Seas 2 Feet or Less

The National Oceanic & Atmospheric Administration, (NOAA) is a division of the United States Department of Commerce. NOAA enriches life through science. The organization reaches from the surface of the sun to the deepest depths of the ocean as they strive to keep citizens informed of the constantly changing environment around them. From daily weather forecasts, severe storm warnings and climate monitoring, to fisheries management, coastal restoration and supporting marine commerce, NOAA’s products and services support economic vitality and effect more than one-third of the U.S. gross domestic product.

National Weather Service

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The National Weather Service is where marine forecasts are developed. Photo: Steve Dougherty

NOAA’s roots date back to 1807 when the nation’s first scientific agency, The Survey of the Coast, was established. Since, NOAA has evolved to meet the needs of a changing nation and not only maintains a presence in every state, but has emerged as an international leader on scientific and environmental matters. Today, NOAA’s dedicated scientists use cutting-edge research and high-tech instrumentation to provide citizens, planners, emergency managers and other decision makers with the reliable information they need when they need it.

When you read a NWS marine forecast calling for “SEAS 2 TO 4 FEET,” the forecasted wave height refers to “significant wave height,” not the actual wave height boaters may encounter.

For mariners across the country, NOAA’s National Weather Service (NWS), one of its many agencies, is the resource we most often turn to for accurate marine weather forecasts. By visiting the National Weather Service online (, local mariners obtain marine forecasts by text and graph format for every region across the country. Updated forecasts can also be sent directly to a cell phone or PDA. Visitors to the site also take advantage of sea surface temperatures, real time sea conditions, view NEXRAD Doppler radar images, they study predicted tides and currents, review all sorts of weather charts and so, so much more.

Don’t Blame The Messenger…
Having a better understanding of just how the National Weather Service does its job will help you comprehend why boaters sometimes encounter crisp, six-foot seas during a predicted forecast reading, “SEAS 2 TO 4 FEET.” Mariners must take into account that while accurately forecasting weather over land is challenging, forecasting accurate sea conditions is even more difficult and still far from a perfect science, but is rather a lengthy equation containing many bits of data. Contrary to what some may believe; meteorologists are not out standing on every beach reporting what they see and feel.

Your local marine forecast all starts with wind speed and direction. This information is forecast with the help of computer models for each particular geographical zone. Determining wind speed and direction, which is vital to forecasting accurate sea states, comes with its own unique set of challenges. The computer model starts with a snapshot of the current state of the atmosphere, a snapshot that is often far from picture perfect due to the fact that important marine areas are typically under-sampled. The computer models utilize the snapshot to produce wind forecasts by solving numerical equations. These equations represent, in some ways, approximations that could very well contain errors. Another limitation the computer models face is “spatial resolution.” If the phenomenon responsible for causing the weather is smaller than the spatial resolution of the model, then the computer model may have trouble accurately resolving the equation.

The next consideration is how the forecasted winds generate waves. This is where “fetch” becomes a very important term. Fetch is the uninterrupted distance over which the wind blows without significant change in direction. The work being done by the wind over the water across the fetch area is called “frictional drag,” which can also be looked at as the transfer of energy from the wind to the water. The result is building seas and/or generation of waves. For any given wind speed and direction, as a general rule, the greater the fetch, the more energy the wind transmits into the sea.

For any given location, the resulting waves generated by local winds are called “wind waves.” However, the actual wave heights at any given location are the result not only of wind waves, but also of waves (energy) that have been propagating across the ocean from a fetch area far away. These components are called “swells,” which could, in fact, be generated by a weather system hundreds or even thousands of miles away. Swells are typically responsible for creating rough surf conditions and noticeable beach erosion. When swells are significant, your local NWS marine forecast may indicate swell size along with the predicted wave height. The swell size however, is already calculated into the forecasted wave height. In other words, a NOAA marine forecast reading “SEAS 4 FEET WITH A 2 FOOT SWELL” does not technically mean six-foot seas. The swell size is simply provided as a safety precaution for boaters who may be effected by wide spread heaves and the potentially dangerous, rougher than normal conditions they can generate near shore.

