Stacks of flukey S'ly swell, and another from the E/NE

Ben Matson picture
Ben Matson (thermalben)

Sydney Hunter Illawarra Surf Forecast by Ben Matson (issued Mon 12th Dec)

Forecast Summary (tl;dr)

  • Tiny surf early Tues
  • Late Tues pulse of small, flukey, low confidence S'ly swell
  • Better chance for this small S'ly swell early Wed with offshore winds
  • Super flukey but potentially good S'ly swell for Thurs
  • Solid S'ly swells Fri/Sat/Sun
  • Long range E/NE groundswell building Thurs/Fri, peaking Sat, easing Sun
  • Plenty of swell for next week

Recap

Solid S'ly swells built across southern NSW on Saturday, reaching 4-6ft at south facing beaches into the afternoon, then easing steadily through Sunday, but with much smaller size at remaining beaches. Both mornings saw light offshore winds ahead of developing sea breezes through the day (SE tending E'ly Sat, NE on Sun). The south swell was all but gone this mornig, relaced by a weak NE windswell in the 2ft range (some 3ft sets were observed south from Sydney). Early N/NW winds gave way to a fresh E/NW breeze this afternoon.

This week (Dec 13 - 16)

The source of today's NE swell will dissipate overnight, so we probably won't see much more than a few tiny leftover sets early Tuesday at the swell magnets. 

The next round of swell will be acute from the south, sourced from W/SW gales exiting eastern Bass Strait from this evening onwards, in association with the same change that rocketed across the coast this afternoon.

Whilst wind speeds out of Bass Strait are strong and the fetch is broad, its alignment is more west than I'm keen on, so I'm keeping my expectations pegged a little lower from this source than usual. Additionally, the arrival of the swell is expected late Tuesday, ahead of an overnight peak and a steady easing trend early Wednesday, so it's quite likely it'll do all of its main work under the cover of darkness. 

Therefore, expect small residual energy for most of the day, ahead of a very late flush of south swell up into the 1-2ft range at south swell magnets, with an outside chance (i.e. low confidence) that the Hunter might pick up some 2-3ft sets. Further reducing the likelihood of waves in this region is the even later arrival time, due to the extra travel distance. 

And to cap it off, Tuesday's generally light variable flow will be around to a moderate NE swell breeze by the time the swell arrives.

Wednesday morning is probably a safer bet as we'll resume a moderate offshore W/SW flow and there may be a further small reinforcement from a secondary fetch just south of Tasmania. But it's not worth getting your hopes up about.

The outlook for the second half of the week became a little more complex in the last day or two, with a second swell source now sizing up on the synoptics.

On Wednesday afternoon, a new Tasman Low is expected to rapidly develop east of Bass Strait, slingshotting S/SW gales around its western flank as the system remains slow mnoving to the E/SE over the next couple of days. This will generate several pulses of S'ly swell for the region, though initially they'll be quite flukey due to the low's proximity to the coast.

Thursday has the potential for a large event on the Far South Coast, but I fear the axis of the evolving low relative to potential shadowing from Green Cape (see below) may restrict surf size across Sydney, Hunter and Illawarra coasts - I'm not normally one to sit on the fence, but I really want to revise Thursday's numbers in more detail on Wednesday. Right now it's plausible we could see anything from 2ft up to 6ft+ but it's too early to make a call. It's a tricky low and anything is possible.

Friday's size prospects are a little better, as it's likely the low will be further into the Tasman Sea by this time, and so we can be more confident of a useful spread of size. 5-6ft sets are likely at south facing beaches, under a moderate southerly breeze.

While this is going on, a developing tropical low south of Fiji is forming a broad E/NE fetch about its southern flank, and will generate a quality swell from the same direction (see below. model data from tonight). We'll see this energy fill in on Thursday, building steadily Friday towards a peak on Saturday, with sets in the 4-5ft range at most open beaches - though they will be very inconsistent. 

Let's take a closer look on Wednesday with the availability of scatterometry data. 

This weekend (Dec 17 - 18)

Lots of swell due this weekend from both the Tasman Low, and the sub-tropical low south of Fiji. 

The south swell will be undergoing a third or fourth pulse by this time, but the specifics aren't clear as confidence isn't high as to where the low will be (by Fri/Sat) and how strong the winds will be around its western flank. Still, wave heights are quite likely to remain elevated (i.e. in excess of 6ft). Southern quadrant winds are likely but strengths are yet to be determined.

The E/NE swell from the sub-tropical low will probably reach a peak on Saturday and then trend slowly down, but we'll see plenty of influence both days. 

Let's pick things up again on Wednesday once the models start to resolve a little closer together.

