A Gridded Version of the National Hurricane Center Official Forecasts to Support Operations at National Centers and Weather Forecast Offices: Part I: Model Formulation

A gridded representation of the Tropical Cyclone forecast advisory Message (TCM), based on the modified Rankine (Depperman, 1947) wind model and a wavenumber one asymmetry proportional to the storm translational speed, has been developed and is being tested and validated by the National Hurricane Center (NHC) and the National Weather Forecast (NWS) Office (WFO) in Miami during the 2019 tropical season. The model has been set up to run for the entire Atlantic and eastern North Pacific basins. The code was adapted from NHC’s wind speed probability model, so the new wind field program is referred to as the WTCM. Validation is being conducted against recent storms, including Hurricanes Harvey, Irma, Maria, Florence, and Michael (as well as 2019 season storms). The model uses a logarithmic wind equation (Holton, 1979) to reduce wind speeds over land by calculating roughness parameters based on land use and wind speed and direction. The inflow angle is determined by an empirical formula that depends on surface roughness. The validation methodology consists of generating a two-dimensional time dependent wind field of the storm history from the model based on the best track rendition and validating the generated time series for multiple sites against observations. This paper will present a detailed description of the algorithm components and their execution. The validation methodology and the results are presented in a companion paper (Part 2). A two-dimensional time dependent rendition of NHC’s official forecast would provide National Centers and WFO forecasters with objective guidance for their tropical cyclone wind field forecasts. The resulting wind field can be used to provide wind forcing that drives the Nearshore Wave Prediction System (NWPS; Van der Westhuysen, A. J. et al., 2013, 2014) providing wave guidance that is consistent with official forecasts. Envisioned as a replacement for the current labor-intensive methodology used by National Centers and WFOs, the WTCM has the potential to significantly streamline tropical cyclone wind forecasts preparation while keeping them consistent with official NHC’s forecasts.