WEATHER TIP #164 UNDERSTANDING THE LIMITATIONS OF COMPUTER WAVE MODELS In recent years, many surfers have gained Internet access. This has made it possible for the public to access information previously only available to professional meteorologists and oceanographers. Computer-generated wave forecast models are popular, but it is important to understand what these models can and cannot do. Universally recognized laws of physics determine the motions of our atmosphere and oceans. Now, computer models of the atmosphere routinely attempt to predict pressure and wind patterns up to a week in advance. In turn, the output of these atmospheric models is used to generate wave forecasts for the world's oceans. In principle, these high-tech models sound great, but the results are often less accurate than many surfers expect. For the purpose of the discussion, I'll use the popular U.S. Navy WAM model. But other computer wave models have similar limitations. 1) Poor knowledge of initial conditions. Numerous buoys are located near the coasts of the United States and Europe; these provide very accurate measurements of wind, pressure and sea state. But in other regions forecasters rely heavily on passing ship observations. If there aren't enough reports, a computer model will run with an inaccurate initial analysis. 2) The forecast skill of atmospheric models quickly decrease with time. In the Northern Hemisphere, a typical low is predicted within 100 miles of its actual position 24 hours later. But by five days later, the actual average error is more than 500 miles. That may not be a big problem if a storm is in the middle of the ocean. But if a low is near land, a 500-mile margin error may make the difference between a major swell or no waves. Furthermore, the models often predict ocean gale intensity poorly. If the wind forecast is off by 10 knots, the resulting wave forecast height will be off by 30 to 50 percent. If the swell source is 2,000 miles away, the expected swell arrival time will be a day off. 3) All prediction models have a hard time with tropical cyclones. No current model is able to predict tropical cyclone formation consistently. Once a hurricane forms, the models often fail to depict the storm size and intensity correctly. The resulting wave forecast is sometimes much too high and once in a while, it is much too low. 4) The public version of the WAM model shows only dominant swell direction. In many cases two or more swell fields are present at the same time, and the secondary swell may be the most important for surfers. For example, the models often show a dominant easterly trade wind swell in regions such as Hawaii or Puerto Rico. But a secondary distant northwest ground swell will not show up. 5) The model shows the significant wave height of combined seas. That is the result of interacting wind waves and swell of varying directions and periods. In some cases, the combined sea height is much higher than the ground swell size aimed at you. That may lead an inexperienced person to over-predict a swell. 6) The WAM doesn't show you the wave period. This factor is needed to compute swell travel time. Wave period multiplied by 1.5 equals swell forward speed in knots. 7) The model doesn't tell you local wind conditions. You will need to get this info from local reports. The most accurate surf forecasts are the result of accurate analysis, careful evaluation and lots of experience. The wave models are just one of the tools used by the most successful forecasters. For further discussion of the WAM model, check out the Fleet Numerical Meteorology web site at www.fno.com Written by Chris Zawasky