Large cyclone swell building Thursday onwards
South-east Queensland and Northern NSW Surf Forecast by Ben Matson (issued Monday 12th February)
Best Days: Thurs onwards: building long period E'ly cyclone swell that's likely to reach a sizeable peak early next week.
Recap: Good E/SE swell pulsed through the weekend, peaking a little earlier than expected (late Sat thru’ early Sun) and the expected N’lies also cropped up late Saturday morning and through the afternoon, ahead of schedule. Wave heights eased slightly through Sunday afternoon and further today, with generally light variable winds.
This week (Feb 13 - 16)
There’s not much quality surf in store over the short term.
A weak trough lying across Southern Qld and extending out into the western Tasman Sea off the Mid North Coast will create periods of northerly winds for the next few days, mainly across SE Qld and Far Northern NSW (there’s a greater chance for light variable winds at times south from Yamba).
We’ll see a small undercurrent of swell from the east most days, best suited to open beaches but I can’t see much more than the occasional 2ft set.
Elsewhere, s small front cleared east of Tasmania yesterday and it's sent up a small south swell due later Tuesday (Mid North Coast) though no great size is expected. A strong front exiting eastern Bass Strait later Wednesday will generate a small south swell for Thursday afternoon, and 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’ly 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 - though it will be best aimed towards Southern NSW during this period.
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 current 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 hurricane-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.
Anyway, Thursday should start to see the leading edge of infrequent sets through the day, building from 2-3ft to 3-5ft by the afternoon across most open coasts. I don’t think there’ll be too much of a time delay across the region, though its possible we may see a slight lag across the Mid North Coast. I think it'll be very, very inconsistent though.
Winds look tricky in SE Qld on Thursday with an easing N’ly flow, but it should be light and variable south from Yamba to Seal Rocks.
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 4-5 to 6ft+ during the day but you will likely have to be very patient for the bigger waves.
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 periods of smaller surf between sets which may be quite deceiving.
Light N’ly winds are possible about SE Qld and Far Northern NSW coasts, though a late S’ly change is on the cards for the Mid North Coast.
This weekend (Feb 17 - 18)
At this stage a weak pressure pattern is expected all weekend with a high the Bight and another supporting high in the Southern Tasman Sea, as STC Gita motors through our swell window at strength. So, winds should be locally light, though they are shifting around between model runs. Right now we’re looking at S’lies which is ideal for the points.
We’ll be seeing building surf zone during this time from STC Gita's developments just south of Fiji later this week. There are likely to be several embedded pulses in energy from Saturday onwards, so it’s hard to map out a distinct weekend trend.
At this stage I’ll put in some ball park number of somewhere in the 6-8ft range Saturday potentially building to 8-10ft Sunday, but will refine it on Wednesday and Friday as we draw closer to the weekend. Obviously, a groundswell of this size and strength will only favour a handful of protected locations so options will be limited.
Also, there’ll be some strong long period S’ly groundswell in the mix from late Saturday afternoon (Mid North Coast) through Sunday (remaining Northern NSW Coast), 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 lunchtime and afternoon.
I wonder how the combined swell trains will react at exposed swell magnets in Northern NSW? 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, though mainly across Southern NSW as the system will move south of our swell window through Sunday.
Actually, that’s not entirely true - it will move south though there’s every chance we’ll see a SE fetch on the bottom flank of the low aimed our way. But, southward tracking lows are no good for swell potential up north, so the upside is that we’re likely to see a peak in size on Monday across SE Qld before it starts easing. This peak may be delayed across more southern regions, depending on the speed of STC Gita as it transitions through the central Tasman Sea.
Obviously, small deviations in track and strength will affect the size outcome here but there’s no reason to discount the possibility of some exposed locations maintaining 10ft+ surf on Monday (originating from STC Gita’s position later Saturday and Sunday), 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.