The Weather of September 2015

    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 and September, a record since 1946, the total rainfall in September was only 87.9 millimetres, a deficit of about 73 percent comparing to the normal figure of 327.6 millimetres. The accumulated rainfall of 1619.1 millimetres since 1 January was about 27 percent below the normal figure of 2233.1 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 rain was heavier in the morning of 2 September 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 4 September 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 7 September as another trough of low pressure moved across the coast of Guangdong. However, generally fine conditions soon returned the next day as a relatively dry easterly airstream became established over the coast of southeastern China.

    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 13-15 September. 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 16 September. 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 21 September. More than 30 millimetres of rainfall were generally recorded over the territory, with rain particularly heavy over parts of the New Territories where rainfall amount exceeded 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 24 September. The weather then deteriorated in the afternoon on 26 September 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 27 September 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.