The significant wave height forecast map layer displays the NOAA/National Weather Service's official coastal and Great Lakes for 12 hours periods out to three days. The forecast is obtained from the NWS' National Digital Forecast Database (NDFD).
The significant wave height is defined as the average height (trough to crest) of the one third highest waves valid for the indicated 12 hour period. The spatial resolution of the wave forecasts is 5km (3.1 miles). The units are in feet.
The wave heights are indicated by different colors at 2 feet increments up to 20 feet and then at 5 feet increments up to 40 feet.
The forecast is obtained from the NDFD four times per day at 0015, 0615, 1215, and 1815 UTC. Depending on geographic region and present weather conditions, the NDFD may be updated more frequently. In addition, the NWS plans to include significant wave height forecasts for offshore and high seas areas in the NDFD.
The NWS' National Digital Forecast Database (NDFD) consists of gridded forecasts of sensible weather elements (e.g., cloud cover, maximum temperature, significant wave heights). NDFD contains a seamless mosaic of digital forecasts from NWS weather forecast offices working in collaboration with the NWS' National Centers for Environmental Prediction (NCEP) around the country.
Each NWS Weather Forecast Office (WFO) prepares gridded NDFD forecasts for a specific area of responsibility. When these locally generated forecasts are merged into a national mosaic, occasionally areas of discontinuity will be evident. Staff at NWS forecast offices attempt to resolve discontinuities along the boundaries of the forecasts by coordinating with forecasters at surrounding WFOs and using workstation forecast tools that identify and resolve some of these differences. The NWS is making progress in this area, and recognizes that this is a significant issue in which improvements are still needed.