Sydney, Hunter and Illawarra Surf Forecast by Ben Matson (issued Friday 11th December)

Features of the Forecast (tl;dr)

• The weekend will probably be spoiled by onshore winds
• Building E/NE swells Monday will become very large by late afternoon, though likely still under an onshore flow
• Size will slowly ease from Tuesday, with improving conditions as winds become light and variable
• Plenty of great surf with light winds for the second half of next week
• Interesting long period cyclone swell on the cards for the long term

Recap: Small, easing S’ly swells and a morning of light offshore winds on Thursday preceded a gusty S’ly change that reached Wollongong bang on 10am as forecast (Bellambi at 10:25am) before crossing the harbour and swinging the anemometer at North Head at 12:25pm (around half an hour behind schedule), thereafter building southerly windswells into the afternoon. Choppy S/SE windswells have dominated today with 3-4ft sets and fresh S/SE winds. 

This weekend (Dec 12 - 13)

A series of complex tropical systems are forming to our north and they’re going to heavily influence our surf, wind and weather over the coming days. 

Unfortunately, the weekend is looking a little shabby, mainly thanks to a persistent onshore airstream. It won’t be terribly strong enough Saturday (though certainly just enough to spoil conditions) however Sunday will see wind strengths increase from the east, a direction where there’s really nowhere to hide. 

Wind speeds will generally be lighter south from Sydney but still holding from the eastern quadrant. 

As for surf, we’ve got a small southerly swell due both days, from a front that pushed under Tasmania on Thursday. Size may reach 2-3ft at south swell magnets but it’ll be bumpy.

Strengthening E’ly winds feeding into a broad coastal trough across Northern NSW and SE Qld will supply a spread of E/NE swell across our region, building from 2-3ft early Saturday to 3-4ft into the afternoon, holding this size into Sunday before a possible late pulse into the 4-5ft range.

Whilst the synoptic onshore flow will be quite established all weekend, these kinds of tricky coastal troughs often allow for pockets of lighter winds here and there so I can’t rule out the odd window of fun lumpy waves.

But if you have to hedge your bets one way or the other, it’s more likely to be a pretty ordinary weekend of waves, unless you’re well south from Sydney where we’re likely to see lighter winds both days, and a greater chance for extended periods of variable wind conditions. 

Next week (Dec 14 onwards)

A deepening trough in the central/southern Coral Sea over the weekend will slide to the south, strengthening an infeed of easterly quadrant gales (E/SE nearer the mainland, E’ly thru E/NE further east in the the central/northern Tasman Sea).

As the trough moves south, we’ll see light variable winds develop north of the trough axis. But… the timing on the position of the trough is very hard to be confident in right now.

At this stage, Monday is more likely than not to be under the influence of locally freshening E’ly winds, and building swells that should push north of 8-10ft at times by the afternoon (smaller earlier). Most of the size will probably be centered around the Hunter region, and it’ll become slightly smaller as you head south from here, but it may not be until Tuesday that we see light winds envelop the Sydney/Hunter/Wollongong regions.

And, it’s worth pointing out that we'd ideally see a moderate to fresh synoptic offshore around this time, to iron out the wobble from (by then) four and a half days of onshore winds. This isn’t likely, so we will probably experience lumpy/glassy conditions across most coasts on Tuesday, slowly improving with each hour. 

Tuesday should pick up a similar level of size as per late Monday, before easing slowly into the afternoon and further into Wednesday (say, 4-6ft open beaches). Thursday and then Friday will probably level out with ‘background’ 3-4ft sets, thanks to the slow decaying nature of the source fetch in the western Tasman Sea, including a broad supporting ridge north of New Zealand. Variable winds are expected around this time too, with no major synoptic influence apart from a mild southerly change around Friday. So, banks pending, there should be some really nice waves for the second half of the week.

Looking further ahead, and just to rewind the clock back to this coming weekend: it’s very likely that within the next 24 hours we’ll see one, possibly two tropical cyclones develop north and north-west of Fiji (RSMC-Nadi is monitoring two tropical disturbances: TD01F and TD02F, see below - JTWC has actually named one as a cyclone (04P) however their threshold criteria is different). 

We’ll probably end up with one dominant tropical cyclone by Sunday, and this is then expected to intensify to Cat 4 or Cat 5 early next week, pin-balling between Fiji and Vanuatu for a few days under a weak steering influence.

However this will all occur inside the swell shadow of New Caledonia - the cyclone is not expected to properly slip south into our swell window until the end of next week or the weekend (of which there’s then a 2+ day travel time for any swell to reach our region).

That being said, there’s a few small regions of partial clearance where small long period E/NE or NE energy could reach the mainland. Model guidance suggest the main energy could start to arrive next Sunday (18+ seconds!) but there’s actually potential for an arrival much earlier than this - but we really need a few more days to asses the early stages of the cyclone’s genesis, to see if its wind field matches that of the model guidance. It’s a really tricky system to have confidence in. 

So, whilst the coast is feeling the effects of large swells under the local trough early next week, we’ll also be able to firm up the size and timing for potential long period cyclone swell for the end of next week the following weekend and beyond.

Have a great weekend, see you Monday!