Around three and a half weeks ago, I was chatting with Jamie Mitchell about the long range forecast for Hawaii. He’d just spent some time with Chris Owens, and Chris had predicted “An Eddie swell in a couple of weeks.” I found that pretty interesting, because I couldn’t see anything on the long-range forecast that suggested Hawaii had a big swell in its future. In fact, the Peahi contest had run a few days before, and the swell that had fed that contest appeared to have sucked the life out of the North Pacific.
But Chris had told Jamie that we’d have an Eddie swell for Christmas, or New Years at the very latest—and three weeks later, he came very close to being right. The Eddie was put on yellow alert, t-shirts were printed and sold, and Kam Highway was in gridlock for two days. As it turned out, the Eddie ended up getting cancelled the day before it was scheduled to run, but the swell was still the biggest of the season thus far, with Buoy 1 hitting a frightening 17 feet at 19 seconds at one point.
More surf science, HERE.
Once things settled down, I decided I wanted to know what sort of crystal ball Chris and Jamie were looking into when they forecasted this swell. Jamie had mentioned the state of high pressure over the north Pacific, as well as a shift in the jet stream, but I needed to know more, so I got in touch with MSW forecaster and science man Tony Butt for his input.
“An accurate two-week surf forecast for anywhere, let alone somewhere as fickle as Waimea, is a very difficult thing to do. As I’ve said before, long-term forecasts are a three-way playoff between accuracy, precision, and length, so if you want to make really long-term forecasts with any acceptable degree of accuracy, the statements you make can’t be very precise but we can mitigate that.
“Now, swell depends on surface weather systems that generate wind across the ocean surface. These weather systems depend in turn on the flow high up in the atmosphere: the upper airstream or jet stream. The upper airstream is slower to change than the surface flow—more stubborn, if you like—which means its behaviour can be predicted further into the future. For example, whether the upper airstream is likely to be more or less strong, weak, southerly or northerly shifted, straight or containing a large meander can sometimes be predicted over a week ahead.
“Waimea happens to thrive on very large, relatively short-period swells, generated by lows that deepen close to Hawaii. Swells generated a long way away that diminish with distance but arrive with long periods, are ideal for spots such as Peahi because the intensity of the focusing increases with period. Waimea, however, doesn’t have that kind of bathymetry, and longer period swells tend to be sucked away by the outer reefs such as Outer Log Cabins and (to a lesser extent) Alligators.
“If the upper airstream is shifted more south than usual, and is particularly strong, this will be favourable for the generation of powerful storms that come close to Hawaii. And, therefore, favourable for big surf at Waimea.
“Of course, in the end the Eddie nearly ran, but it didn’t. Apart from raw, closely-generated swells, Waimea is also very sensitive to swell direction and a host of other factors that really can’t be predicted so far ahead. The fact that the final decision isn’t made until the same day simply adds to the magic of the most legendary big-wave event on the planet.”
For an explanation about the jet stream and how it impacts your surf session, see HERE. Also, under our charts tab at the top of the page, you can select your region and choose jet stream to track what's happening nearby or across the globe. There are wind charts and swell charts too, which are our most commonly used visual for showing the intensity of swells. Let us know if there's anything surf related you'd like explaining and we'll dig into the topic. For everything else surf science related, including lots of articles from Tony, head over to our Making the Call section.