The Weather of September 2013

    Due to the heavy rain episodes in the early part of the month and the rainfall associated with tropical cyclone Usagi in late September, it was wetter than usual in September 2013.  The total rainfall of the month was 454.2 millimetres, about 39 percent above the normal figure of 327.6 millimetres.  The accumulated rainfall since 1 January was 2673.0 millimetres, about 20 percent 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 20 and 21 September.  

    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 4 and 5 September as an easterly airstream converged with a southerly airstream in the vicinity of the Pearl River Estuary.  The rain was particularly heavy on 5 September 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 over southeastern China, local weather improved gradually with sunny periods on 6 September.  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 13 to 16 September.  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, tropical cyclone Usagi over the seas east of the Philippines intensified into a super typhoon on 19 September.  Under the influence of the subsiding continental airstream associated with the outer circulation of Usagi, local weather became dry and very hot on 20 and 21 September.  The temperatures at the Hong Kong Observatory rose to a maximum of 34.7 degrees on the afternoon of 21 September, the highest record for September since 1969.  The daily mean temperature of 31.2 degrees recorded at the Hong Kong Observatory on 21 September was also the highest record for September since 1884, tied with those set on 22 September 2008 and 1 September 2010. 

    Usagi passed the Luzon Strait on 21 September 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 23 September.  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 22 September.  Winds reached gale force in many places of the territory with heavy squally showers on the night of 22 September and in the early morning of 23 September when Usagi came closest to Hong Kong.  Moreover, the storm surge induced by Usagi caused minor flooding in some of the low lying areas in 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 24 and 25 September.  A cold front formed over the northern part of southern China on the morning of 25 September 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 26 and 27 September.

    Tropical cyclone Wutip developed to the west of Luzon moved across the central part of the South China Sea from 26 to 29 September and intensified into a severe typhoon on 29 September.  Wutip made landfall over the coast of central part of Vietnam in the evening of 30 September.  Locally, affected by a band of clouds and rain associated with Wutip, it was cloudy with a few rain patches on 28 and 29 September.  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.    

Radar echoes showing Severe Typhoon Usagi skirting within 100 kilometres to the north of Hong Kong on the night of 22 September 2013

    Nine 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 summarized in Tables 1.1 to 1.6.  Monthly meteorological figures and departures from normal for September are tabulated in Table 2.