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Under the general prevalence of a subtropical ridge of high pressure, the weather in the first half of July 2012 was drier and warmer than usual. However, the rainfall deficit was more than compensated by the heavy rain episodes in the latter part of the month brought about by the southwest monsoon and Severe Typhoon Vicente. Overall, the monthly total rainfall was 467.8 millimetres, about 24 per cent above the norm. The accumulated rainfall since January 1 was 1,395.6 millimetres, about 5 per cent below the normal figure of 1,473.3 millimetres for the same period. The mean temperature and relative humidity for the whole month were both near-normal.
Under the influence of a subtropical ridge of high pressure, the weather in Hong Kong was mainly fine and hot apart from a few showers for the first two days of the month. With winds turning light, it was sunny and very hot during the day on July 3 and 4. The temperature at the Observatory rose to a maximum of 33.8 degrees on July 4, the highest in the month.
Affected by a trough of low pressure over the northern part of the South China Sea, local weather became mainly cloudy with showers on July 5 and 6. With the return of the subtropical ridge extending westwards from the Pacific Ocean, it was sunny and hot with a few isolated showers for the ensuing six days.
Dominated by a southwesterly airstream, the weather remained hot with some showers from July 13 to 17. Affected by an active southwest monsoon, there were heavy showers and squally thunderstorms on the morning of July 18. More than 30 millimetres of rainfall were recorded over most parts of the territory. Under the prevalence of the anticyclone over southern China, local weather turned mainly fine and very hot for the next three days.
Meanwhile, Tropical Depression Vicente entered the South China Sea on July 21 and intensified into a tropical storm that night. It stalled over the northeastern part of the South China Sea on July 22. Affected by the outer rainbands of Vicente, the weather became mainly cloudy with a few squally showers and thunderstorms on that day. Vicente began to edge towards the south China coast to the west of the Pearl River estuary on July 23, undergoing rapid intensification into a typhoon in the afternoon and further to a severe typhoon. With the approach of Vicente, local weather deteriorated significantly on that day with heavy squally showers. Local winds also strengthened generally in the evening and reached gale to storm force in some places. Hurricane force winds affected the southwestern part of the territory in the small hours on July 24, necessitating the issuance of the Hurricane Signal No. 10 for the first time since 1999. Vicente made landfall near the coastal areas of Taishan about 130 kilometres west-southwest of Hong Kong before dawn on July 24 and weakened gradually over land during the day. As Vicente moved away, local wind and rain abated gradually that afternoon. Between July 23 and 24, over 200 millimetres of rainfall were recorded at the Hong Kong Observatory.
Affected by a broad monsoon trough, local weather remained unsettled with heavy showers and squally thunderstorms from July 25 to 27. With the weakening of the trough, the weather improved gradually with sunny periods on July 28. A ridge of high pressure brought generally fine and very hot weather to the territory for the next two days. Under the influence of the outer subsiding air mass of Typhoon Saola over the seas east of Taiwan, it was very hot in Hong Kong on July 31. However, thundery showers that developed over inland Guangdong moved in to affect the territory towards the evening.
Four tropical cyclones occurred over the South China Sea and the western North Pacific 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 normal in July are tabulated in Table 2.
Ends/Thursday, August 2, 2012
Issued at HKT 19:08
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