South-east Queensland and Northern NSW Surf Forecast by Ben Matson (issued Monday 8th May)

Best Days: Tues/Wed: reasonable chance for a solid NE cyclone Tues, easing from Wed. Most size in SE Qld. Plenty of concurrent S'ly swell in Northern NSW too, from a Tasman Low. Thurs PM/Fri: rebuilding E'ly swell. Sat: easing E'ly swell with light winds. 

Recap: A steady trade swell kept SE Qld and Far Northern NSW humming over the weekend and into today with sets around the 2-3ft+ mark. Northern NSW also saw a small south swell early Saturday but it eased during the day and locations south of Yamba have seen much smaller surf for the last 48 hours, though a fresh S’ly groundswell is now starting to show across the region. SE Qld has seen a slight increase in surf this afternoon, which could be attributable to the leading edge of cyclone swell form STC Donna, but is more likely just a reinforcing trade swell (buoy data doesn’t show anything significant in the period data).

Late afternoon sests at Noosa - most likely trade swell

Fun evening options at Alex Headland
 
This week (May 9 - 12)

We’ve got a very tricky forecast ahead. So, let’s start with the (relatively) easy stuff.

A strengthening ridge across SE Qld will generate a short range SE swell over the coming days. The models have pulled back its strength and also aligned it slightly more meridionally (north/south than east/west) which isn’t as good for surf prospects. The Sunshine Coast should pick up 2-3ft sets from this source over the coming days, with marginally smaller surf on the Gold and Tweed Coasts and very little size south of Byron or Ballina.

A supporting ridge south of STC Donna - from New Caledonia to the waters south-east of Fiji - is generating new E’ly swells that will concurrently build across our coasts from about Tuesday afternoon and Wednesday onwards. This fetch looks deceptively good on paper, though this is mainly due to the presence of the cyclone (which is now located inside the swell shadow of New Caledonia, and is not generating any direct swell at the moment). Once the cyclone and its shadowed fetch is removed from the equation, we essentially have a bog-standard, 25kt trade flow that should be worthy of 3ft+ sets across most regions through the middle of the week. 

However, as STC Donna tracks south over the coming days and undergoes extra tropical transition, we’ll see core winds pick up within the swell window. This is expected to occur late Tuesday (SE of new Caledonia) which should create a fresh pulse of E’ly swell arriving late Thursday and into Friday with sets anywhere between 3ft and maybe 5ft, depending on exposure. I’ll revise these figures on Wednesday once (by then, ex-)TC Donna moves into the Northern Tasman Sea and we can confirm wind speeds and the forward track. 

And we also have a Tasman Low east of Tasmania that’s generating strong southerly swells for Northern NSW. In actual fact, the latest model guidance has come back in line with a slightly more prolonged swell event from this system, meandering the low about the Southern Tasman Sea up until Thursday. However, this system has already reached peak intensity, so even though it’ll remain within a good region of our S and SE swell window for the next few days, the largest surf we’ll see will be on Tuesday afternoon, with smaller (but still strong) surf padding out Wednesday, easing further into Thursday and Friday. 

South facing beaches should pick up anywhere between 4ft and (at a best case scenario) 6ft by late Tuesday, though most beaches will see considerably less size due to the acute swell direction. Expect smaller surf from this source during the rest of the week. 

This low will also dictate local winds across Northern NSW all week, resulting in mainly moderate to fresh S’ly winds, possibly SW for periods early morning. Across SE Qld, winds will be focused initially by the developing ridge - so, more S/SE throughout the days following the early SW breeze in isolated regions. Winds should ease off everywhere by Friday.

And now to the most important feature on the synoptic chart: Severe Tropical Cyclone Donna.

STC Donna reached Cat 4 strength over the weekend, and was actually upgraded to Cat 5 this morning, with average wind speeds of 110kts and gusts to 140kts. That's about 260km/hr. Fortunately it moved just west of Vanuatu, compared to Friday's model runs which suggested a direct hit.

Although STC Donna is now located well and truly inside the swell shadow of New Caledonia, it did briefly skirt the southern periphery of a very narrow, rare swell window just north of New Caledonia over the weekend. Unfortunately, the weekend’s scatterometer (satellite) passes didn’t align with the centre of the cyclone, so it was only late yesterday where we managed a brief pass over its eastern sector, which confirmed surface wind speeds of 50-60kts. Secondary wind estimation techniques (via JTWC) have confirmed Cat 5 strength winds around the core of STC Donna today, though the short fetch length of these core winds means they won't contribute theoretically higher swell periods. 

Now, the difficulty with the associated surf forecast is that it relies predominantly on model guidance. And in the examples below, we can see that even the models can’t be trusted - checkout the different between GFS (left) and ACCESSG (right) - this is the initialisation data for Sunday morning, (which should be the same!).

GFS - the wind model underpinning our wave model - has core winds of 60kts within our swell window, yet the BOM’s ACCESS G has barely 40kts, positioned outside of our swell window. Other models have fallen somewhere between the two. 

This means estimating surf size really comes down to confidence and experience with the various model’s tendencies, and I do feel relatively comfortable with GFS, especially give it picked up surface wind speeds quite well. However there are a considerable number of shoals and other surface obstacles between STC Donna's Sunday position and the SE Qld coast, that are likely to influence how much size we see, and to what degree of consistency we see in the surf zone. 

As such, I am sticking with Friday’s forecast for a solid swell event on Tuesday - in fact, increasing it slightly - with a building trend expected all day, and set waves pushing 5-6ft+ across exposed parts of the Gold and Sunshine Coast by late Tuesday. 

Because of the structure of the cyclone, the various shadowing obstacles in its path and the reasonable travel distance, set waves are likely to be very inconsistent - and with the coastline south of Byron Bay less favourably aligned for this swell direction, we stand a greater chance of seeing a much wider variation in size from beach to beach across Northern NSW.

As mentioned on Friday, think of this as the reverse of what you would an acute south swell: some exposed headlands and points may see large waves, but sheltered northern corners (there’s an infrequently used term!) will see much smaller surf. But in general expect easing size with increasing southerly latitude. And the complexity also lies in the presence of a strong south swell too. The combined swell trains from the S and NE could see occasional rogue waves of considerably greater height at times. 

Wave heights will then ease from Wednesday morning though the early session could still see a considerable percentage of the size seen late Tuesday. This swell will rapidly transition into the secondary E’ly trade swell by Wednesday afternoon and then level out into Thursday ahead of the next increase in E’ly groundswell from the latter states of (ex-)STC Donna late Thursday and Friday.

This weekend (May 13 - 14)

Nothing major is expected this weekend, especially in context against what’s in store for the coming days. 

Friday’s solid E’ly swell will ease through the afternoon and this downwards trend will continue into Saturday. Early morning should still see 3ft sets across most open beaches but it’ll probably be down to 2-3ft by the afternoon and an inconsistent 2ft on Sunday. 

A small south swell will also push up the Northern NSW coast on Saturday but no major size is expected, possibly some 2-3ft sets at south facing beaches but smaller surf elsewhere, easing into Sunday. 

Conditions should be generally OK with mainly light winds under a weak pressure gradient. 

Next week (May 15 onwards)

Redeveloping trades are expected early next week with a possible coastal trough which suggests more short range E/SE swell for the SE Qld region for most of next week. More on this in Wednesday’s update.