It's been a while since the North Atlantic had a moment like this. With last year almost making records for the lack of huge Atlantic surf we can't help but get excited by a chart with that iconic black blob at its heart. So how do we break down the odds?
We've spoken time and again about the process of separating fantasy charts from the sort of swells that have the chasers packing their bags. Here we're going to walk you through using some of the MSW tool set for that process. This swell is a perfect example - the chart couldn't be clearer. It's unambiguously huge. But it's also ten days out. So how do we work out our odds?
The first thing to understand is that predicting the atmospheric conditions that create the winds that make swell is difficult. It's complex and non-linear and the text book example of the 'chaos theory' that tells us that the further we look into the future the exponentially less likely something is to happen. To dig into what this means for our storm, the first thing we want to do is to look at the atmospheric charts to see if we can spot how far into the future the low pressure starts to properly form:
Here we can see that, although we can track the disturbance that causes this system to an earlier time, the storm doesn't really start to take shape for almost a week. There's no firm cut off for accuracy here, but we should definitely be nervous beyond the five day mark. Typically, for confidence to the level of advising a big wave event to consider an amber status, we'd normally look to be no more than three days from storm formation with a good agreement between models. We're compounding the risks here because of the complex interplay of land and ocean in the area in which the storm is forming.
Our next step will be to see how the Euro atmospheric model feels about the same storm (our default model uses US modelled winds). We can do that by clicking the 'ECM' button on the chart page and MSW Pro users can click the multi charts button to lay the two side by side.
You'll see that as the storm starts to form properly on the US model it remains less clearly defined on the Euro model (right). Beyond this, the Euro model has it stalling and fading rather than moving aggressively into the Labrador sea.
Which model do we trust? Both. They use slightly different initial inputs and physics but it's not normally a question of one being better than the other, it's more a question of them offering us two opinions on a scenario. Here we've got something of a split decision which should give us pause for thought. 50 / 50 odds? It's probably not that simple but that'd be a good starting point.
What next? One of the most powerful tools we have is our multi-model forecast. We know that the long range forecast is subject to change based on even small differences in the weather right now, that's the butterfly effect. We know this will be a problem because even measuring the current weather accurately across the globe is a challenge. One tried and tested way to estimate for this issue is to run not one, but many different models and input slightly different starting conditions to each one. If the atmosphere is likely to behave consistently despite this variation, we know that we've got good odds. Alternatively, there may be several future alternatives that we can usefully explore. Magicseaweed makes 20 separate models available for this purpose. In fact, this is how the 29% rating we're currently giving to this swell is calculated. Click on the percentage on any forecast hour and MSW Pro users can explore in detail all the possible futures the model sees:
You can see from this list of possible Nazare forecasts that the spread is wide. We're probably close to that 50 / 50 mark in terms of huge swell and we're at significantly lower odds on the sort of conditions that really could see the largest ever wave ridden at a location that we're confident has yet to show its real teeth. None the less, there's more than a hint that we're going to see a swell in a range that'll be interesting.
Final call? This is a 'one to watch' rather than one to book flights for. There is a tantalising possibility of the sort of swell we've yet to see ridden at Nazare in particular. When will we be clearer? Every day, little by little, but the real crunch point will be on Thursday or Friday next week as that low pressure does, or doesn't, actually start to follow the plan.