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A wet and relatively cooler September

     Due to heavy rain episodes in the early part of the month and rainfall associated with tropical cyclone Usagi late in the month, September 2013 was wetter than usual. The total rainfall for the month was 454.2 millimetres, about 39 per cent above the normal figure of 327.6 millimetres. The accumulated rainfall since January 1 was 2673.0 millimetres, about 20 per cent above the normal figure of 2233.1 millimetres for the same period. While the month was overall slightly cooler than normal, the approach of Usagi also brought very hot conditions and high temperatures on September 20 and 21.

     Under the influence of a trough of low pressure over the northern part of the South China Sea, it was mainly cloudy and showery in Hong Kong on the first three days of the month. Local weather deteriorated further with occasional heavy rain and a few squally thunderstorms on September 4 and 5 as an easterly airstream converged with a southerly airstream in the vicinity of the Pearl River Estuary. The rain was particularly heavy on September 5 with more than 150 millimetres of rainfall over the eastern part of Hong Kong Island and the central part of Kowloon.  

     With a subtropical ridge establishing itself over southeastern China, local weather improved gradually with sunny periods on September 6. It remained generally fine and hot for the ensuing six days. A broad area of low pressure over the northern part of the South China Sea brought a few showers to the territory from September 13 to 16. Under the influence of the northeast monsoon over the coastal areas of Guangdong, the weather was windy with sunny periods for the next three days.  

     Meanwhile, over the seas east of the Philippines tropical cyclone Usagi intensified into a super typhoon on September 19. Under the influence of the subsiding continental airstream associated with the outer circulation of Usagi, local weather became dry and very hot on September 20 and 21. The temperatures at the Hong Kong Observatory rose to a maximum of 34.7 degrees on the afternoon of September 21, the highest record for September since 1969. The daily mean temperature of 31.2 degrees recorded at the Hong Kong Observatory on September 21 was also the highest for September since 1884, tied with those set on September 22, 2008 and September 1, 2010.

     Usagi passed the Luzon Strait on September 21 and weakened into a severe typhoon that night. Usagi tracked west-northwest and moved across the northeastern part of the South China Sea the next day, making landfall near Shanwei at night. Usagi skirted within 100 kilometres to the north of Hong Kong in the small hours of September 23. It moved further inland afterwards and weakened into an area of low pressure over Guangxi in the afternoon. Locally, with the approach of Usagi, winds strengthened gradually and the weather deteriorated with squally showers during the day on September 22. Winds reached gale force in many parts of the territory with heavy squally showers on the night of September 22 and in the early morning of September 23 when Usagi came closest to Hong Kong. Moreover, the storm surge induced by Usagi caused minor flooding in some low lying areas of Hong Kong. As Usagi moved away, local winds moderated gradually and rain eased off during the day.  

     Affected by a moderate to fresh easterly airstream, there were a few showers on September 24 and 25. A cold front formed over the northern part of southern China on the morning of September 25 and moved across the coastal areas of Guangdong on the next morning. Under the influence of the northeast monsoon behind the cold front, local weather became mainly fine and slightly cooler on September 26 and 27.  

     Tropical cyclone Wutip developed to the west of Luzon and moved across the central part of the South China Sea from September 26 to 29, and intensified into a severe typhoon on September 29. Wutip made landfall over the coast of central part of Vietnam on the evening of September 30. Locally, affected by a band of clouds and rain associated with Wutip, it was cloudy with a few rain patches on September 28 and 29. Under the combined effect of the northeast monsoon and tropical cyclone Wutip, the weather was windy with occasional rain on the last day of the month.

     Nine tropical cyclones occurred over the South China Sea and the western North Pacific in the month.

     Details of the issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from the normal for September are tabulated in Table 2.

Ends/Thursday, October 3, 2013
Issued at HKT 18:16


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