Random Swell Period Question

This was actually directed to the forecasts on www.wavewatch.com but I thought I would post it here too.


Hi,

I new to surf forecasting and I'm trying to get a better handle on wave period. From my reading I have found out that any swell with a period longer than 13 seconds is considered sizable. However when I review your surf forecasts for period the scale starts at 15 seconds. If you can help me out that would be greatly appreciated.

Regards,
Stuart

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My response:

Stuart,

I am stoked that you are getting into Surf Forecasting...and that you are paying attention to the swell period. Personally I think swell-period is sometimes the most overlooked and underrated attribute of a swell, and in many cases it can really "make or break" the surf shape of an incoming swell.

To answer your specific question...yes a 13-second swell can be a substantial swell, but you have to combine in the other factors, namely the distance of the storm, swell-size, and swell-direction.

For example, if you have a very closely located storm sending you surf it is possible to get a very large swell but with very short-periods. Since the storm is so close the waves don't have a chance to decay, (which is when they bleed off energy as friction as they move through long-distances in the ocean), and so you get the large, but usually disorganized, waves.

On the other end it is possible to have a very small swell with long periods. Sometimes when a storm is very far away you can have the "meat" of the swell decay completely away leaving you with just the long-period forerunners. So even though the swell is coming in at 18- or 19-seconds it doesn't actually produce much surf...or is very inconsistent in the few waves it does produce.

The very over-simplistic and general rule of thumb for forecasting (and swell period) is this...the bigger the swell-height AND the longer the swell-period the bigger the waves will be on the beach.

Hope this helps,

Adam