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Hawaii 7-Day Forecast Graph

  • Published:02/17/08
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As of 1540 Hawaii time Saturday Feb 16:

Why does the Hawaii N. Shore forecast dashboard show Monday 8am 4-7ft faces on the while the deep water waves icon that shows the swell period and direction at 6 or 12 hour intervals shows Monday 8am 5ft 15sec 321 deg? Is the deep water icon for buoy 1's location? If it is not it seems confusing that 5ft at 15 sec would only produce 4-7 faces.

Same question for Tuesday: dashboard 8am: 8-14ft faces / Deep Water waves: 8am: 10ft 15sec 311deg. Seems unlikely that 10ft at 15sec would only produce 14 faces esp from 311 deg...

My own suspicion is that the swell won't be here until later on Monday as history has shown that Lola is optimistic about arrival times. Especially on swells that travel long distances from the originating storm...(but that's a whole 'nother story).

I scanned the blogs but didn't find anything directly on point....

Thanks for taking the time to answer, hope it's not a silly question....

Hi Bill,

First of all I think you're right in that our surf heights calculation might be slightly low for Hawaii at the moment and we are working on that correction in the dashboard, but the deep water swell heights are accurate.

A couple of other things are in play here as well... In looking at the raw data it appears for your time, Monday morning at 8:00AM, the deep water swell height was about 4.5 feet for the Oahu, North Shore Dashboard. To keep things somewhat simple and to avoid barraging everyone with too many numbers, decimals, etc., we round off the wave height numbers to the closest whole number keep it simple. But we do use the actual original deep water swell height numbers to calculate the average surf height for most spots in the area. So if the actual deep water swell height was calculated to be 4.501023', we would use that number to calculate 7.43967' faces but round it to the closest whole number of 7' for the average maximum, even though it would actually be closer to 7.5 feet. Again, we're offering everyone so many numbers we don't want to add the complexity of decimals.

Also, the growth of deep water swell heights to breaking wave surf heights is calculated by a number of factors including the deep water swell height, the swell period, the swell direction, and the local bathymetry. As a simple example, a 4 foot swell at 15 second period might result in surf of 7 foot faces, but the same 4 foot deep water swell at 20 seconds could result in surf of 12 foot faces or more in some areas. As a rule, the longer the swell period, the more the swell will grow from deep water through shallow water into larger surf, especially with swell periods 16 seconds and above. (See other postings about swell period) As always there are exceptions depending on the local bathymetry.

Secondly, you're absolutely right in the sense that there will be bigger waves in other areas. But from a large scale communication standpoint, if we forecast the biggest waves at the biggest spots for a region, that is all people will remember and we will be wrong for 90% of the other spots in the region. So our strategy is to target the biggest waves at "most" of the better spots to realize the maximum accuracy for most people. And yes, you will find slightly larger waves at the very best spots. As surfers we always seem to be stoked if the surf is bigger than forecast, but we get really pissed if it comes in even slightly smaller than forecast. I don't even want to get started on the psychology behind the work that we do...

Regarding the LOLA swell model Dashboard location, you can always locate where the exact location by clicking the Global Swell Tracker tab just below the dashboard, next to the Virtual Buoys. This will display a Google map with the exact location of the swell data point in the model.

Regarding the timing of the swell arrivals, I've been watching the swells very closely on the North shore over the past few months as we have been forecasting for all the Triple Crown events and other events, and in my experience the swell arrival times have been very, very good. You can also go look at the LOLA Archives section yourself to compare the swell model data with the buoys. The Waimea buoy is often a good gauge but keep in mind the longer period swells will sometimes show a few hours earlier up at Pipe, Sunset and Velzyland as the long period swells wrap in harder over the outer reefs toward those areas. Sometimes Waimea and the other spots further west won't show the swell until the swell period drops a little so the waves will pass over the outer reefs a mile offshore without being wrapped in further north. A little too complicated to go into here but a good option would be to always check the cameras to see what is happening versus the buoys.

Don't forget to check out the Surfline Forecast as we have more detail about the swell and surf heights in that section, beyond what is displayed in the LOLA swell model. The swell models have become an invaluable tool since the mid 90's, but they aren't 100% accurate all the time, and our forecasters always refer to the original wind and swell charts, satellite data and other information to compare their manual forecasts with the models. The Surfline Forecast will always be the last call for a swell forecast, but the models can certainly fill in a lot of areas of communication.

Hopefully this helps and you score great surf!


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