Tropical Cyclone Wind Gust Guidance for National Weather Service Operations

The National Weather Service (NWS) is tasked with forecasting the 1-minute sustained surface winds and 3-second wind gusts from tropical cyclones (TCs) over water and land. The wind gusts forecasts are critical in decision support briefings to emergency managers since hurricane wind gusts can occur outside of areas with sustained hurricane winds, as was dramatically illustrated by the landfall of Hurricane Beryl along the Texas coast in July of 2024. In addition, structural damage, power outages, and other wind hazards are typically associated with gusts. The National Hurricane Center (NHC) official forecasts include maximum wind gust estimates out to 5 days and local NWS Weather Forecast Offices (WFOs) provide gridded fields of sustained winds and wind gusts out to 7 days. Despite the importance of these gust forecasts, however, very little objective guidance is available to NWS forecasters.

As part of the Hurricane Forecast Improvement Program (HFIP), a wind gust parameterization is being developed for the Hurricane Analysis and Forecast System (HAFS). The HAFS parameterization is based on a modified version of the parameterization from the fifth generation ECMWF reanalysis (ERA5), where gusts are diagnosed from the instantaneous turbulent and convective components. A large database of surface sustained winds and 3-sec gusts from ASOS and WeatherFlow stations was assembled from more than 20 recent U.S. landfalling hurricanes. Saildrone observations of sustained winds and gusts data are also included for coverage over the open ocean. This database is being used to validate and calibrate the HAFS gust parameterization.

In addition to the HAFS parameterization, statistically-based gust guidance is being developed using simple multiple-regression and machine learning methods. The statistical gust models are being trained using the same database collected for the HAFS parameterization and will be used in combination with the HAFS parameterization as input to NWS wind hazard products. The wind gust database, HAFS and statistical gust forecast model results, and the planned path to NWS operations will be described.

Preliminary results from HAFS showed that modifications to the ECMWF parameterization were necessary to remove a high bias in the gust factors when the model results were compared with observations. Pending forecaster evaluation, the gust parameterization will be implemented in the operational HAFS as part of the post-processing.

For the statistical models, the most important factors for estimating gust factors are the sustained wind speed, the surface roughness, the distance of the station from the tropical cyclone center and the latitude of the tropical cyclone center. Once the most accurate statistical gust model is developed and validated, it can be added to the parametric 2-D surface wind model being developed by NHC for use by WFOs and marine forecast centers. The gust parameterization can also be added to NHC’s next generation wind speed probability model, which would provide input for NWS wind hazard risk communication products so they would include information on sustained winds and wind gusts.