The National Hurricane Center (NHC) Next Generation Wind Speed Probability (WSP) Model

The National Hurricane Center (NHC) legacy Wind Speed Probability Model (WSP) estimates the probability of exceeding 34, 50 and 64 kt wind speeds from 1000 realizations that are centered around the official forecast. The WSP model is run on a 50 km grid and the probabilities generated from the model are point specific. For track and intensity, the realizations are based on the previous 5-year error distributions of the NHC and Central Pacific Hurricane Center (CPHC) forecasts. The errors are randomly generated using a Monte Carlo model, with only a weak dependence on the track spread of a selected set of real time dynamical model forecasts. The wind radii associated with the realizations are based on the error distributions of a simple climatology and persistence radii model. The simple radii model includes the initial official forecast radii as input, but not the official wind radii forecasts. Furthermore, the wind probabilities are calculated from the wind radii for each member, but the full 2-dimensional wind field is not calculated. Land surface effects are included in a very simple way, with a constant adjustment factor for inland points, and elevation effects not taken into account.

The next generation WSP model (referred to as WSP 2.0) is based on 1000 realizations of complete two-dimensional wind fields derived from an empirical modified Rankine vortex algorithm referred to as the WTCM. The WTCM includes detailed land surface information in a surface layer model to reduce the wind speeds over land. WSP2.0 will also provide probability parameters not available from the legacy WSP model, including probabilities of exceeding major hurricane force winds (96 kt) as well as wind speed exceedance grids/maps/graphics for several probability levels (namely, 5%, 10%, 25%, 50%, 75%, 90%, and 95%). A preliminary version of the the model using the same track, intensity, and radii perturbations as the legacy model has been run in real time in experimental mode during the 2022, 2023,and 2024 seasons and they are accessible from CIRA’s website: https://rammb-data.cira.colostate.edu/tc_realtime/. The 2022 and 2023 test runs shown on this website are based on the realizations run at 20 km resolution, but in 2024 the resolution of the realizations was increased to 5 km, a significant improvement over the legacy model.

Additional enhancements being developed for WSP 2.0 include: 1) testing new methods to generate radii perturbations, replacing climatology-based perturbations in the legacy model with one that perturbs the official wind radii forecasts; 2) adapting a wind gust parameterization that will enable the model to generate wind gust probabilities; and 3) testing improvements to the methods for calculating the track and intensity perturbations. Possibilities include additional post-processing to improve the smoothness of the track, intensity and radii perturbations in the individual realizations and evaluation of a new machine learning method to include situational dependence and dynamical model input in the perturbations. The development team is also coordinating this work with the Probabilistic Storm Surge Model (P-Surge) developers in an effort to unify and bring consistency to the wind forcing in both WSP 2.0 and P-Surge. Future enhancements include adding elevation adjustments to the WTCM wind model and these could be added to a future version of WSP 2.0 as well. Once the initial model development is completed, a full validation and calibration will be conducted, which will be important for downstream applications.

This presentation will detail the model design, a summary of the main findings during the first 3 years of in-house experimentation, and the path to operations. Preliminary results show that point probabilities over land are much smaller than in the legacy model (as expected). Finally, there is a need to recalibrate downstream applications that are based on the WSP model, such as the generation of wind-based watches and warning, public and marine forecasts, and risk communication products and guidelines. The transition plan is to continue to run the model experimentally for the next two years while development, validation, and calibration continues with an aim to implement it operationally in 2027 at the earliest.