Mystery swell on the Gold Coast
“While southerly swells are always a low percentage game for protected spots and locations north of the border, sometimes exceptionally strong frontal passages - even when poorly aligned - can often produce one day wonders in SE Qld.”
Swellnet Forecaster Notes, last Wednesday, 8th July 2015, describing the upcoming system that eventually produced yesterday’s great surf.
Tuesday delivered an incredible day of waves across Northern NSW, and surprisingly, the Gold Coast. This swell wasn’t unexpected, however its vast reach was certainly underestimated.
Strong southerly lines power up the Tweed Coast. Photo: Ben Matson
A Tasman Low formed east of Bass Strait on Monday, in the wake of a polar outbreak across Victorian and New South Wales over the weekend. Gale to storm force SW winds were reported across Far Southern NSW on Monday morning as the low developed.
However, unlike many Tasman Lows, the fetch on its western flank displayed much less south in direction than normal. In the animated image of surface winds to the right (sourced from our WAMS), you can see a small purple area develop just east of the Far South Coast on Monday morning. The purple colour indicates gale to storm force winds (45-50kts) but as per the arrows, they are SW in direction, aimed away from the coast.
In the next image - six hours later - these winds are now well east of the Sydney coast and more W/SW in direction, aimed even further away from the mainland. Over the following images (all spaced six hours apart) you can see the fetch slowly continue to push outside of the East Coast’s south swell window.
But despite the poor alignment within our window, the surf exceeded expectations. The Forecaster Notes I published late Monday expected a peak of up to 5ft at reliable south swell magnets on Tuesday, with much smaller surf everywhere else, and tiny conditions across most SE Qld beaches.
In contrast, most open beaches in Northern NSW saw 3-5ft surf throughout the day, with larger 6-8ft waves at south swell magnets.
My own observations from the Tweed Coast recorded solid 4ft sets early afternoon, and by mid-late afternoon the swell had reached the Gold Coast with many beaches producing excellent 3ft waves - much bigger than the tiny surf I had anticipated north of the border.
Tuesday’s swell finally reached the Sunshine Coast overnight with similarly straight but slightly smaller surf on offer this morning.
Surfers Paradise beachies reeling off late Tuesday. Photo: Swellnet member palmymick.
Now, when forecasting for any part of the East Coast, there are several rules of thumb to be used whenever there’s a south swell in the outlook.
The reason for this is that the East Coast’s coastal alignment is quite varied, which means certain regions - and more so, specific breaks - can pick up considerably more size than others. Narrow south swells have a tendency to bypass any coast that’s not completely open to the south - this year alone there have been many southerly swells that have produced great waves in Northern NSW but resulted in hardly anything across SE Qld.
To partly explain this, let’s take a look at the NSW coast in more detail. If we split the NSW coast in half (southern and northern), the southern half is slightly better exposed to south swells than the northern half (without wanting to complicate matters, this presumes a southerly swell generated in the Southern Ocean, not off the NSW Coast).
Northern NSW’s broad alignment (off true north) is about 13 degrees, which means it generally faces - on average - slightly south of east. Southern NSW’s broad alignment is about 21 degrees which means it generally faces about E/SE. A little more south, if you will.
But there are local modifications that also need to be considered. In Southern NSW, each region’s coastal alignment varies considerably - the Northern Beaches is around 12 degrees off true north, whilst the Central Coast is 28 degrees and Newcastle is 42 degrees (which helps to explain why the Hunter region almost always picks up more size under a south swell than the Sydney/Illawarra coasts - Newcastle faces almost straight SE). And some specific beaches and reefs are even better aligned for south swells due to their local bathymetry.
In Northern NSW, Ballina is around 38 degrees off true north, which also qualifies its reputation as a south swell magnet. But in stark contrast, the Tweed Coast veers back to the north, with a rough coastal alignment of about 5 degrees. Hence the often used phrase that south swells will be best picked up at beaches “south of Byron Bay”.
Small ruler-edged lines at Burleigh Heads as the new swell started to fill into the Gold Coast around 3pm. Photo: Swellnet Burleigh Heads surfcam.
This makes Tuesday’s event all the more surprising. The swell originated from a narrow fetch inside the Hunter curve, a geographical feature where the coastline juts out from Port Stephens to Seal Rocks. Ordinarily, this will shadow the North Coast from most south swells generated close to the coast, resulting in very small waves.
But in this most recent system, as the fetch tracked into the Tasman Sea, it veered even more westerly in direction - less favourably aimed for the North Coast. And even less so for South-east Queensland.
So why did we see such a strong swell across the North Coast, and clean head high runners across the Gold Coast? I don’t have precise answers right now, but I suspect it’s related to several factors:
(1) core wind speeds reaching 40-50kts as the low reached maximum intensity
(2) the core fetch ‘slingshotting’ through the swell window, setting up a “captured fetch” scenario, which enhances wave heights
(3) the fetch being positioned further south than the target destinations, which allowed the swell periods to grow to their theoretical maximum, which translated to larger surf at the coast
Over the last twelve years I’ve been forecasting for the East Coast, I’ve seen these kinds of flukey south swells produce memorable surf for SE Qld about half a dozen times. Tuesday’s swell was certainly another one of those curveballs that’ll add valuable knowledge to our forecasting systems. //BEN MATSON
Chasing dolphins on the Tweed Coast. Photo Ben Matson.