Probabilistic Guidance for Tropical Cyclone Inland Wind Watches and Warnings

The National Weather Service (NWS) has provided emergency managers with tropical cyclone (TC) forecast uncertainty information to aid in mitigation activities since the implementation of the Hurricane Strike Probabilities in 1983. The Strike Probabilities only considered track uncertainty and were replaced in 2006 by wind speed probabilities (WSPs), which consider track, intensity, and size uncertainties, and the companion time-of-arrival of tropical-storm-force winds product was added in 2018. This probabilistic information complements the deterministic TC forecasts from the National Hurricane Center (NHC) and associated coastal TC wind (e.g., tropical storm and hurricane) watches and warnings (WWs), which are issued by NHC in coordination with local NWS Weather Forecast Offices (WFOs). The WFOs have the responsibility for issuing TC wind WWs for inland locations while NHC does the coastal WWs. The WWs from NHC and WFOs are typically based on deterministic forecasts and a subjective assessment of forecast uncertainty based on historical forecast errors and model guidance in general.

In this paper, a new method for providing objective probabilistic guidance for the determination of TC wind WWs based on the NHC WSP model will be described. TC WWs are first and foremost risk communication tools, and until now, NHC did not have a probabilistic based objective framework to determine those WWs. The new guidance is called the Wind Hazard Recommender (WHR) and recommends that watches or warnings be issued based on the exceedance of specific wind speed probability thresholds in time for each WW type and discontinued based on a set of lower probability thresholds, with continuity constraints taken into account. This guidance facilitates collaboration between offices and consistency between the products issued by NHC and WFOs. The WHR was demonstrated and evaluated in testbed experiments in 2021 and 2022, implemented in the NWS Advanced Weather Interactive Processing System (AWIPS) in 2023, and subsequently used experimentally in the Atlantic basin and operationally in the East Pacific basin over the 2023 hurricane season. The WHR development, implementation, and some WHR WW verification of the 2023 hurricane season will be described. The WHR provides a direct connection between probabilistic forecasts and wind WWs, and could be generalized in the future to incorporate information from dynamical model ensemble systems in concert with the statistically based ensembles from the WSP model.