Sustained sizeable cyclone swell ahoy!
Sydney, Hunter and Illawarra Surf Forecast by Ben Matson (issued Monday 12th February)
Best Days: Fri onwards: building long period E/NE cyclone swell that's likely to reach a peak early-mid next week.
Recap: Easing S/SE swell Saturday didn’t offer much size away from south facing beaches, however the expected E’ly swell for Sunday arrived earlier than expected, showing through Saturday afternoon and producing good 3ft surf across open beaches. This swell persisted into Sunday though eased throughout the day. Today we've seen smaller, easing leftovers.
This week (Feb 13 - 16)
There’s not much quality surf in store for the next few days.
A small front cleared east of Tasmania yesterday and its sent up a small south swell due on Tuesday though no great size is expected. Also in the mix will be a minor E’ly swell from the residual fetch that occupied the waters north of New Zealand late last week and into the weekend. But again, no major size or strength is likely.
A weak trough off the coast will direct light to moderate onshores into the coast at times though there’ll be periods of light variable winds. Aim for the exposed beaches if you have to, but make sure you’ve got a suitable board for the small, weak surf.
A strong front exiting eastern Bass Strait later Wednesday will generate a small south swell for Thursday morning however it looks like an associated southerly change will clip the coast around the same time, delivering bumpy conditions to those south swell magnets picking up any size (2-3ft sets). Further south swells are then expected through Friday and the weekend from the parent low well south of Tasmania, but this will be greatly overshadowed by a more significant E/NE groundswell.
Severe Tropical Cyclone Gita formed in the Tropical South Pacific on Friday afternoon, and has since strengthened to category 4 status whilst undertaking an impressive southward curve (from an eastward track), before recurving again to the west.
STC Gita is a very large, dangerous system that will likely reach Cat 5 this evening and persist at this strength for the next few days. STC Gita is in a very favourable environment for sustained development, with warm Sea Surface Temps (28-29 degrees), moderate wind shear and good outflow, and is being steered westward along the northern perimeter of a subtropical ridge further south.
In short, this is a very large, extremely powerful cyclone entering our far E/NE swell window, with an expected forward track that is as-good-as-it-gets for long range groundswell potential.
Rather than verbalise the numerical guidance - we can all see the purple blob on the screen, and observe the impressive virtual buoy height and period figures for your local coastline - what I’d like to do is highlight the things I like about this system, and the things I don’t like. Because really, at the end of the day we’re looking at a very large, long lived groundswell event that’s going to light up a small percentage surf spots that will likely be tackled by an even smaller percentage of surfers. Overall, this won't be a swell event suitable for beginner or even intermediate surfers. It’s very likely to be a sustained, sizeable and powerful round of energy with strong currents and dangerous conditions for all but the most experienced surfers.
So, what do I like? Well, what’s not to like? There are so many fascinating aspects as to how this system is being modelled, I’m not sure where to start.
First of all, TC Gita initially recurved to the west way out in the far reaches of our South Pacific swell window, and it did so at comparatively low strength (Cat 1). As it moves through our swell window aimed towards the East Coast, it will gain strength, which is useful though only if other conditions are adequate.
When looking at the forward track of any weather system, you ideally want it to be heading to your coast at a speed that is similar to the speed of the swell it is generating, because this enhances swell production in a phenomena known as Captured Fetch (resulting in much larger waves than you'd see from a regular system). Initially, it appears that STC Gita will move too slow for this to happen, however from about Friday onwards its expected to veer to the SW from a position just SE of New Caledonia, and then accelerate into this desired speed range. This is right up through our mid range swell window, which to be honest couldn’t be timed much better.
STC Gita is very broad and relatively linear in strength; sometimes an ill-placed supporting ridge will focus the strongest winds (all we require for surf potential) into an unfavourable quadrant. However this looks pretty even across the board for much of the model run so far. So, that's a good thing too.
Whilst the slow moving nature of STC Gita may initially restrict its Captured Fetch potential, by the same token it will also allow the swell to move ahead of the main fetch. For example, the leading edge of this event - generated over the weekend well to the east of Tonga - will arrive some time on Thursday, but at that point STC Gita will be positioned south of Fiji, generating fresh energy that won’t arrive until Saturday. What this means is that STC Gita will be working on a very active sea state (which enhances swell potential), compared to a system that moves too quickly through the swell window into essentially ‘dead’ water (if the synoptic system is set up that way).
