Tropical cyclones (TCs) are a type of severe weather system that commonly affects Hong Kong. On average, approximately six TCs come within 500 km of Hong Kong in a year. These TCs can bring strong winds, heavy rain, and severe storm surge to Hong Kong and neighbouring regions depending on their track and intensity. To warn the public about potential strong wind hazards brought by TCs, a numbered TC warning system was introduced in Hong Kong in 1917. The current 1-3-8-9-10 numbering scheme for TC warning signals has been in use since 1973 (Lui et al., 2018) and is very well known by the public in Hong Kong.

Hong Kong was directly hit by two TCs within a few weeks in 2023. First, in early September 2023, Super Typhoon Saola (2309) was bringing hurricane force winds to Hong Kong, which was a rather difficult case in terms of TC forecasting due to its relatively small circulation and the interaction with Tropical Cyclone Haikui (2311) near Taiwan. The forecasting aspect of Saola has been summarized in a paper by Chan et al. (2023). Then about a month later, in early October 2023, Hong Kong was directly hit again by Severe Typhoon Koinu (2314). In He et al. (2023), an observational study had been made about Koinu. Since Koinu might bring about hurricane force winds to some places in Hong Kong, the TC warning signal No. 9 was issued for Hong Kong, meaning that gale or storm force wind is increasing or expected to increase significantly in strength (Lui et al., 2018). Under the successive strikes of TCs Saola and Koinu, this was the first time since 1999 that TC warning signal No. 9 or above was issued twice within a single TC season.

While Saola brought about big challenges on operational forecasting, Koinu was even more difficult to forecast, as far as its impact on Hong Kong was concerned due to a number of factors. As such, given todays’ technology, it is basically technically infeasible to give advance warning for the public about how strong local winds over Hong Kong could become, say, one day or even several hours ahead, which has now become the expectation from the general public. This manuscript will review the forecast difficulties of Koinu, aiming to help in the following several aspects: first, a documentation for later reference by forecasters in Hong Kong, which may also benefit other parts of the world; second, a stimulation for further study for this case in the research community, hopefully to address and resolve the forecast difficulties in the future with the advancement of technology; and third, a call for the operation of numerical weather prediction (NWP) models with higher spatial resolution and the availability of more observations over the vast ocean, namely, South China Sea in this case.

As in the case of Saola, one major challenge in forecasting the track and intensity of Koinu is its rather small and compact circulation. In fact, Koinu was even smaller than Saola. From the surface isobaric pattern when Koinu was close to Hong Kong (Figure 1(a)), its circulation was just around less than 200 km. Given such a small system, it would be rather difficult for global NWP models, with a spatial resolution as high as 9 to 10 km nowadays, to properly resolve the TC and forecast its movement and track.

The second complication is the impact of the weak northeast monsoon. In early October, it is climatologically the commencement of northeast monsoon affecting southern China. Weak monsoon flow, with slightly cooler and drier air from the continent, may occasionally reach the coast of southern China, but not yet strong enough to dominate in this region. The interaction between the northeast monsoon and a TC has long been a problem for TC warning in Hong Kong (e.g. in the case of Tropical Cyclone Nalgae in 2022, as documented in Chan et al. (2023b)). On one hand, the cooler and drier continental air might cause weakening of the TC as it gets close to the south China coast. On the other hand, the east to northeasterly monsoon winds to the northern flank of the TC and the southwesterly flow still prevailing over much of the South China Sea might enhance the horizontal shear over the region, thus favouring the intensification (or at least maintaining the intensity) of the tropical cyclone along the inter-tropical convergence zone (ITCZ). It would be a rather difficult problem for global NWP models to correctly predict which of the two competing factors would become dominating.

Thirdly, with the arrival of weak northeast monsoon at the surface, the upper flow at the middle troposphere (e.g. 700 hPa to 500 hPa) would be generally westerly. As it is still in the autumn, the westerly flow is not particularly strong (as comparing to winter time). There may be occasions of perturbations in the westerly flow, especially at the lower latitude of around 20 degrees North. Such westerly short waves may “drag” the TC, resulting in slight northward movement or at least quasi-stationary situation for the TC for a certain period of time. As such, the movement of the TC is not so steady. Given that global NWP models are still struggling to pick up short waves in the westerly, the TC track forecasting becomes rather difficult. At the same time, the vertical shearing resulting from the westerly short wave might cause gradually weakening of the TC, but it would be rather difficult to precisely predict when and where the weakening would occur. As shown in the satellite image (Figure 1(b)) and radar picture of Hong Kong (Figure 1(c)) when Koinu was close to the territory, there was an extensive cloud band and rain band to the north and northeast of Koinu, suggesting the passage of a westerly wave/trough in the middle troposphere, which partly explains the quasi-stationary or even slight northwest movement of Koinu as well as gradual weakening of the cyclone at that time. The accurate forecast of the timing of the impact of such westerly waves remains a major challenge for NWP models.

The forecasting aspects of Koinu in 2023 over the northern part of the South China Sea are summarized in this paper, namely, the track, intensity, local wind and rainfall forecast. The conventional deterministic global models are found to have moderate performance only. On the other hand, the newly emerging AI models and a regional model, appear to be better in forecasting the track and intensity of Koinu respectively. While these results are based on one case study only, more cases and examples would need to be accumulated in order to fully assess the performance of the new technologies.

From Saola and Koinu, it appears that regional model has reasonable performance in the forecast of heavy rain in association with a tropical cyclone. Wind forecast still remains rather challenging. Moreover, Koinu had a small circulation and underwent a rather irregular track over northern part of South China Sea, namely, firstly dipping to the south and later moving a bit to the north (in a horizontal “S” shape) when it came close to Hong Kong. All these posed great challenges to the warning service for this tropical cyclone. Such subtle but important changes of its movement were not well captured by available models, neither conventional nor AI, with only a weak indication by the model forecast of Fengwu which gave not-so-accurate time and location of the changes in the movement. It is hoped that the present paper could stimulate further researches to improve the track and intensity forecast of this rather difficult system, which form the basis for tropical cyclone warning service, providing better support on early alerts with longer lead time.