Observational and Forecasting Aspects of Super Typhoon Ragasa in Hong Kong

Abstract

Super Typhoon Ragasa in September 2025 ranked the second-strongest tropical cyclone (TC) in the South China Sea since records began in 1950. Its closest approach to Hong Kong was only about 120 km yet its impact on the territory was not particularly severe. This work analysed the meteorological perspective using observations and model forecasts, where Ragasa's high wind region skirted just around the door step of the territory of Hong Kong. This work also serves as a valuable case study for near-coast eyewall replacement of TC; and for meteorological authorities that pursue early warnings to intense TC by showing that (i) observational data, especially near-sea-surface dropsonde data, are crucial for determining the spatial extent of hurricane winds associated with Ragasa, and (ii) high-resolution regional model was used to capture subtle changes in Ragasa's track, intensity, and radius of maximum wind as it approached Hong Kong, thereby providing essential information for assessing its potential impact on the territory. Furthermore, the performance of location-specific wind forecasts over Hong Kong is discussed. A regional ensemble prediction system with data assimilation of all-sky radiance from satellites is shown to provide the smallest error on local wind forecasts among other global and regional deterministic models for Ragasa.