Winter, How's it looking

[oldman]

I'm looking for a nice little 2' from late Monday to Wednesday for central coast.

Could be a bit later but I'd be disappointed if there wasn't a little something in the water on Tuesday from the east.

I've got just the beach for that one. Wonder how the banks are doing?

[SAsurfa]

I'm using GFS Old's.

The low is currently predicted to form mainly to the north-east of the North Island with only a brief burst (12hrs or so) of 30-40kt+ winds within our swell window.

This would give Wednesday's swell that may be seen late Tuesday, but before this I think you'd be lucky to get much above 1 to maybe 2ft.

South swell looks like the one to keep an eye on1

[Donweather]

OK, so now I'm confused Craig. If you're using GFS, can you explain to me how an open ocean swell of 18-20ft just to the N/NE of NZ late Sunday/early Monday correlates to 2ft, perhaps 3ft in Sydney next Wednesday? My calcs are indicating a 5-6ft@12 sec E/SE groundswell to arrive in Sydney next Wednesday.

And I'll be the first to put my hand up and acknowledge I'm no expert when it comes to forecasting south swells in Sydney, but for me, the forecast south swell next Wednesday looks way to west of south for any real significant (one to watch) impact on the coastline, other than offshore south facing bombies. Once again, this is based on latest GFS forecasts (and EC looks even more W/SW fetch orientated)....but I'm happy to be proven wrong on this as I don't have the experience on these south swells in Sydney.

[SAsurfa]

Have you looked at the winds around the forecast low Don, I never look at the open ocean swell created at the source of a system.

If you look at the source winds you'll understand why Sydney won't pick up much of this swell at all.

And after forecasting Sydney for 2 years or so I've got a pretty good idea on how these fronts that look like they're pushing away from us perform around Sydney (it's pretty amazing actually).

Have a look at the width and strength of the fetch, should see a lot of swell spreading radially out from this progression and persisting into Thursday/Friday and even the weekend.

[Donweather]

Craig, I look at both wind and wave models and run my own numbers to verify or otherwise.

If we take your numbers above, of say 12-18hrs of 30-40knt+ winds, let's take an average of 15hrs@35knts, and noting the fetch length isn't too shabby either (around 200nm), then once again hand calcs would indicate an open ocean swell of the order of 18ft is not out of the question.

And as for the south swell, I'll take your word on that as you've got way more experience in that department than this black duck!!!

[oldman]

Thanks gents,

I'm thinking, sure a little hopefully, that the nor east fetch lines up for a while before the low actually forms, and that should be enough to send a little something on its merry way before the main part of the swell arrives, ergo, hoping to see some signs late Monday.

It does appear that the north island NZ cuts off the low pretty soon after it gets serious.

As for the south swell, I'm with Donw in that it has a lot of west in it, but by god it's a bit of a monster, so I would expect a fair amount of refraction/radial spread that will last for a few days, and maybe a fair bit more. The centre of the thing gets down below where my charts cut off which are half way to Antarctica as it is. Lord help me but somewhere is gonna get some waves from it. No doubt Craig looks at more maps, and much better ones, than me.

Plus it's his job!

I've got a pretty good idea on how these fronts that look like they're pushing away from us perform around Sydney

I think I know in particular some of the recent ones that are fresh in your mind. I've got to say that one of them, which was declared a hoax, really had bugger all fetch to it and so was never going to do that much in my mind. And another one which I reported to Craig the swell event that occurred up at the Central Coast where I was, which was quite different to the reports taht were being generated for Sydney.

I'll let you boys know how it goes.

[SAsurfa]

Unfortunately the outlook is even less inspiring off the low above New Zealand now in 00z with the winds on the south-eastern flank being weaker and I wouldn't expect much above 2ft now

Also the S'ly swell on Wednesday has been downgraded slightly, but the secondary pulses on Thursday and Friday still look good.

Still a long way out on these S'ly swells though.

[Donweather]

Yep, fecking GFS has killed the fetch now in the 00z run.

