Small weekend o' tradeswell, potential solid NE cyclone swell Tuesday

Ben Matson picture
Ben Matson (thermalben)

South-east Queensland and Northern NSW Surf Forecast by Ben Matson (issued Friday 5th May)

Best Days: Sat/Sun/Mon: average trade swell in SE Qld. Fun S'ly swell early Sat across the Mid North Coast. Tues/Wed: chance for a solid NE cyclone swell. Should also see a strong S'ly swell in Northern NSW. 

Recap: Nothing amazing for the last few days. Building S’ly swell across Northern NSW and building trade swell across SE Qld. Mainly fresh winds from the southern quadrant, with isolated W/SW winds early morning. 
 
This weekend (May 6 - 7)

Anchored trades through the Coral Sea will maintain a small but steady supply of E/SE swell across SE Qld and Far Northern NSW this weekend. No great size is forecast but we should see surf around the 2-3ft mark across open Gold and Tweed Coasts, up towards 3ft+ on the Sunshine Coast. 

Moderate to fresh SE winds at times will render the beaches quite bumpy, but we should see early pockets of SW winds in some regions (low chance for this on the Sunshine Coast though, due to its closer proximity to the trade belt). 

Across Northern NSW, surf size from this trade swell source will taper off with increasing southerly latitude, however early Saturday morning should pick up some small S’ly swell from today’s building episode (which produced lovely 3ft+ sets across the Southern NSW coast). The trend will be down during the day but early morning may see occasional 2-3ft sets at south facing beaches south of Byron, with smaller surf elsewhere due to the swell direction.

The Far North Coast (north from about Yamba) is likely to see some influence from the local trade flow, so open beaches will generally be a little wind affected. But south from Coffs, the pressure gradient will be lighter so local winds will be light and variable with afternoon sea breezes. 

Smaller but similarly clean waves will then pad out much of Sunday in Northern NSW. Model guidance has a minor pulse of S’ly swell in the mix into the afternoon (0.5m at 14.4 seconds at Coffs Harbour around noon) but it’s slightly SE of S (173 degrees) so isn’t sourced from the developing low/front east of Tasmania on Sunday (the swell from this is not due until Monday anyway). The peak period of 14-15 seconds suggests a polar origin, south or south-east of the broader Tasman region. 

I can’t recall anything noteworthy in our far swell windows over the last few days of data observations, and (truth be told) haven’t had the time this afternoon to hindcast every possible region it may have originated from - so gut feel leads me to believe there won’t be much in the way of quality surf around the Northern NSW coast on Sunday. But with the model guidance of this size, it’s just big enough not to completely rule out a late pulse of S/SE groundswell south from Yamba on Sunday afternoon

If I’m wrong, and there ends up being clean waves bigger than 1-2ft before 3pm on Sunday, I’ll punish myself to a morning of brutal hindcasting on Monday morning, to work out where it originated from. Otherwise, I’m comfortable in the notion that Sunday (probably) won’t deliver much in the way of quality south swell for Northern NSW.

Next week (May 8 onwards)

Crikey - the long term surf forecast got a heck of a lot more difficult over the last few days. 

As has been discussed over the last week or so, TC Donna developed on Wednesday night and is now pushing just north of Vanuatu at the moment. It’s seen some small, but significant changes within the model guidance over the last three days, as per the chart below - this is a snapshot of every model run since Wednesday night, showing what it’s predicted for Sunday morning each time. By looking at the model's evolution this way, we can more accurately assess the broader model trends. 

In this case, we’ve shifted from a scenario on Wednesday where TC Donna was expected to remain completely outside of our swell window due to the swell shadow afforded by New Caledonia (Wednesday night's model run), to a situation where it's now a possible swell generator for many parts of the East Coast (Friday morning's run). 

The GFS model has essentially shifted Sunday morning’s position further to the NW, which is now well and truly inside the swell window for SE Qld. We do have to bear in mind that we are still a few days away from this happening and that things could very well change between now and then. In particular, there is disagreement from the other models, which have the cyclone much weaker in strength, and still inside the swell shadow of New Caledonia.