Next week (Dec 19 onwards)

It's way too early to have any level of confidence, but the models are suggesting more dynamic developments in the Tasman Sea.. i.e. plenty of swell in the lead up to Xmas.

Comments

Panman's picture
Panman's picture
Panman Monday, 12 Dec 2022 at 6:49pm

Going to be a few giant wedges with those 2 swells running

Craig's picture
Craig's picture
Craig Tuesday, 13 Dec 2022 at 7:55am

I'm loving these crisp, cold summer mornings. Summer is still yet to kick in properly across southern NSW, but this weather is divine.

joesydney's picture
joesydney's picture
joesydney Tuesday, 13 Dec 2022 at 8:31am

I asked this question in the earlier forecast but thought I would chuck it here.

What’s the threshold for calling something a ground swell? Fetch, period, size, distance travelled ?

Genuine question we use the term “groundswell” but what does that mean ? (Apart from solid waves! )

thermalben's picture
thermalben's picture
thermalben Tuesday, 13 Dec 2022 at 8:45am

Good question. Normally it'd be related to swell periods, for example anything below ten seconds might be windswell, anything above groundswell.

Of course, in real life this doesn't always fit the narrative (that windswells are weak, and groundswells are strong). I've seen punchy surf at 9-10 seconds, and comparatively slow, lacklustre surf at 15 seconds. Individual numbers don't tell the story.

Same goes for the source of the swell - normally, windswells would be generated locally and groundswells would be generated a long distance away. But sometimes this isn't always the case (for example, distant trade swells are generated a long distance away, but display much lower periods than what we expect for groundswells).

When using these terms in my notes, I'll try to use them to best describe the situation, but sometimes I'll alternate in order to differentiate between individual sources (i.e. call one event a 'swell', and another a 'groundswell').

Hope that didn't make it more confusing!

Craig's picture
Craig's picture
Craig Tuesday, 13 Dec 2022 at 8:46am

Great question.

It all depends on the swell period, ie frequency.

Swells cover a wide spectrum, from high frequency, capillary and windswell waves to lower frequency groundswell, infragravity waves, tsunamis and then tide cycles.

So depending on the frequency, the swell is classified as windswell, mid-period or long-period.

Windswell could be classified up to 8-9s, mid-period swell from 9-13/14s and then groundswell from 15s onwards.

Where-is-albo's picture
Where-is-albo's picture
Where-is-albo Tuesday, 13 Dec 2022 at 10:26am

Is there a maximum achievable swell period for waves formed by wind? Highest recorded?

Craig's picture
Craig's picture
Craig Tuesday, 13 Dec 2022 at 11:33am

Not sure on the highest recorded but I've seen 25s on the buoys before.

The maximum theoretical swell period from an open ocean storm is limitless in a way.

IE it depends on the strength of the swell generating winds, how long they blow for and over what region (fetch). The greater all of these parameters come, the larger the swell period that emanates from the storm.

These very large swell periods are usually observed thousands of kilometers away, once the swell trains have sorted themselves out into organised sets and groups.

stunet's picture
stunet's picture
stunet Tuesday, 13 Dec 2022 at 2:22pm

In 'Surf Is Where You Find It' by Gerry Lopez there's a story of he and Darrick Doerner surfing Pipe in the '80s under a straight west swell at 26 seconds.

The chapter is called 'The Mother of all Pipeline Swells'.

joesydney's picture
joesydney's picture
joesydney Tuesday, 13 Dec 2022 at 3:49pm

I was lucky enough to be on the north shore for this swell, back when you would listen to the radio and they would call out the bouy readings……it just kept going up and up !

https://www.swellnet.com/news/rearview-mirror/2018/01/28/condition-black...

MrBungle's picture
MrBungle's picture
MrBungle Tuesday, 13 Dec 2022 at 3:52pm

That sounds terrifying. Paddling through the massive lumps of swell at 26 second intervals sounds scary enough.

joesydney's picture
joesydney's picture
joesydney Tuesday, 13 Dec 2022 at 3:58pm

I was watching live when Jason Magers copped it on the head - it was so impossibly big and we all thought for sure he was dead. The talk at the time was that it was Joel Fitzgerald in the water.

Thegrowingtrend.com's picture
Thegrowingtrend.com's picture
Thegrowingtrend.com Tuesday, 13 Dec 2022 at 1:27pm

have a look at the swell in the north Pacific massive ground swell

Wong's picture
Wong's picture
Wong Tuesday, 13 Dec 2022 at 10:57pm

One thing today's little swell did was over perform all the way into the midday high tide up here on a Central Coast beachie

thermalben's picture
thermalben's picture
thermalben Wednesday, 14 Dec 2022 at 6:06am

Unreal.. how big?