Probabilistic Guidance for Tropical Cyclone 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. 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 issues the coastal WWs. The WWs from NHC and WFOs are typically based on a subjective assessment of forecast uncertainty based on historical forecast errors and model guidance in general.

In this presentation, a new method for providing objective probabilistic guidance for the determination of TC wind WWs based on the NHC WSP model will be described. The new guidance is called the Wind Hazard Recommender (WHR) and recommends that TC wind 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. In general, the probability thresholds for issuance of TC wind WWs are 20% higher at the coast than they are inland to account for the high bias the WSP model has over land. The WHR was implemented into NWS forecast operations in 2024 to facilitate collaboration between offices and consistency between the products issued by NHC and WFOs.

The history of TC wind WW thresholding, application to forecast operations, WHR WW verification, and future work to re-calibrate thresholding ahead of the next-generation WSP model will be described. The WHR provides a direct connection between probabilistic forecasts and wind WWs. Until now NHC did not have a probabilistic-based, objective framework to determine these wind WWs, which are a risk communication tool at the center of the NWS Decision Support Services. The WHR will also provide a mechanism for WW improvements through the WSP model, which will reduce/expand the recommended WW areas as forecast uncertainty decreases/increases, and as new capabilities such as situationally dependent dynamic ensemble forecast systems are incorporated into the next-generation WSP model.