So, using the GFS-derived outlook as a baseline (as our surf model graph uses this data for its predictions), we can see that - according to the Gold Coast data point to the right - we’d be looking at the leading edge arriving on Tuesday morning (peak swell periods of around 16 seconds), building towards a peak late afternoon with surf size around 5-6ft. 

In actual fact, I think the global wave model is somewhat undercalling this event - both swell size (2m) and likely periods (give core wind strength) - so if the GFS model comes off as currently expected I think we could add another couple of feet to our model-generated surf height prediction.

But we must also factor in that GFS is the more optimistic model at this point in time. So let’s wait and see how this evenings (and this weekend’s) model runs stack up. In all likelihood I suspect we’ll probably end up with 4-6ft surf out of the NE through Tuesday afternoon, with size then easing slowly through Wednesday. It'll likely be all gone by Thursday due to TC Donna moving back into the swell shadow of New Caledonia. 

As for other sources, a building ridge across the coast early next week will generate a secondary source of short range trade swell for the region, probably 2-3ft across the Gold and Tweed Coasts Monday and a little more through Tuesday and Wednesday (slightly higher on the Sunny Coast) but with fresh, gusty SE winds accompanying, only protected locations will be surfable - and they’ll be a little undersized with the trade swell, prior to the arrival of the NE groundswell - if indeed it eventuates. 

Elsewhere in Northern NSW, this theoretical NE groundswell should likely line up within an acute part of the swell window. Most of the coast is likely to see some degree of swell from this source, but you need to consider it as you would a directional south swell - in which we often see very large ranges in wave heights between south swell magnets and protected southern corners. 

So if you reverse the specifics, you’ll find the same outlook for Northern NSW - a handful of well placed NE swell magnets could pick up 4-6ft sets (later Tues/Wed), but most locations will be considerable smaller. 

And it only gets more complex from there: we also have a strong Tasman Low developing overnight Sunday that will generate a concurrent solid S’ly swell for south facing beaches in Northern NSW. This could supply an equal level of S’ly swell (4-6ft south facing beaches) through Tuesday and Wednesday. Monday should see a late building trend across the Lower Mid North Coast. 

So, with two potentially solid swells from opposing directions through Tuesday and Wednesday, your job will be to find somewhere that works with the expected local winds (prevailing southerly all week, early W/SW in many locations, though fresh SE at times in the Far North) and scout out some fantastic A-frames.

How’s that for complexity?

Have a good weekend, let’s take a closer look on Monday.

Comments

donweather's picture
donweather's picture
donweather Friday, 5 May 2017 at 7:31pm

Isn't your model combining the NE groundswell and E/NE Tradewind swell on Tues 6pm in your image above Ben?

thermalben's picture
thermalben's picture
thermalben Saturday, 6 May 2017 at 7:45am

Possibly Don - though the WW3 model actually outputs six individual swell trains (we only display the "top" three, as it's rare that there are more than three important concurrent swell sources).

As such there's a very real chance that the E/NE trade swell has been shunted to number four by the model (replaced temporarily by the S'ly groundswell from the Tasman low), and that it's still actually in the mix. We have archive logs of this so I can check when I get back into the office.

But, the best way to consider this, is to estimate whether the potential combined numbers look extraordinary or not: yes, the jump (in Hsig) from 12pm to 6pm does raise a red flag, but the easing trend through Wednesday is plausible (and, the E/NE swell is back in the mix, at a similar size as before the arrival of the NE swell, so it's not 'combined' thereafter). The arrival times and phases are within expectations too.

Also, this is a Cat 3-4 cyclone with potentially very good alignment for our region - so the fact is that we SHOULD expect a rapid J-curve on the upwards phase of the swell event. 

So to me, the model is fine in this instance. 

Sprout's picture
Sprout's picture
Sprout Saturday, 6 May 2017 at 9:28am

Busy crowd for 2-3ft bumpy swell bit random. Surely the locals are due a good quality mid-week swell by now, come on Huey.

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 9:27am

Almost every satellite pass missed the core of STC Donna over the weekend. But yesterday evening we finally got some data form the eastern flank of the low.