And lastly, the large distance from STC Gita to the mainland (and all of the factors above) means that the cyclone won’t concurrently influence our local weather as the swell makes landfall. Even the tail end of the model runs currently have a continental high pressure system deflecting it south through the Tasman Sea, away from the Australian mainland. This doesn’t guarantee favourable conditions, as we’ll see local systems influencing our winds, but most often the biggest swell events on the East Coast are generated by ECLs, Tasman Lows or Tropical Cyclones sitting in close proximity to the mainland, and delivering funky winds and weather.
The only things I don’t love about the current model guidance is the supporting ridge to the south of STC Gita. It’s good, and extends the underlying E’ly fetch a reasonable distance back into the South Pacific, but it could be better, especially over the coming days.
Also, it's very important to remember that despite reaching Cat 5 over the coming days - when it's positioned in our swell window - the fetch length of these hurricance-force winds will be relatively short. So, when estimating surf size, you're best off assessing the regional wind charts without the cyclone, to establish the background wave field. Then, you can consider how much additional size and strength the cyclone will add to the mix.
So, enough waffle.. how big is it gonna get?
I'm really not quite sure. I haven’t see a Severe Tropical Cyclone of this size modelled in this way before in such a distant-though-encroaching region of our swell window, and any one of the factors I mentioned earlier could have a small or large impression on eventual surf size.
But, underneath Thursday’s S/SE breeze and mid-range south swell, we should start to see the leading edge of very inconsistent sets through the afternoon. Probably just 2-3ft or so and very inconsistent (the wave buoys may perk up into the 15-16 second range), but there should be some late distinct sets.
Into Friday a much more prominent increase will occur. But we need to remember that Friday’s long range E’ly swell will have been generated several thousand kilometres away, and will thus be extremely inconsistent. Sets should lift from an infrequent 3ft+ to 4-6ft during the day but you will likely have to be very patient.
These figures are largely inline with our model, which I think is picking up this system pretty well. However, it's worth pointing out that the biggest sets will originate from the strongest winds around the core of the cyclone, so not only will they be very inconsistent, there'll be lengthy periods of smaller surf around the 2-3ft mark between sets.
Winds look good for the morning though a late S’ly change is on the cards.
This weekend (Feb 17 - 18)
At this stage a weak pressure pattern is expected all weekend with a high in the Bight and another supporting high in the Southern Tasman Sea, as STC Gita motors through our swell window at strength.
However we’ll be seeing building surf zone during this time from its developments just south of Fiji. There are likely to be several embedded pulses in energy from Saturday through the following five or six days, so it’s hard to map out a distinct trend.
At this stage I’ll put in some ball park number of somewhere between 6 and maybe 8ft for both days (out of the E/NE), and refine it on Wednesday and Friday as we draw closer to the weekend.
Oh, and BTW there’ll be some strong long period S’ly groundswell in the mix from Saturday afternoon through Sunday, from the parent low south of Tasmania on Thursday. South swell magnets should see 4-5ft+ sets from this source at times, likely largest Sunday morning.
I wonder how the combined swell trains will react at exposed swell magnets? Crikey.
Next week (Feb 19th onwards)
Monday, Tuesday and Wednesday are very complex, and very interesting as well.
Assuming the model guidance holds - and to be honest, for a long range system they’ve done very well over the last week or so - we’re looking at a peak in size early/mid next week, with super consistent energy trending from the E/NE to the E and then E/SE as STC transitions through the central Tasman Sea as an extra-tropical system.
Obviously, small deviations in track and strength will affect the size outcome here but as it currently stands there’s no reason to discount the possibility of some exposed locations reaching 8ft+ on Monday (originating from STC Gita’s position on Saturday), with Tuesday and Wednesday possibly seeing even more size thanks to an even closer position to the mainland.
But we’re way out in the fantasy chart timeline here so I’d prefer to keep assessing each model run as to how the overall trend is performing.
Looks like I have my work cut out for me over the coming days!
See you Wednesday.