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Why did the last SSW swell arrive so much later than most forecasts had shown?

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Adam Wright
by
(Tuesday) 6.11.13

Kevin Asked: Hey Adam, Curious what your take is on why the swell arrived so much later than most forecasts had shown. It looks like the swell periods were about as expected, so why did the swell travel so much slower to get to SoCal?

Adam Wright – Solspot Forecaster:

Kevin that is a pretty good question…in fact, while the swell was first arriving Austin, (my forecasting partner in crime), and I were having some pretty good conversations about that exact thing.

The funny thing about these sort of swells, particularly when they are “running late”, is that there are actually a lot of reasons why/how it happened this time. Depending on how you look at it some of these reasons are rooted directly to the physical world (the one that we can measure, track, and eventually break down into some sort of hard math/science) and some that are related to our own perceptions and expectations of the swell that are little harder to pin down.

In this case I think the main issues were mostly physical ones that slipped into a couple of blind spots that can affect Southern Hemi swells. The short answer is that the storm, while nice and strong, appears to have taken a more northerly track at first, which pushed the main focus of the initial energy toward Hawaii, but more importantly kept the best part of the storm’s fetch (for the West Coast) shadowed by the South Pacific Islands. This error in tracking was compounded by the forecast charts that were predicting the center of the storm moving into a more open area of the ocean…so while we were seeing all of these great winds/sea-heights the storm wasn’t quite lined up for the West Coast.

The more detailed “why do I think this is most likely the case” is based on how the swell behaved as it first arrived…the very first part of the swell came in with some very longer-period energy that started to arrive late on Wednesday…it wasn’t very big or consistent but it was registering on the buoys that evening.

That is a pretty common behavior for a big swell that has tried to push through the SPAC island shadow…you will still see long-period swell arriving when it is supposed to but the ‘guts’ of the swell aren’t there to really flesh out the surf. Another very annoying property of swells like these is that they almost always trick the computer swell models…the models are built to factor in things like swell-height and swell-period in order to get deepwater swell heights, but when you have a swell that is made up almost entirely of long-period energy but none of the supporting lower-periods it gives the computer a false positive. Sometimes the better models will recognize this is happening and will adjust, but honestly I think it miss-calls it 90% of the time on the bigger swells. (like this one).

Fortunately by Thursday we did see a real change in the swell…the periods were still very long, indicating the storm was still very strong at the time this portion of the swell was formed, but you could tell this energy was coming from a more open part of the swell window. The swell direction shifted slightly more to the SSW and there were some good-sized waves hitting by mid-morning. From there the swell behaved about how we thought it would, (just about 12-18 hours behind), building steadily through the day and eventually peaking on Friday, holding into Saturday.

A follow up question to your original one is ‘knowing that there are blindspots can’t you compensate for those and keep the forecast on track’…my short answer to that one is yes we do, but we are limited in a lot of ways due to lack of information from that part of the Pacific.

One thing to keep in mind is that is much easier to do a ‘post-op’ on a swell…we have all of the data at that point, everything from satellite information right on down to photographs/videos of the waves hitting the beach. When forecasting a swell from this part of the Pacific you have to work with a variety of limited resources. There is almost no land in the South Pacific (except for a handful of scattered islands), so you rely heavily on satellites, ones that only measure a fraction of the millions of square miles of ocean that you need data from…and even that information is compiled in a way that it is hard to break it out into a consistent timeline (to measure the lifespan of the storm). You also need to use the computer models to help with some of the guidance…unfortunately when a storm doesn’t behave the way the ‘forecast model’ thinks it is going to, you get an error that creeps in and starts to corrupt the accuracy. When you get down to the core of it…it is basically “bad data in….bad data out”, we human forecasters try and watch for these errors but are hamstrung by the lack of measureable information coming from the region.

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