Sydney, Hunter and Illawarra Surf Forecast by Ben Matson (issued Friday 16th February)

Best Days: There'll waves every day though Sun/Mon will likely be too large for exposed spots, and will therefore favour novelty breaks. Tricky winds, Mon AM offering the best conditions.

Recap: Plenty of south swell Thursday with sets around 3ft (bigger in the Hunter) and workable conditions as the onshores eased through the middle of the day. Today we’ve seen easing S’ly swell and a building E/NE swell from Severe Tropical Cyclone Gita, the morning saw very inconsistent 3ft sets with light winds, and it’s now pushing a solid 4-5ft, though afternoon onshores are causing some problems. For reference, Far Northern NSW and SE Qld has seen some 6ft sets today, so swell wise, everything is pretty much on track from STC Gita at the moment. 

Today’s Forecaster Notes are brought to you by Rip Curl

Decent sets at Manly this evening

This weekend and next week  (Feb 17 onwards)

It’s very hard to stare down the barrel of a significant new long range groundswell - especially one that’s displaying synoptic characteristics you’ve never seen before - and not panic. 

The best way to install confidence in the forecast is to go back through the data, AKA do some hindcasting (something we’d normally do at the completion of the swell event). And this is where things get really interesting.

Take today’s surf, generally reported as reaching 4-5ft across Sydney beaches this afternoon, and up towards 6ft in the northern parts of the state - nicely within Wednesday's forecast expectations. 

Now, taking Far Northern NSW as a reference point - seeing that it has seen the most size, and a slightly earlier arrival (and, that’s where I live too, so can also personally verify the conditions) - peak swell periods across the region’s buoys have been in and around 14 seconds. Yes, there is longer period energy in the mix - but without real-time access to more granular data, we’re best sticking with oceanographic standard metrics for this discussion. 

So, a quick calculation has 14 second swell periods travelling at around 39km/hr. Over twenty four hours, that’s a travel distance of about 936km. This is a quick and effective way to go back through the weather charts to find the ‘source’ of today’s 4-6ft surf. In order to do this, there are pre-requisites and assumptions required, but the rough calculations are good enough for our purposes. 

So, by looking at twelve hourly intervals (468km travel each block), we can track back the origin of this swell to a position somewhere just south/south-east of Fiji around Tuesday - three days ago - when it was around 2,800km (or 1,500nm) from the Tweed Coast. And we can assess other points along STC Gita’s track, to assess its characteristics via satellite winds (scatterometry), and see exactly what might have generated today’s waves.  

In doing so, its quite perplexing to see that STC Gita didn’t look that impressive. At that point of its lifecycle - from Monday to Wednesday - STC Gita was relatively small, with a very narrow radius of > gale force winds. As discussed in Monday’s notes, the supporting ridge to the south wasn’t particular strong, and the primary fetch length - generating the bulk of the swell - wasn’t very long either. 

In short, it’s not a system that in isolation that you'd get overly excited about, nor would consider had the potential to generate 4-6ft surf. Obviously, other factors have helped build today's swell prospects (most of which have been discussed in Monday’s and Wednesday’s notes). 

But, this little analysis is important because it sets a useful benchmark for the remainder of the event. And that's because STC Gita is still positioned SE of New Caledonia, and will continue to traverse a very favourable part of our swell window for another two days. 

That’s right - the surf we see on Saturday will be from STC’s position S and SW of Fiji on Wednesday, and Sunday’s waves will be from STC Gita’s position Thurs/Fri and early Saturday, immediately SE, S and then SW of New Caledonia. 

Let me paint the picture a little more clearly. Below is the forecast position of STC at lunchtime Sunday, a little under one thousand kilometres east of Far Northern NSW and another three hundred more from the Sydney region, displaying core winds of 60-70kts still within our swell window. 

The (very large) swell generated by this part of STC Gita won’t arrive until Monday. Well after the weekend has been and gone. 

