of the Web, but surf websites were already a dime a dozen. A particular forecasting site - the one that might have something to do with Harry Potter’s claim to fame and marine flora - gave Cape Town a five star rating one day.

A solid swell and land breezes were forecast. What’s not to like? Big waves and offshore wind! The problem? There was quite a bit of both.

I  stood at Long Beach watching Sunset Reef thunder at 25 feet, while a galeforce southeaster tore the tops off giant peaks to turn them into 100 foot spumes of spray, whiplashed backwards.

I saw a salt-caked surfer stagger past wide eyed, trying to control his board as it bucked like a bronco in the wind, which must have been 45kts. I am sure they’ve changed their internal algorithm now. Too much of a good thing is not necessarily good.

Rule #2 Always check for the existence of other swells in the ocean. The models give credence to the primary swell. This is significant wave height: the average height of the biggest one third of the waves. Other swells are not shown. WW3 outputs secondary swell data but few take notice. Besides, this information can be sketchy. Often a localised windswell will be given priority over a smaller groundswell. This often happens with northeast winds on the East Coast and the southeaster in Cape Town.

When I give my swell courses, I also tell people to be wary of the final figures spat out by the model. The data can be accurate, especially a couple of days before, but the glitch is what arrives with your primary swell. How do find this out? Read on McDuff.

Rule #3 Skunkage is inevitable, but reduce it by widening your sources and types of information. Calling an epic day is not easy. Surf forecasting tools don’t give you parameters like “quality” or “consistency”. And of course they don’t say: at 9.30am a six metre Great White will brush against your leg.

There are so many variables to compute, particularly from a week out, that you might as well throw bones and burn incense. Fortunately, there are other tools. For me, the most valuable tool (especially for spots with funny angles of approach like Durban) is the good old synoptic chart. Yes, the original contour map of air pressure on the surface of the sea. It was first published as a gray-scaled one dimensional image in newspapers - one frame in the animation - that is now published as three-hourly animated, colour-coded forecast charts going up to at least seven days.

These charts come directly from the Global Forecast System (GFS), which provides the raw data for WW3, so you’re in effect skipping the middle man when you study this chart. The GFS surface pressure charts provide a graphic rendition of where and when certain winds will blow at what velocities for how long. I only look at the end data on forecasting apps to “prove” what I calculated in the first place.

By looking at the root, the source of the end data, I feel I have a head start. By having the focal point of winds in a storm in front of me, I am able to form a much clearer picture of those hard-to-know variables like direction, quality and consistency, but that is not easily learned. A few decades obviously helps.

What shape is the storm? How strong is it? Are there other storms? Where are they situated? How fast are they moving? Obviously, herein lies the rub, to understand the charts, you must understand how wind creates waves. You need to understand basic mathematical rules that govern how wind velocity determines swell size and power (given a minimum fetch distance and duration).

The rent in the space-time continuum precludes me from explaining that here, but suffice to say that you need more ammo when checking the model outputs. If the data corroborates with the wave model in your brain, not your screen, good surf lies ahead.

Rule #4 Ah yes. How to turn your brain into a computer model. Develop your own feedback loop, or validation cycle. Build your own profile of theory vs reality. Take your prediction and hold it up against actual conditions. Rinse. And repeat. Do it enough, and you improve accuracy and reduce skunkage.  

We are all different. We surf different spots. We like different kinds of breaks, from soft and mellow to hollow and hard, some of which are shielded from open ocean ambient swell. This is a custom profile unique to each surfer.

When you get good at it, you’ll even be able to understand the rare event of a major upgrade in a swell, when it’s positively giant, and way bigger than it was forecast. Dynamic Fetch, when the storm moves with the swell and locks it in, is usually to blame for that one, but that’s another story.

Rule #5 Only pull the trigger when you feel safe that the predictions are correct, or close to it. The accuracy of WW3 is dramatically curtailed the longer the forecast period. That’s why the GFS plots give you a head start. GFS predicts storms before they even form, and gives this wind data to WW3, which predicts the waves by modelling the effects of this wind on the ocean.

However, if the storm hasn’t formed yet, you’re playing with much higher potential for downgrades. The genesis of the storm is proof it exists, and the maths all falls into place.

This extra layer of uncertainty brings a lot of volatility to the end prediction modelled by WW3. When that happens, it can be way off the mark.


Hone accuracy by expanding your sources. Get to grips with how wind creates waves. Then study the wind charts from GFS that come even before the WW3 models. Be realistic about how accurate your forecast is going to be within a realistic time frame. Within three days is usually good enough to pull the trigger, especially if the storm has formed, even if its in its infancy. And finally, build your own custom profile of theory vs reality at your spot to populate your personalised feedback loop. Better info means better waves.