The National Hurricane Center (NHC) Two-Dimensional Wind Forecasts

A gridded representation of the Tropical Cyclone forecast advisory Message (TCM) has been undergoing development, validation, calibration, and testing by the National Hurricane Center (NHC) and the Cooperative Institute for Research in the Atmosphere (CIRA) since 2019. The code was adapted from NHC’s Wind Speed Probability (WSP) model but enhanced to include a more general parametric wind model and a land surface model and is referred to as the WTCM, or the gridded TCM. During this period, the WTCM has been tested operationally at National Centers and Weather Forecast Offices (WFOs) to populate their wind grids in the AWIPS Gridded Forecast Editor (GFE). The feedback collected so far is very positive and it is anticipated that the WTCM will be transitioned to operations in the next couple of years. Work is also underway with the National Blend of Models (NBM) team to incorporate the WTCM into the NBM to improve consistency of the surface wind fields near tropical cyclones with the NHC official forecast.

This paper will present a technical description of the algorithm design and a summary of the validation and calibration conducted over the past few years. Future enhancements and transition plans to operations will also be described. The WTCM wind field is constrained by the TCM with the vortex center at the location of the TCM track and maximum wind point matching the TCM intensity. The forecast wind radii (34, 50 and 64 kt radii in four quadrants relative to the TC center) are fitted to a generalized modified Rankine vortex model with a wave number 1 asymmetry included. An empirical land surface model based on the log wind profile is used to apply wind reduction over land using surface roughness parameters for 16 land surface types. The computations are performed on a 1 km grid resolution. Validation and calibration for the 2023 WTCM was based on 19 U.S. landfalling TC cases covering the 2011 to 2022 seasons. Overall the model exhibits biases of around 4 kt for all points combined and around 1 kt for land points when fitted to the 19 landfalling cases. The real-time runs of the model also show a low bias. Future enhancements include the addition of elevation adjustments and the addition of a gust parameterization. The model will continue to be run experimentally in 2024 with initial plans to transition to operations in 2025. The model is also being used as part of NHC’s next generation Wind Speed Probability model.