And yes, its also delayed the south swell now to something better come Thurs and Friday next week, but if you ask me, when GFS starts to push out (in time) the peak of the fetch then from previous experience that tends to indicate either it isn't going to happen or it will be severely diluted.

[Donweather]

12z run looking a wee bit better than the 00z run last night, but I do agree that the GFS model has now downgraded the fetch (more so on the SW/S flank) and pushed the core winds to the back of the fetch (SE flank) more aligned with what EC was showing yesterday.

So this has a two fold impact.....delays the arrival of the swell to Wednesday now and reduces the swell heights. So I'm thinking something in the order of an inconsistent 3ft/3ft+ now, with potentially the odd 4ft bomb at the super exposed swell magnets. This is for SE Qld. Sydney looks to be a similar size in the 3ft mark, but probably not until later Wednesday and more so into Thursday. (Note my previous comments about GFS pushing back the developments of the fetch tending to indicate it's not going to happen or be downgraded!!!).

So yes, I'm now eating humble pie!!

[Donweather]

Craig, Steve, anyone else.....do you guys know if the swell models consider water depths when evaluating swell decay? For instance, is the water across the top of NZ shallower and do the wave models take this into consideration. My gut feel is no, they don't, as this would just complicate the calculations to the next level.

If they don't, I'm still a little confused as to why the wave models are showing such small swell from the E/SE to hit Sydney compared to say Nth NSW/SE Qld. My calcs, including great circles over the top of NZ, are still showing Sydney to be bigger than 1-2ft from the E/SE on Wednesday (probably later on Wednesday now) and briefly into Thursday morning. I really can't see why it wouldn't be seeing the same swell heights as expected to hit SE Qld/Nth NSW, perhaps a touch smaller....so I'd be thinking something in the 3ft/3ft+ class come later Wednesday and briefly into Thursday for Sydney. Of course this is also likely to coincide with some of the S'ly groundswell, albeit very west of south on Wednesday and Thursday, hence only likely to be seen at the most south facing swell magnets and offshore bombies.

Actually, in saying all of that, I think the wave models have also undercooked the E/SE swell forecast for Wednesday afternoon and Thursday in SE Qld/Nth NSW also.

[SAsurfa]

They do Don, and it's a bottom friction calculation from memory. Also there are obstruction grids ingested into the model which account for sea mounts, or land masses that are smaller than the models resolution.

[Donweather]

Serious!!! Wow, I'm impressed. So in saying that then, does anyone know how deep the waters just off to the north of NZ are?

[alakaboo]

http://www.niwa.co.nz/our-science/oceans/bathymetry

Don, all the bathymetry you need should be on that page. Big files and images so I can't show them.

[Donweather]

Thanks alakaboo. Greatly appreciated.

Looks like the west norfolk ridge could be a killer for those fetches then!!!

ftp://ftp.niwa.co.nz/bathymetry/NZBathy ... _image.jpg

Anyone know what the contours are measured in, in the above image? ft or metres below sea level?

[alakaboo]

they're in metres Don.
NZ's been metric for a while.

[oldman]

Aye carumba,

By golly, that low pressure system under east OZ is a freaking monster, and looking at the latest runs will be sending swell up the east coast for days, and I'm expecting fairly large swell at that.

But we will only be getting the edge of it, somewhere is going to get some outrageous swell in the coming week or more.

[Donweather]

But we will only be getting the edge of it, somewhere is going to get some outrageous swell in the coming week or more.

And that would be NZ, Fiji and New Cal. As per usual, most locations to the north of Cape Byron will seriously struggle to see any of this swell, although the period looks sensational so that might assist with something surfable in SE Qld.

[Grooter]

Winter is looking pretty farken good for Victoria at the moment

Wave buoys in Bass Strait are reporting a 3+ metre average swell today and it the Sorrell buoy has been climbing since Monday too.

Winds may kill all the fun but protected locations are going to be good. I might avoid the weekend if I can, swell events like this pull them all out of the woodwork