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  • Another east-landing typhoon

  • Wednesday, 3rd August 2016

Since last Wednesday (27 July) when convective clouds began to develop gradually over the seas east of the Philippines, the Observatory had been keeping a close watch on the development and movement of this system. As it developed into Tropical Cyclone Nida, the Observatory started to issue its forecast track last Friday. At that time, there were still large discrepancies among major computer forecast models. Some predicted Nida making landfall over eastern Guangdong or even Fujian, while some forecast Nida heading in the general direction of Leizhou in western Guangdong. Although the European Centre predicted many times that Nida would make landfall near Hong Kong, it also once forecast Nida moving to the Taiwan Strait. As for the ensemble model of European Centre, different members forecast vastly different tracks (please see the last blog : Landing east or west). It was indeed difficult to choose a forecast track among numerous scenarios for the information of the public. Based on various kinds of meteorological information and past experience, the Observatory finally forecast that Nida would edge rather close to Hong Kong and make landfall just to our east, and kept this professional judgement all along.

Figure 1 shows the forecast track for Nida issued by the Observatory based on available information in the morning of 30 July, expecting Nida to make landfall to the east of Hong Kong on 2 August. Nida's actual landfall location was only 60 km away from this forecast track, a distance roughly of the size of Hong Kong. With the average error of 72-hour forecast for past tropical cyclones being hundred-odd kilometres, or even more than two hundred kilometres, the Observatory's forecast track for Nida was indeed rather accurate.

Figure 1

Figure 1      The forecast track for Nida issued by the Observatory on 30 July (red line) and its actual track (black line).

While expecting Nida landing to our east, the Observatory remained aware of the forecast uncertainty. Last week when the Observatory advised the public of the storm surge brought by Nida, other possible scenarios were also mentioned: if Nida moved slightly further to the south, it could directly hit Hong Kong or skirt just to its south, and the territory would be more severely affected by storm surge. It turned out that Nida skirted to the northeast of the Observatory and sea levels over most parts of the territory exceeded 0.5 m above the normal tide levels on Tuesday morning, bringing flooding to many places. Fortunately Nida was an east-landing typhoon as expected by the Observatory and the other possible scenarios did not materialize, otherwise flooding over Hong Kong could be more serious and may even be close to the severe flooding caused by Wanda in 1962!

Regarding the intensity of Nida, the Observatory kept forecasting that Nida would gradually intensify when it moved across the northern part of the South China Sea after considering the favourable factors such as ocean and atmospheric conditions. In order to assess the strength of Nida more accurately, the Observatory collaborated with the Government Flying Service to send a fixed-wing aircraft to the centre of Nida on Monday afternoon. Hurricane force winds were recorded over the intense convections in the vicinity of the eyewall, confirming that Nida had reached the typhoon strength. When the western part of Nida's eyewall affected Hong Kong (Figure 2), local winds strengthened significantly, particularly over the western part of the New Territories and the harbour. With the large eye of Nida moving into the northern part of the New Territories in the early hours of Tuesday, winds dropped for a while over the region. Nevertheless, the possibility of strengthening of local winds still existed as the strength of Nida was evidenced by aircraft observation and the southeastern part of Nida's eyewall was expected to bring southwesterlies which were less sheltered by the terrain. Colleagues in the Observatory's forecasting office kept a vigilant eye and monitored closely the need to issue the Increasing Gale or Storm Signal No. 9. In the end, Nida made landfall near Dapeng Peninsula and gradually weakened under the influence of landmass. Local winds did not further strengthen when the southeastern part of Nida's eyewall affected Hong Kong. Higher signal was not necessary for Nida finally.

Figure 2

Figure 2      Radar image at 1 a.m. on 2 August, showing that the western part of Nida's eyewall was affecting Hong Kong.

L.S. Lee & Y.H. He

Last revision date: <25 Aug 2016>