Big Sunday Swell Periods
Posted on 02/27/08

Hello,

Regarding the big swell last Sunday, I was wondering how it could have such a long interval if it developed
only 600 miles off the coast?

I was checking the California Buoy on Saturday night and it jumped up to 34ft @ 20 seconds in a matter of an hour
It had 20+ second intervals on all the Local East and West Channel Buoys when it started to hit on Sunday.

I was under the impression that it would need a longer fetch of wind to create those longer period swells?
I realize the wind speeds were significant, but this one seemed to instantly jump up to long period swell in a really short amount of time.

Any info would be appreciated as it is interesting to figure out, especially on these larger swell.

Thanks, Mike

 

Mike,

Sustained 60 knot+ winds in a storm only need about 20 hours and a fetch of about 600 miles to generate a massive swell with the peak swell energy the 20 second period band. Sustained 45 knot+ winds in a storm may need 48 hours and a fetch of over 1,000 miles to create a similar swell. As a rule, the harder the wind blows, for the longer time, over the longer fetch or distance of ocean, the more energy that is transferred into the ocean from the wind - which results in larger waves developed and longer swell periods between waves. It's a sliding scale. Also keep in mind that there is also tremendous swell energy in the surrounding 12-22 second period bands as well. All of this wave energy combines to create an estimated "significant wave height" at a specific location, which is comparable to an observer noting the highest 1/3 of the highest waves during a period of time.

As discussed in our earlier blogs about how the buoys measure swells, they will report the combined significant wave height and the "single dominant swell period band" of the entire swell spectrum. So in the case of the California Buoy on Saturday night, a report of 34 feet at 20 seconds does NOT mean that it is a true single swell of 34 feet with a swell period of 20 seconds. At that buoy during that report there was a significant 15 foot south/southeast swell also running that was generated by the pre-frontal winds of the storm. When the west swell began to also arrive at the buoy, the significant wave height jumped up to 34 feet, but from the combination of opposing swells that were actually going against each other.

Once again, the single dominant swell in the water at the buoy was the west swell that was in the 25 foot range with the peak energy in the 20 second period band. As the storm was so close to the California Buoy, it became practically impossible for the buoy and LOLA to separate the different wave trains from each other in the midst of the extremely stormy seas. Once the swell was able to travel outside of the storm area however, the buoys closer to the coast like Harvest 071 attached, did a good job of separating the wave trains so LOLA could pick out the exact height and periods of the individual swells.

Once again, this is how Surfline's LOLA is unique in how we display the buoy and swell model information. Significant wave height and dominant swell periods are confusing numbers, and as surfers we need to know exactly the size and period of individual swells so we can accurately forecast surf heights everywhere along the coast.

Hope you scored! Sean...