A stormy and wet July

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     With a trough of low pressure lingering over the south China coastal region early in the month and the active tropical cyclone activities over the northern part of the South China Sea in the latter half, July 2017 was cloudier with more rain than usual. The monthly total rainfall was 570.0 millimetres, more than 50 per cent above the normal figure of 376.5 millimetres. The accumulated rainfall recorded in the first seven months of the year was 1 759.8 millimetres, nearly 20 per cent above the normal figure of 1 473.3 millimetres for the same period.

     An active southwest monsoon brought cloudy and showery weather to Hong Kong in the first three days of the month, with some isolated heavy downpours affecting mostly the New Territories. Under the influence of a broad trough of low pressure over the coast of Guangdong and the northern part of the South China Sea, the unsettled weather lasted till July 8 with occasional heavy showers and squally thunderstorms despite some interludes of sunshine. With the setting in of the subtropical ridge over southeastern China, showery activities gradually eased off after July 9. Apart from some isolated heavy showers affecting the Sai Kung area on July 12, hot and mainly fine weather prevailed in Hong Kong over the next five days.

     Meanwhile, easterly winds along the coastal areas of Guangdong gradually strengthened as an area of low pressure near Hainan Island eventually developed into a tropical cyclone named Talas on July 15. Even though Talas moved away from Hong Kong and headed towards the coast of Vietnam, local weather turned cloudy and showery again on July 16. The weather deteriorated further over the next couple of days as enhanced easterly flow in the wake of Talas brought outbreaks of heavy rain and squally thunderstorms to Hong Kong that necessitated the issuance of rainstorm warnings on July 17 and 18, with more than 400 millimetres of rainfall recorded over the eastern part of Hong Kong Island and the northeastern part of the New Territories during the period. The lowest temperature in the month, 24.4 degrees, was recorded at the Hong Kong Observatory on July 17.

     A mixture of sunshine and showers prevailed from July 19 to 22, culminating in a very hot day on July 22 as two tropical cyclones, Roke and Sonca, hovered over the northern part of the South China Sea. With Roke making landfall over the eastern part of Hong Kong in the morning on July 23, local weather deteriorated with outbreaks of heavy squally showers. Showery weather continued to affect the territory on the following day as Roke soon dissipated inland and Sonca headed towards the coast of Vietnam.

     The establishment of an anticyclone over southeastern China brought fine and very hot conditions to Hong Kong on July 25 and 26. With yet another tropical cyclone brewing over the northern part of the South China Sea, local weather turned cloudier with isolated showers on July 27. The developing tropical cyclone was named Haitang and headed towards Taiwan immediately following the passage of another tropical cyclone, Nesat, that moved in from the western North Pacific. With both cyclones passing at a distance to the east of Hong Kong, subsiding air over the Guangdong region led to prolonged sunshine and very hot conditions in the territory towards the end of the month. Under oppressive heat and hazy conditions, temperature at the Hong Kong Observatory soared to the month's highest level of 34.8 degrees on July 30.

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

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

Ends/Wednesday, August 2, 2017
Issued at HKT 12:36

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Table 1 (Warnings and Signals issued in July 2017)

Table 2 (Figures and Departures from Normal - July 2017)