What is NDBC SeaState and how is it generated?.

The accuracy of a wave model forecast can be critical when it is used to make operational decisions. If the forecast five days out shows workable wave heights and wind speeds and a decision is made to stay in the field, then it is inconvenient, costly, and potentially dangerous when the actual wave heights and wind speeds are much higher. More importantly it biases the opinions of your offshore managers in using the forecast as even remotely reliable. So, what is a quantifiable measure of how accurate a wave model is? We use the real-time data from NDBC wave buoys - wave heights, wave periods, wave directions, wind speed and wind direction - to validate the accuracy of a wave model's ability to forecast the sea state.

For wave heights the accuracy is judged by whether the forecasted wave heights are within 1.5 foot (0.45 m) of the actual wave height recorded by a wave buoy. This is a high bar, but it is justified for offshore operations. It is our judgement that looking at the bias of a wave model over weeks, months, years, etc. is worthless. We look at five specific time periods, 0-3 days, 3-5 days, 5-7 days, 7-10 days, and 10-15 days. If 90% of the forecasted wave heights are within 1.5 foot of the wave buoy during the time period, then that is classified as an excellent forecast. if 80 - 90% of the forecasts are within 1.5 foot then the forecast is very good, 70 - 80% is good, 60 - 70% is poor, and anything less than 60% is statistical noise. On average, wave height forecasts from NOAA's GFS model are very good to excellent out to 3 days, good at 3 - 5 days, poor from 5 - 7 days, and statistical noise for any forecast longer than 7 days. Forecasting any weather condition more than seven days out is usually fruitless.

For reference the WW3 operational wave and wind forecasts use the NCEP/EMC global deterministic wave model unified with the Global Forecast System (GFS). The WAVEWATCH III spectral wave model is one way coupled to the atmospheric forecast model. In addition, surface ocean currents from the Global Real-Time Ocean Forecast System (RTOFS) are input to the wave model. The model is run by NCEP four times a day: 00Z, 06Z, 12Z, and 18Z and produces hourly forecasts out to 120 hours and every 3 hours from 120 to 384 hrs (5-16 days). There are three native computational grids, one for the arctic, one for one for the northern hemisphere (15S to 52.5N), and one for the southern hemisphere (10.5S to 79.5S) and four post-processed grids.