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September 2009 was hotter and wetter than usual. The mean temperature was 28.8 degrees, 1.2 degrees above the normal of 27.6 degrees. There were 10 Very Hot Days with daily maximum temperatures of 33 degrees or above in the month, breaking the records for the Septembers in 1963 and 1969. The mean minimum temperature of 26.9 degrees was also the highest for September since records began. The total rainfall of 486.3 millimetres in the month was about 69% above the normal figure of 287.5 millimetres. The accumulated rainfall since January 1 was 2,027.3 millimetres, about 6% below the normal figure of 2,161.2 millimetres for the same period.
Under the influence of a ridge of high pressure over southern China, the weather in Hong Kong was fine and very hot for the first eight days of the month. Affected by an easterly airstream, the weather became cloudy with showers on September 9. Meanwhile, an area of low pressure over the northern part of the South China Sea intensified into a tropical depression and tracked west-northwestwards. It further intensified into a tropical storm named Mujigae on September 10. Locally, winds strengthened gradually and there were some scattered squally showers on that day. Affected by the outer rainbands associated with Mujigae, the weather remained cloudy with occasional rain on September 11. The rain eased off while Mujigae made landfall over northern Vietnam and weakened on September 12.
A tropical depression over the western North Pacific entered the northern part of the South China Sea on September 13. It intensified into a tropical storm that evening and was named Koppu. The outer rainbands of Koppu brought a few thundery showers to Hong Kong that evening. Koppu intensified into a typhoon on September 14 and the weather became cloudy with squally showers. Local winds also strengthened gradually towards the evening. Gale force southeasterly winds with maximum gusts up to 151 kilometres per hour were recorded at Cheung Chau Beach in the late evening of September 14. Koppu made landfall over the western coast of Guangdong in the morning of September 15 and weakened into a tropical storm in the afternoon. The rainbands associated with Koppu brought heavy squally showers to the territory on that day. With Koppu moving further inland and dissipating, the showers eased off gradually on September 16.
A ridge of high pressure brought generally fine weather to Hong Kong from September 17 to 20. A cold front formed over the northern part of southern China in the morning of September 21 and crossed the coastal areas of Guangdong that evening, bringing a few thundery showers to Hong Kong. The northeast monsoon behind the cold front brought generally fine weather condition from September 22 to 27. In the mean time, Tropical Storm Ketsana entered the South China Sea on September 26 and intensified into a severe tropical storm the next day. It gained typhoon intensity while crossing the northern part of the South China Sea on September 28. Under the combined effect of Ketsana and the northeast monsoon, local weather was windy with rain on September 28 and 29. Ketsana made landfall over the coast of central Vietnam in the afternoon of September 29 and weakened gradually afterwards. The weather was rainy in the last day of the month.
Nine tropical cyclones occurred in the western North Pacific and the South China Sea 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 figures in September are tabulated in Table 2.
Ends/Monday, October 5, 2009
Issued at HKT 19:09
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