Spectral Density Composites for Aiding Oahu, Hawaii Northern Shore Surf Forecasts

During the high surf season for northern shores of Hawaii, large to giant surf creates coastal hazards. The National Weather Service in Honolulu issues High Surf Advisories when heights approach the high season average and High Surf Warnings for exceptionally large episodes. The issuance of a warning sets into motion actions for the civil defense, select transportation and commercial sectors, and water safety personnel. Thus, the forecasters need strong evidence prior to making the call. One of the most important tools used for fine-tuning a surf forecast is buoy data. Wave sensors of a buoy 407 km west-northwest of Oahu provide roughly a 6–12 hour lead time for the most common swell directional band from 295–345°. Typically a forecaster focuses on the measured significant wave height and period. Giant episodes have deep water characteristics of 5–7 m significant wave heights and 14–25 second dominant periods. However, if the wind in the vicinity of the buoy is greater than 21 knots, the significant wave height is augmented by the local seas, creating less confidence in the interpretation of these data. To better understand the potential magnitude of an imminent swell, the spectral density as a function of wave period is analyzed. This study creates composite signatures of spectral density based on historic cases of surf categorized by magnitude. The composites can be used as guidelines to improve north shore, Oahu surf forecasts.