Swell Alert: A Multitude of Swell Trains Converge along the Eastern Seaboard

21 May 2020 0 Share

Ben Macartney

Chief Surf Forecaster

Issued Thursday, 21 May 2020

The notion that no two waves are the same is often quoted by surfers – and the same idea can also be applied to weather systems. Over the course of this week we’ve seen a unique synoptic pattern take shape, featuring the almost simultaneous development of three, disparate swell sources. Among them resides one very large and slow moving low pressure system that’s set to emerge as the primary swell-bringer across the NSW coast over the next few days.


An extended run of large South to SSE swell sets in across the NSW coast over the next few days. Photo: Dan Bielich.

An extended run of large South to SSE swell sets in across the NSW coast over the next few days. Photo: Dan Bielich.


  • Deep low forming off the southern NSW coast drives a steep rise in South swell from Newcastle and Hunter coasts south on Friday.

  • Large South swell generated by the low builds throughout the weekend and gradually transitions SSE in direction Sunday/ Monday.

  • A long-range pulse of high-period southerly groundswell moves in beneath larger SSE swell this weekend.

  • A tropical low located northeast of New Zealand sets up an additional round of ESE groundswell, most evident across southern Queensland and northern NSW coasts throughout the weekend and early next week.

Sydney, Newcastle and the South Coast 
Thursday’s dreamy run of 2 to 3ft easterly swell and light autumn winds belies all hell breaking loose over the next 12 to 24 hours. A deep low pressure system forming over the southwest Tasman Sea is already entering the initial stages of a rapid and extremely steep increase in south swell across the region on Friday morning. Winds turned south at 20 to 35 knots across the South Coast on Thursday afternoon and they’ll continue to ramp up in strength overnight, reaching 30 to 45 knots early Friday, with gusts up to 50 knots anticipated across Sydney during the afternoon.

Although there’s a strong SW bias in local winds across the coastal fringes, the broader fetch area exhibits a strong southerly bias, predominantly south of Ulludulla. These impressive wind-speeds, coupled with the point-blank range of the fetch, will result in a classic J-curve spike in south swell; most likely reaching 6 to 8ft+ across Sydney’s southern exposures into the afternoon – perhaps a foot or two bigger across Newcastle.

Friday sees a large spike in south swell, arising from a Tasman low still in its infancy. Source: BOM.

Friday sees a large spike in south swell, arising from a Tasman low still in its infancy. Source: BOM.

Friday’s strong spike marks first chapter of an extended run of large and energetic surf, brought about by the Tasman low as it hovers over southern Tasman Sea for several days to come. They key models are now falling into much closer step surrounding the lifecycle of the low; projecting slow movement of the storm, roughly 250 nautical miles off the southern NSW coast throughout Friday and early Saturday, before it drifts further east to southeast throughout the remainder of the weekend. The formation of this system is linked to an active cold front sweeping across Victoria and southern NSW, interacting with a broad low pressure trough shifting off the southern Qld and northern NSW coast. The end result is a large and slow moving storm, enduring over the southern Tasman Sea for several days.

It's now clear the extended duration of an associated southerly quarter fetch will support large South and SSE swell all weekend; mostly likely peaking in the 8 to 12ft realm across exposed areas on Sunday. While maxing out the majority of exposed breaks, the swell will open up a wider range of still big and challenging conditions inside semi-sheltered breaks out of the direct path of the swell train; the same venues predisposed to the strong SW to SSW winds prevailing as the low sits offshore.

The close proximity of a 30 to 45kt SSW fetch off the NSW coast sees a rapid transformation in conditions throughout Friday. Source: BOM.

The close proximity of a 30 to 45kt SSW fetch off the NSW coast sees a rapid transformation in conditions throughout Friday. Source: BOM.

While the SSE swell will eclipse any secondary swell-trains, there are indeed two notable, longer period groundswells in the mix over the outlook period; generated by more remote sources impacting disparate parts of our long range swell windows. The first of these is an intense mid-latitude low, positioned deep below the mainland. The storm intensified dramatically as it stalled some 1,450 odd nautical miles south of Kangaroo Island on Wednesday morning, and has since migrated further eastward through our long-range swell- window, well below Victoria and Tasmania.


The low supports a broad swathe of 40 to 45kt WSW to SW winds, responsible for generating a vast area of 30 to 40ft+ seas and swell below Tasmania. The upshot is a powerful round of long-range, high period southerly groundswell; the leading edge starting to materialise at 18 second intervals on Friday arvo, ahead of the bulk of energy arriving on Saturday 23rd . Interestingly, this first wave-train overlaps with a second, high period pulse generated by the same storm system on Saturday; in all likelihood resulting in strong 3 to 4ft+ sets or bigger across southern exposures. As mentioned, this swell-train will be thoroughly masked by the much larger and more proximate SSE swell arising from the Tasman low.

Southern Queensland and northern NSW
The impact of the Tasman low will be comparatively diluted north of Seal Rocks. Both the initial SSW fetch forming along its western flank and the secondary ESE/SE fetch taking shape early next week are both relatively removed from – and more indirectly aligned with northern NSW and southern Queensland.

However, further compounding the forecast is a deepening tropical low now developing northeast of NZ. This system is on track to give rise to an impressive, albeit short-lived 30 to 40kt Easterly fetch inside our long range east swell window throughout Thursday, before it slips southward, gradually falling inside NZ's swell shadow on Friday. For southern Queensland coasts this should see a strong pulse of mid to large Easterly groundswell emerging as the primary source of surf as it arrives on Sunday and Monday, before easing pretty quickly into Tuesday and Wednesday.

From Monday to Wednesday we’ll see the Tasman low emerging as the primary supplier of waves as large South groundswell transitions SSE, facilitating better refraction north of Byron from Monday onwards. That will see several days of still mid to large surf, with a wide range of heights contingent on each locations’ exposure to the southerly direction.


The reverberations of the Tasman low will continue at lower levels across the region mid to late next week – but at this point we're still seeing some fine divergence among the global models surrounding the final stages of the storm's lifecycle. Recent EC runs maintain the low over the central Tasman Sea, somewhere off the Mid North Coast early to mid next week. This would see an additional ESE fetch setting up over the southern Tasman Sea as a cradling high moves in south of the low on Tuesday, before rapidly weakening as the low dissipates on Wednesday. In contrast, GFS run move the low further northeast, resulting in a shorter-lived, weaker SE fetch - again dissipating on Wednesday. However it pans out, it presents good potential for a reinforcing SE swell Wednesday, Thursday next week.

Tags: Swell Alert , Ben Macartney (create Alert from these tags)

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