Summer. How's it lookin?

[oldman]

This one is locked in the memory bank and will be recalled on in the future

Absolutely Craig. A very interesting swell (non) event.

I can't recall seeing one like that that peaked well below expectations and disappeared like it was a locally generated windswell.

Interesting, as Spock would say.

[Donweather]

Olds, it looks as if today's secondary pulse is missing Sydney and hitting Newcastle, most likely due to the period of the swell steering it away from the Sydney region. This is backed up by the reports with 1-1.5ft waves in Sydney and 3-4ft surf up at Newy, also the Mid-North-Coast is picking up more than us.

Craig, interested to hear a little more as to what you mean by "most likely due to the period of the swell steering it away from the Sydney region"?

[SAsurfa]

I think this has been discussed in here before and on Swellnet but basically when you have different swell periods, they "feel" the ocean bottom at different depths and depending on the period, some swells may be 'amplified' towards certain regions due to the bottom topography, while others may be steered away.

[Kunji]

So are you saying that the swell needs to be a certain length from a certain direction to feel the sydney breaks? If so, what are the numbers?

[2nd Reef]

We don't really know that information Coops. As it stands it's only a theory that the size discrepancy is being caused by nearshore bathymetry. Yet, as it's a regular, well-observed occurrence it seems it has to be due to the ocean floor (and not, say, the swell-producing properties of any one storm).

The most obvious example is longer period (10 sec+) south swells and how they hit certain regions much harder than others. Shellharbour seems to cop it, while Wollongong and the Coal Coast don't get as much. Cronulla goes off, while the Northern Beaches goes without. Newcastle also reacts well to those swells.

Each of those regions has a north-south running coastline and, at least on paper, they should all get as much swell as each other.

It's been a theory brewing and it came to the boil during the Big Saturday swell that saw Vooey and Barrack off tap. That swell had periods of approx. 14 seconds so the effect was amplified again. Voodoo and a few other Bate Bay reefs were 15 foot while the largest wave on the Northern Beaches was probably 8-10 foot at Long Reef. Makaha was 8 foot. Newport Reef was a thick six. RP's smaller again and Queenscliff Bombie didn't break. It just didn't make sense...

Hence the only viable explanation at this point (and remembering it's been something we've observed for a while) is that it's due to bathymetry: certain areas have bottom contours that are conducive to attracting and steering longer period swells to the coast. The long period wave energy is travelling deeper in the water so it feels the floor earlier.

The answer to your question is that we don't know what the optimum period or perfect direction is, however it's something to be aware of the next time a similar swell shows on the map. Of course it would make sense that swells from every direction are susceptible yet we don't get enough north or east swells to really observe what locations operate best and how they compare.

Waves Misbehavin' article here

[pinhead]

Here's an example of how water depth effects refraction in Victoria. The pic shows tide times overlaid on the swell direction read out from the Pt Nepean Bouy. You can see the swell comes in more from the south about an hour after low tide and more from the west about an hour after high. It would seem the swell is refracting around Cape Otway to the west and bending in more when the tide is low.

[Donweather]

I think you're clutching at straws if you're trying to compare the swell earlier this week to the solid groundswell we saw earlier this year, both in terms of swell being deflected by undersea topography. The swell last week was a locally generated near shore system with swell periods not much greater than 9-10 seconds (in the Sydney region). The monster Antarctic swell was a pure highly unusual east coast groundswell exhibiting swell periods in excess of 14-15 seconds.

Two extremely different swells in my opinion. Yes one will be highly affected by undersea topography. The other would hardly, if at all be affected. The fetch/swell direction for last week's swell was the main reason Sydney missed out. Pure and simple lads.

For example, a 10 sec period swell will bottom drag at approximately 80m. A 14 sec period swell will bottom drag at approximately 160m. Massive difference in my opinion.

[Donweather]

Looking at the compact fetch around the Tasman low (ASCAT) and hindcast models, I reckon Nth NSW might just see something sneak under the radar on Saturday. Could be something in the inconsistent 3-4ft class at south facing beaches me thinks come Saturday but perhaps not at dawn.

http://manati.orbit.nesdis.noaa.gov/win ... 0_noaa.png

Might be a little hard to decipher in the E'ly windswell junk mind you?

[Donweather]

SE Qld Weekend Warriors, here tis:

http://www.surfphotos.com.au/weekend-su ... ary-8th9th

[diggerdickson]

Are you referring to the onion ring Tasman low, Craig? I wouldn't have said it's a done deal just yet. Still some reasonable model divergence in the location and deepening of the low next Monday/Tuesday from what I can see?

ahhhh, ive got monday off to Hey trev, you not in town are you, if you are might come down mate, let me know

[Donweather]

Ummmm, digger, you're about 2 weeks behind with that post of mine you've quoted.

[Scroty]

I was just checking seabreeze and was wondering if someone can explain the following to an unlearned one such as myself.

Looking at the 7 day wave direction and period forecast it has:
Newcastle between 5-8 feet and 7-9 seconds period from the east
Pt Perp between 5-7 feet and 7-9 seconds period from the east
Sydney between 2-4 feet and 7-9 seconds period from the east

Why is sydney forecast to be so much smaller?

[Donweather]

Why is sydney forecast to be so much smaller?

From my observations it appears to be an error as other wave model websites that use the GFS input data are certainly not showing the same disparity for Sydney.

[daryl]

that size sounds fine for an ol fella
but geez the period seems tooo low

[SAsurfa]

Scroty, that is just raw model output from GFS and it looks as if they have tweaked the Sydney region to represent what the surf heights will more than likely be at the beach.

But where is that Sydney point, because if its a general point for open east facing beaches that's fair enough, but say you get a south swell, Bondi might be 4ft and Manly 1ft. So that reading would make no sense to the general user..

[diggerdickson]

Ummmm, digger, you're about 2 weeks behind with that post of mine you've quoted.

Oh well, I think I know somewhere that will be okay, the swell seems to be coming up anyway, monday the go I tell ya don, Im just being optimistic

[Donweather]

Scroty, that is just raw model output from GFS and it looks as if they have tweaked the Sydney region to represent what the surf heights will more than likely be at the beach..

Craig, I didn't think seabreeze was this sophisticated and if it was, why have they only just done it for the Sydney point?

[Donweather]

Start praying people!!!

http://www.surfphotos.com.au/please-huey-please