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September 2015 was marked by sunny and warm weather with below normal rainfall. The monthly mean temperature of 28.4 degrees was the seventh highest for September on record and 0.7 degrees above the normal figure of 27.7 degrees. With no tropical cyclone affecting Hong Kong and necessitating the issuance of tropical cyclone warning signals in August or September, a record since 1946, the total rainfall in September was only 87.9 millimetres, a deficit of about 73 per cent compared to the normal figure of 327.6 millimetres. The accumulated rainfall of 1 608.0 millimetres since January this year was about 28 per cent below the normal figure of 2 233.1 millimetres for the same period.
Affected by a trough of low pressure, the weather in Hong Kong was mainly cloudy with showers and isolated squally thunderstorms on the first three days of the month. The rainfall was heavy in the morning on September 2, with more than 40 millimetres of rainfall recorded over many parts of Hong Kong.
With the trough weakening and a ridge of high pressure setting in over the south China coastal areas, the weather turned sunny and very hot in Hong Kong on September 4 as daytime temperatures soared to a maximum of 32.9 degrees, the highest of the month. Local weather became cloudy again with occasional rain and a few squally thunderstorms on September 7 as another trough of low pressure moved across the coast of Guangdong. Affected by a relatively dry easterly airstream established over the coast of southeastern China, the weather in Hong Kong soon turned generally fine the following day.
As the northeast monsoon prevailed over southern China, the weather in Hong Kong remained mostly fine apart from some isolated showers in the next four days. With Tropical Storm Vamco moving towards central Vietnam, monsoon winds were enhanced over the south China coastal waters and local weather became rather windy on September 13 to 15. As a rainband extended over the coast of Guangdong and the northern part of the South China Sea, the weather turned mainly cloudy with a few showers on September 16. Local weather became mainly fine again except for a few morning showers over the next four days, as the northeast monsoon was gradually replaced by a maritime airstream from the south.
With a trough of low pressure forming over the inland areas of Guangdong and edging towards the coast, outbreaks of heavy rain and squally thunderstorms affected Hong Kong on September 21. More than 30 millimetres of rainfall were recorded generally over the territory, with particularly heavy rainfall over parts of the New Territories exceeding 100 millimetres.
With the weakening of the trough, the weather was a mixture of sunny periods and showers over the next couple of days before fine and hot conditions set in on September 24. The weather then deteriorated on the afternoon of September 26 as intense thunderstorms and heavy rain associated with a trough of low pressure brought more than 30 millimetres of rainfall to Hong Kong. The rain was particularly heavy over the eastern part of Kowloon, with rainfall exceeding 70 millimetres. The weather remained mainly cloudy and showery on September 27 before a dry continental airstream brought fine and hot conditions towards the end of the month as Super Typhoon Dujuan swept across Taiwan and landed over Fujian.
Five tropical cyclones occurred over the South China Sea and the western North Pacific in the month.
Details of issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from normal for September are tabulated in Table 2.
Ends/Monday, October 5, 2015
Issued at HKT 18:55
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