18 Sec Period for Sydney

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S4MS
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18 Sec Period for Sydney

Post by S4MS » Wed Apr 10, 2013 11:05 am

3ft, 18 seconds predicted for Sydney on Saturday by coastalwatch and magicseaweed? Pretty rare to get that sort of period in Sydney, isn't it?

What does this mean for my Bronze Medallion wet assessment? :shock:

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steve shearer
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Re: 18 Sec Period for Sydney

Post by steve shearer » Wed Apr 10, 2013 4:15 pm

not that rare. It's just with more eyes upon measurements we're becoming more aware of period signatures.
Most long period swells in the Tasman are highly refracted though, which is why most of them are small.

Last years Fiji swell had swell periods of 21 seconds here on the North Coast.
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godsavetheking
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Re: 18 Sec Period for Sydney

Post by godsavetheking » Wed Apr 10, 2013 4:33 pm

I don't know if it's the same for you as it is for us, but magicseaweed's period forecast broadly corresponds to the dominant rather than the mean period. so on a decent swell the dominant is typically around the 18 or 19 second mark whereas the mean (for us anyway) rarely gets above 11
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steve shearer
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Re: 18 Sec Period for Sydney

Post by steve shearer » Wed Apr 10, 2013 6:49 pm

sorry mate but that makes no sense.

A swell will have a period signature .....that signature will change over the course of a swell but it won't be between 18-19 seconds and 11 seconds at the same time.

I think what you mean is a dominant swell period signature and a secondary swell period ie a lower period windswell component.

Of course you can have multiple swell sources running but they will have different swell period signatures and different directions.
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godsavetheking
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Re: 18 Sec Period for Sydney

Post by godsavetheking » Wed Apr 10, 2013 7:13 pm

Yeah of course you're right. What I was getting at is that, for us, if the buoy you mainly rely on only records mean period (as our main indicator one does) you're unlikely to ever see recorded the periods that magicseaweed shouts about.

Our secondary buoy that's tucked closer inshore records both the dominant (which "corresponds to the period of the larger waves that occurred during the measurement time period") and the mean. Today's readings give a dominant of 12ish secs and a mean of 8ish, the 12ish seconds being broadly in line with what msw was forecasting for today. For me, the dominant period is an interesting extra piece of info but the mean period (plus swell height obviously) is what I use to extrapolate at-the-beach wave height
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Mon Sep 04, 2023 10:48 pm
To tell the truth eggs are one thing I avoid buying when I’m grocery shopping. So you may say I’m an intermittent egg buyer who usually lets someone else buy them.

Nick Carroll
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Re: 18 Sec Period for Sydney

Post by Nick Carroll » Thu Apr 11, 2013 6:58 am

magicseaweed, huh.

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Re: 18 Sec Period for Sydney

Post by Donweather » Thu Apr 11, 2013 9:12 am

Nick Carroll wrote:magicseaweed, huh.
Well to be quite frank Nick, I've found MSW to be way more accurate of late than Surfline. Particularly since Surfline upgraded their LOLA forecasting display, I've found it to be over estimating wave heights considerably, even in the 1-3 day forecast range.

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