Surf Forecasting

Surfers are lucky because our government funds a very comprehensive set of tools that anyone with an internet account can access. It’s called the National Oceanic and Atmospheric Administration or “NOAA” as we like to call it. A large part of NOAA that is of the most interest to surfers is the network of coastal buoys. These are the absolute key to becoming a knowledgeable surf forecaster. Any and all of them can be read by going to the National Data Buoy Center. Once there, look at the geographical map for your area. You want to pay particular attention to buoys that indicate swell that will likely impact your swell window.

When you identify a particular buoy, check its readings. This is where the basic math comes into play. The speed of the swell is calculated by multiplying the average period by 1.5. For example, a swell with an 11-second period travels at 16.5 knots because 11 x 1.5 = 16.5. That’s not the whole picture though. To see how fast individual waves within the swell train move, simply double that. This is because individual waves move at twice the speed of the wave train. So, in the case of our example, each individual wave is traveling at 33 knots. Now, if you want to convert knots to miles per hour (MPH) we simply take that value and multiply it by 1.2. That means that individual waves in our example are moving at almost 40 MPH.

Now that we’ve figured out some of the basic math, let’s take a real word example and see how this all plays out. Say for instance you wanted to surf on the east coast - in New Jersey. The most telling buoy for most NJ breaks is the 44004, located 200 nautical miles East of Cape May. If the buoy were showing a 12-second swell, you can say it will take about nine and a half hours to reach you because 12 x 1.5 = 18 Knots and 18 x 1.2 = 21.6 MPH, so then 200 / 21.6 = 9.2 hours.

Okay, now that we’ve got the math down, we have more work to do. Every good waterman knows that swell alone doesn’t make it break into rideable surf. What we have to account for next are the other influential factors that cause waves to break. In this case, we’re referring to the bottom contour and how it influences wave formation at our break. In technical circles this is known as Bathymetry. To find out when a swell starts to drag on the sea floor, we take the swell period, 12 seconds in our example above, square it and then multiply it by a factor of 2.56. This would mean that the swell hitting our 44004 buoy would start to drag on the ocean floor at a depth of about 368 ft because 12 x 12 = 144 and 144 x 2.56 = 368 feet. This is an important factor to take into consideration, especially on the east coast, because we seldom get really strong ground swell. It’s usually wind swell generated in or near our local forecast area. This effect typically generates what’s referred to as a hit-and-run swell.

The simple fact is a long-period ground swell packs more of a punch than short-period wind swells because they contain more energy below the oceans surface. Wind swells on the other hand contain most of their energy near the ocean surface, so ground swells have the greater potential to grow in size as they start to drag on the ocean floor. A three foot wind-swell with an 11-second period may produce a four foot breaking wave, but the same three foot wave with an 18-second period may produce a much larger breaking wave depending on the bottom contour. This is because waves begin to topple, or break, at about 1.3 times their height above the oceans surface which means that a 3 foot wave will break in a little over 5 feet of water where a 10 foot wave will break in about 13 feet of water.

Lastly, with the exception of tidal influences, which likely have a huge effect on whether your break turns on or not, there are advanced factors you might consider when forecasting such as combo-swells, frontal boundaries, surface winds, wave decay and so on, but they’re beyond this basic tutorial. If you’re interested in these, search for them on the Internet and you’ll find plenty of information to expand your new found knowledge.

We hope with this basic tutorial you too can predict when your next swell will arrive and how big it will be.