So, now that I’ve highlighted some background info (which typically goes on behind the scenes here), it makes the actual forecast even more complex. That’s because the models are still not picking up the proper long period energy from STC Gita, either now, nor over the weekend, nor early next week. 

As I’ve been discussing over the last week, this is somewhat of an unprecedented synoptic event, from a Qld/NSW surf forecasting point of view. In fact, I have no doubt that Severe Tropical Cyclone Gita is going to become Exhibit A in a meteorological course module at some unsuspecting university down the track. The way this system is developing is still unlike anything I have ever seen in this basin; much more like the pattern of a long range typhoon in the North Pacific.

How much can we trust the model data? Estimated significant wave heights are probably not too far off for this event (remember, there’s very little windswell contamination in the mix), but periods are of great concern to me. 

However, the prospects of a good captured fetch developing from today onwards - influencing the swell potential from Sunday through Monday - is also likely being poorly resolved, so I’m ditching the models and going with my gut. I can’t in good conscience ignore a Cat 3 cyclone aligned really well within our swell window, aiming 50-60kt winds at such close range, and simply issue a size forecast that is also theoretically possible from a standard winter Tasman Low or an E’ly dip within a fully developed trade flow.

That being said, I am going to slightly temper Saturday’s size potential, for the morning at least. Given what we’re seeing today - swell energy generated around Tuesday south of Fiji - there wasn’t any major enhancement of the system over the following 24 hours, so it’s more likely that wave heights will remain steady around 4-5ft across most exposed coasts for Saturday morning (smaller at south facing beaches). 

A stronger increase is due into Sunday morning, building into the 5-6ft+ range at most exposed coasts. But, we have two upwards curveballs around this time: the afternoon should see a more prominent increase as the peak energy starts to filter through, 6-8ft+ from the E/NE, with an even larger pulse then expected into Monday, around 8-10ft+. And to be honest, if we end up receiving reports of wave heights coming in bigger than this, I’ll be less surprised than if it comes in half the size. 

The other curveball is a solid long period S'ly groundswell due Saturday lunchtime across the South Coast, arriving into Sydney very late afternoon and holding into Sunday with sets in the 4-5ft+ range. However, I doubt you'll be able to decipher the individual swell trains.

Now, there’s a few caveats to discuss. And that is: the angle of the swell direction (E/NE) and the long swell periods associated with this cyclone swell means that there’ll be an unusual distribution of size across the coast. We don’t see many of these kinds of swells at all, so without analogous events to compare to, it’s hard to know which coasts will benefit the most. 

As mentioned in Wednesday’s notes, if you’re particular coast doesn’t seem to be responding as you’d expected, it’ll be worth a drive somewhere else (for example, south facing beaches won’t pick up all of the E/NE energy). Local bathymetry is the key to scoring under these events and it’s one of the least understood aspects of the surf zone, relative to difference swell directions, sizes and periods - simply because we don’t get enough practise, like we do with our endless diet of southerly swells. 

Although I’d still prefer some elasticity in the size and timing of this event - it may move around over the weekend as we get confirmation on STC Gita’s actual wind field - Sunday and Monday really do look to offer the largest surf from this event. Surf size will trend downwards from Tuesday onwards, though it certainly won't be small to begin with. 

As for winds, there’s no major strength expected for the next few days though onshores are likely for the most part. Monday morning looks the best (!) under a weak troughy/variable pattern ahead of a developing S/SE flow into the afternoon (South Coast) and Tuesday (everywhere else) as a ridge pushes up the coast.

The backside of ex-TC Gita (as it transitions through the Tasman Sea) looks less interesting than it did a few days ago, mainly due to its accelerated SE track. We’ll see a brief SE pulse Tues/Wed but it won’t be anything amazing in the size department. So, expect a return to more moderate sizes from Wednesday onwards; light winds are expected to finish the working week too.

Have a great weekend!