Anatomy of a 22 second Swell

You don't often see wave periods in this end of the scale. This is one of the longest period I have ever seen. It started travelling three days ago from a storm that formed rapidly at a location just north of the Georgia Islands. It reached 948mb at its centre during the peak of its formation around 8am on Sunday.

The end of the fetch was roughly 2,350 nautical miles SW of Cape Town. Wind speeds reached hurricane force. A forerunner swell radiated from the initial cyclogenesis at 23 seconds. If you extrapolate the swell speed, it arrived during the day Wednesday, three days later.

The main part of the swell was a 50ft pulse that followed soon after, but by the time the 22 second swell arrived in Cape Town after 72 hours travel time, the 'brunt' of the swell was lagging eight hours behind the forerunners, and had decayed in size by at least half, if not more.

That would mean - in simple terms - the swell reached 20 feet at midnight to 4am this morning, but began subsiding immediately afterwards. The crew that hit the surf at Dungeons early this morning had a few 'maybe' 18 foot sets with long gaps. These continued to grow smaller. By 11am, the sets were down to 12 foot with half an hour lulls. 

From the hindcast of this storm, it appears it didn't last long enough. The pulse was shortlived. Forerunner sets can be really tiny, due to extreme winds and therefore a lot of energy transfer, but not enough time on the sea to really create a massive swell.

Mark Sponsler, the man behind www.stormsurf.com said that the longest period he had seen was 33 secs: "Really it was 33 sec energy that was part of the leading edge of at 25+ sec period swell. There was a few forerunners in the 1-2 ft at 33 secs range. This was back in the early 2000's here at Mavericks".

Yesterday, the forerunner swell was barely noticeable. Cape Town was flat in the morning. As the day wore on, a few 2-3' sets with 45 minute lulls began to pulse in the arvi. You may have thought it was flat unless you examined it with patience. 

Distance and travel time had decayed the swell, and it was smaller than you might think. Slowly but surely, the sets gathered momentum but the brunt of the swell only arrived after dark towards midnight. By 8pm in Camps Bay, I saw a 10ft set chunder into Glen Beach. I counted a 20 second period at that point. 

The surf appears to have peaked after midnight. Due to the shortlived life of the storm - it only lasted 24 hours - this was a short pulse, and the meat (the peak) probably came through with at least 20ft sets but only during a 4-6 hour period and it looks to have between midnight and 4am, and then it was gone.

Big waves can be funny like that. After that, it was a waning 15ft swell with extremely long gaps, that flattered to deceive.