Development of Wind Gust Parameterizations for Tropical Cyclone Surface Wind Prediction Models

The National Weather Service (NWS) is tasked with forecasting the sustained surface winds and wind gusts from tropical cyclones (TCs) over water and land. The wind gusts forecasts are critical in decision support briefings to emergency managers since high wind gusts can occur outside of areas with hurricane and tropical storm watches and warnings, which are based on sustained winds, and because structural damage, power outages, and other wind hazards are typically associated with gusts. The National Hurricane Center (NHC) official forecasts include gust estimates out to 5 days based upon the maximum sustained winds while local NWS Weather Forecast Offices (WFOs) provide gridded fields of sustained winds and wind gusts out to 7 days. Despite their importance, operational wind gust forecasts rely on climatological gust factors because there is no reliable model guidance available for TCs. For example, 99% of the operational NHC wind gust forecasts for the past several seasons were based on a lookup table that is a simple function of the sustained wind.

To address the lack of TC wind gust guidance, new parameterizations are being developed for the dynamical Hurricane Analysis and Forecast System (HAFS) and the experimental WTCM, which uses a parametric wind model to predict the 2-dimensional surface wind field consistent with the NHC official forecast. For the HAFS model, an improved version of the physically-based gust model from the fifth generation ECMWF reanalysis (ERA5) is being used, where a diagnostic parameterization produces instantaneous gust magnitudes from the turbulent and convective components of gusts. Simpler methods are being used for the WTCM, where the gust factor (the ratio of the maximum 3-sec gust over the past minute to the 1-minute sustained wind) is estimated from the sustained wind, latitude, distance from the TC center, surface roughness and several other factors. Machine learning methods are being used for the WTCM gust parameterization to account for nonlinear relationships between the gust factor and the predictors. ASOS and WeatherFlow observations from 22 recent U.S. landfalling TCs are being used to develop and validate the gust parameterizations. Preliminary results from the HAFS and WTCM gust forecasts will be presented, including validation statistics. Future improvements and the potential for transition to NWS operations will also be described.