Already we’ve learned that real time sea conditions are the result of wind waves and swells interacting at any given location. Mariners must now take into account that what you have in the ocean is indeed a wide spectrum of waves, and each is not the same. All of this leads us to the following question. What exactly is it that mariners see when they read their local marine forecast from the NWS?” The answer may come as a shock. When you read a NWS marine forecast calling for “SEAS 2 TO 4 FEET,” the forecasted wave height refers to “significant wave height,” not the actual wave height boaters may encounter.

By definition, significant wave height is the average height (trough to crest) calculated by a computer model of the largest one-third of waves in the entire predicted spectrum of waves. This guestimate, in fact, is the measurement that is forecasted to mariners, not necessarily a real time report taken at any given time at any given place.

Dr. Pablo Santos, Science & Operations Officer at Miami’s National Weather Service office, says boaters must also realize that the actual mean wave height is 63% of the significant wave height forecasted, and that in theory and assuming the marine forecast is dead on, the most probable wave boaters will observe is about half the forecasted wave height. Although, because we have an entire spectrum of waves to deal with, boaters should expect to encounter one in ten waves up to 27% larger than those forecasted, one in a hundred waves up to 67% larger, and while much less likely, a potentially dangerous wave as high as twice or more of the significant wave height forecasted. This is the sort of wave some seamen call “rogue waves.”

All of this information is vitally important for properly interpreting NWS marine forecasts and is exactly where mariners usually get lost. How many times have you read or overheard a boater complain about an inaccurate marine forecast. The reason is because they were under the impression what they read online or heard on the VHF is exactly what they were going to encounter. Surprise-surprise!

Now factor in that if the catalyst to an accurate marine forecast – the initial wind estimates – are a fraction off, ultimately the entire marine forecast could be substantially inaccurate. Plus, wind direction and velocity unexpectedly shift in the blink of an eye, and we already know computer models aren’t perfect. Add in inaccurate local bathymetry in computer models, such as local reef lines, which primarily affects near-shore sea conditions, and ocean currents, such as the Gulf Stream that play havoc on our offshore arena, and the door for an accurate forecast just slammed shut! Sea breezes, too, pop up during blistering summer afternoons and directly impact coastal conditions. Typically though, sea breezes aren’t strong enough, last long enough or blow over long enough distances (fetch) to increase wave height substantially, rather they force existing waves closer together which results in choppier conditions.

If I have done my job, you should now have a clearer understanding of why sea conditions encountered are not always the sea conditions forecasted. Safe boaters must take into account that even under the best-case scenario, forecasted significant wave heights are just that, nothing more than a computer model prediction of what you may experience with the very likely possibility that there will be variables, both bigger and smaller. This stresses the importance of knowing the limitations of your vessel and its crew and to always plan accordingly.

While NOAA marine forecasts may not always be dead-on thanks to Mother Nature and limits to technology, at least they provide us with an idea of what sort of sea conditions we should expect during any given outing. Imagine for a moment what it must have been like for ancient mariners who steered their ships into the unknown without a clue as to what tomorrow may bring. I say, “NOAA, keep up the good work!”

National Data Buoy Center

Additional parameters the National Weather Service utilizes in their computer models are real time wave heights and wind speeds provided by National Data Buoy Center (NDBC) maintained unmanned weather stations moored at strategic locations in the open ocean. Dissect the state from Vero Beach to Naples, arguably the most popular boating region in the entire country, and you’ll find none of these valuable buoys south of this imaginary line. Without real time information, local meteorologists have no way of knowing what is really taking place off our beaches, and no way of equating potentially hazardous conditions into our local marine forecast. If you are a South Florida boater, write your local congressmen or senator and ask why? If we all yell loud enough, we may be able to persuade the installation of a number of these valuable weather stations.