The Briny Bar - How Does Swellmap Work?

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How Does Swellmap Work?

Printed From: The Fishing Website
Category: Saltwater Fishing
Forum Name: The Briny Bar
Forum Description: The place for general chat on saltwater fishing!
URL: https://www.fishing.net.nz/forum/forum_posts.asp?TID=119822
Printed Date: 04 Nov 2026 at 9:39pm

Topic: How Does Swellmap Work?

Posted By: Kevin.S
Subject: How Does Swellmap Work?
Date Posted: 22 Aug 2016 at 2:27pm

I understand the information it gives, but I'm puzzled by the red-amber-green rating that it gives to conditions. Does anyone know how these ratings are calculated?

For example I was checking the forecast on the Manukau bar yesterday morning. There was a 1m swell with a 0.4m chop and a total sea of 1m. The wind was NE11 gusting 14. The rating was 3, which is "poor to dangerous". Later in the week the forecast was for a 1m swell with a 0.6m chop and a total sea of 1.2m. The wind was NE 8 gusting 10. The rating was 7, which is "calm waters".

In the example I find it hard to believe that a difference of 3-4 knots in wind speed takes conditions from dangerous to flat calm.

Incidentally I happened to be at Hamiltons Gap taking the dog for a walk yesterday afternoon, and would rate the sea as about the flattest and calmest I've ever seen it. Hardly even any breaking waves on the beach.

Replies:

Posted By: OneWayTraffic
Date Posted: 22 Aug 2016 at 3:39pm

It's a bit hard to figure out truly. Met Service does the same. Rating of 1-3 for 10 knots and no swell in the Firth of Thames.

For bars better to look at Swell, tides and wind?

Posted By: strx7
Date Posted: 22 Aug 2016 at 3:47pm

Met service and swell-map are done on the same forecast, and over the past couple of years I've concluded it's abit of a cr@p one. I tend to look at 4-5 forecasts to get a better idea of exactly what is to be expected.

Posted By: Kevin.S
Date Posted: 22 Aug 2016 at 4:01pm

The actual forecast, 1m sea and 11kn wind, seemed about right. I just don't see how they rate that as "poor to dangerous"

Posted By: REIVER
Date Posted: 22 Aug 2016 at 4:10pm

Check that you are looking at the boating forecast and not the surfing one. Been caught out myself on that

Posted By: Steps
Date Posted: 22 Aug 2016 at 5:13pm

In the example I find it hard to believe that a difference of 3-4 knots in wind speed takes conditions from dangerous to flat calm

Did you check the tide/ current direction.. you will often go thru the forecast compare wind and the swell chop/ periods .. no changfe in wind but the swell drops to nothing
Then check if the tide changed

Same applies to the Furth

Posted By: smudge
Date Posted: 22 Aug 2016 at 6:40pm

I use swellmap for Manukau bar crossings and like any forecast it is a guide more than anything. I look at both the harbour entrance and the shelf break. I look at the wind forecast and use the tide times to determine when to cross. Tidal flow is important too. For instance I wouldn't want to cross an hour or two into an outgoing tide on a 4.4m tide especially if there is any sort of swell running. Swell and wind direction all play a part too Kevin. The period between swells also comes into play and the longer the period the more energy is stored up in those waves. Some think the bigger distance between waves gives you more room for error but I see it as more potential to get a big green wall of water standing up either behind you or in front of you. Either way that is scary.

So I guess you can see that it isn't only the wind that they base their rating on. Having said that, I take the factors into account and take those ratings with a grain of salt. I've been out when it's three and been shunted around when it's six.

Most importantly I check for waves breaking at Puponga Pt and if there is significant white water on the western side that's a good rule of thumb that the bar will be a little messy. Two weeks ago, everything looked reasonable but we went right up to the shallows and sat and watched. There was a lot of white water across the south channel and were in two minds as to whether to cross. When the tinny we watched smashing its way out turned around and came back in (same size as our boat with only 2 pob, we made the call to fish inside. Pretty disappointing as we were on a mission, but at least i still have my boat in the garage and sitting here able to pass on my thoughts.

If we were an hour earlier (it would have been a bit dark) the bar would have been fine. The wind picked up pretty strong that day and despite crap fishing in the harbour, two weeks later I have no regrets.

Remember back to when I crewed on your boat on the Coastguard course? They said there to stick to a couple of websites and learn the quirks of each one. I'm a strong believer in that.

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Best gurnard fisherman in my street

Posted By: Derek F
Date Posted: 22 Aug 2016 at 7:43pm

Posted By: green guy
Date Posted: 22 Aug 2016 at 9:13pm

The Answer is it don't

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http://www.facebook.com/hauraki.hillbillys

Posted By: Steps
Date Posted: 23 Aug 2016 at 7:53am

We have been watching these for years and find the best result for us is to go from the isobars on the TV weather maps and watch for general wind directions and trends (ie will wind be be dropping or turning during the day).

Yep..closer the isobars faster the wind...for hauaki as general rule of thumb, if wide enough (in any direction) to be sth of Taupo and Nth of the tip of Nth island, its a go... subject to tide and current direction and strength.
Have current against wide ans can chop up bad.. tide changes and next thing you know it will be dead flat.

Smudge also illustrates local knowledge

Most importantly I check for waves breaking at Puponga Pt and if there is significant white water on the western side that's a good rule of thumb that the bar will be a little messy.
Smudge also illustrates local knowledge
small waves breaking on the ramp at Maraetai means the outside of the island bottom end will be uncomfortable to say the least.

Posted By: hookerpuka
Date Posted: 24 Aug 2016 at 12:55pm

Some forcast worst case scenario like Metservice and Swellmap, while others forecast best case scenario. One thing that stands out is fishermen always like to go off best case scenario and complain when the worst case meant they called off a fishing trip when they could have gone, I'm sure most would agree that erring on the side of caution is a good thing. Met service use a system of grading. which is found on the warnings map. It runs by a percentage of deemed accuracy, from a 30% chance through to a high chance which I think is 70%... People seem to forget that all of them are a prediction. Isobars are great but you have to understand the ISO charts and also if the system is deepening or receeding, otherwise it's like sticking your finger in the air in the morning and expecting the weather to stay the same all day...

I use metservice, Predictwind, Swell map and Metvuw in conjunction with Northport live weather feed to see which one has it right on the day... Good for Whangarei area but not so good for others although Coastguard app does the same thing

Posted By: maxtarakihi
Date Posted: 24 Aug 2016 at 3:23pm

I have been using predictwind.com and find it way more accurate than swellmap for the BOP.I compare the two every trip and find and if they differ much I tend to trust predictwind as it is more pinpoint (you can put in spots on the map where you want to fish)than swellmap

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