Understanding a marine forecast is critical to safe boating. Weather and wave conditions can change suddenly, catching boaters off guard and creating life threatening conditions.
Typical marine forecasts predict wind speed and direction, wave heights, and significant weather. Marine forecasts cover large areas and the forecast elements are often given in ranges. The significant weather may not occur over the entire area or during the entire forecast period. The ranges represent average conditions over a period of time (usually 12 hours) and the actual conditions may be lower or higher than the forecast range at times.
When looking at wave heights, boaters need to understand what is actually forecast. A wave train is made up of many different waves with differing heights. The wave forecast is actually the expected Significant Wave Height. This is the average of the highest one-third (33%) of waves (measured from trough to crest) that occur in a given period. This is measured because the larger waves are usually more significant than the smaller waves. For instance, the larger waves in a storm cause the most beach erosion, and larger waves can cause navigation problems for mariners. Since the Significant Wave Height is an average of the largest waves, you should be aware that some individual waves will be higher. If we take a sample forecast of waves of 2 to 4 feet, this implies that the average of the highest one-third waves will have a Significant Wave Height of 2 to 4 feet. But mariners need to keep in mind that roughly one of every ten waves will be greater than 4 feet; one in every 100 waves will be greater than 5 feet; and one in every 1000 waves will be greater than 6 feet.
As a general rule, the largest individual wave one may encounter is approximately twice as high as the Significant Wave Height.