It's right on the periphery of the swell window, and could go either way - it's now Cat 4 (and expected to reach peak strength today) - I'm hanging in there with my expectations of 4-6ft surf, though it looks like it may be a little delayed (i.e. not start building until later Tuesday, peaking early Wednesday). Will give the model guidance a comprehensive runover this afternoon.

donweather's picture
donweather's picture
donweather Monday, 8 May 2017 at 9:53am

I was watching each ASCAT pass with anticipation over the weekend also Ben and like you we only got this one above to go by. Gonna be very tough to call as the models won't have any real time data to calibrate to so they'll be relying on their wind forecasts rather than actuals. Also, the swell window is insanely small given those reefs off New Cal NW coast and the GBR off our coast too.

I still think the models are double counting the two swells for Wed, so I'm reserving my call to more like 3-4ft+, with the 4ft+ being the sets.

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 10:02am

There were also big differences in model initialisation over the weekend (I saved the images on my home computer, will see if I can upload them tonight). However EC and GFS were both pretty close. 

donweather's picture
donweather's picture
donweather Monday, 8 May 2017 at 11:52am

But if there's no ASCAT data what are they using to initialise?

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 12:12pm

I'm referencing the GCMs at a global level. Every ASCAT pass will miss some parts of the ocean, in this case they just didn't line up with the eye of the cyclone.

donweather's picture
donweather's picture
donweather Monday, 8 May 2017 at 12:14pm

GCMs?

Wow, she looks to have moved super quick over the weekend. ASCAT B latest pass now shows her well behind the swell shadow of New Cal!!!

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 12:19pm

GCM = General Circulation Model (just a term I use to summarise all models). 

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 9:39am

Also, much less chance of a spread down the NSW coast from this system. It's going to be really interesting to see how much size makes it south of the border.

Josh385's picture
Josh385's picture
Josh385 Monday, 8 May 2017 at 9:43am

Hey Ben, is it possible to get the forcast notes built into the Swellnet App? Thanks

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 10:00am

Will do as we overhaul the App in the near future. 

Josh385's picture
Josh385's picture
Josh385 Monday, 8 May 2017 at 10:51am

That would be great

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 10:36am

Nice lines at Currumbin.

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 10:53am

Snapper! Looking nice.

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 12:52pm

Wow, latest wave model is resolving this swell a lot better. Looks like it could easily come in at the upper end of my forecast expectations (6-8ft), peaking on Tuesday afternoon too.

The leading edge of 15 seconds is expected around 6pm, though given core wind speeds over the weekend I wouldn't be surprised if it registers higher (i.e. 18 seconds) and as a consequence, arrives earlier.

donweather's picture
donweather's picture
donweather Monday, 8 May 2017 at 2:01pm

Which location is that for Ben? Your Fraser Island model output clearly shows she's double counting the tradewind swell and NE groundswell on the Wed. I can't post the latest model output for Fraser Island here....maybe you can to show what I mean.

https://www.swellnet.com/reports/australia/queensland/fraser-island/fore...

But it's not double counting on the Tuesday that's for sure and has upped the anti on the NE groundswell that's for sure. Albeit this location is closer to the storm and also well within the swell window compared to those locations further south.

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 2:09pm

My table above was for the Goldy.

I reckon the Fraser Island data may not necessarily be doubling up the trains but could be that the NE tending E/NE swell train has fallen to position number four in the model output. However Fraser Island will certainly see a slightly earlier/faster easing trend than the Gold Coast too (which is reflective in this output). 

thermalben's picture
thermalben's picture
thermalben Monday, 8 May 2017 at 1:59pm

Just a size reference - 'cos if we see the NE swell kick later today (or tomorrow) it'll probably show at Noosa before anywhere else. Currently one foot of glorious logging energy.

crg's picture
crg's picture
crg Monday, 8 May 2017 at 2:39pm

Bring on the Monday night forecast notes!!

udo's picture
udo's picture
udo Monday, 8 May 2017 at 3:41pm

Nearly head high sets at Agnes