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A hot yet stormy August
     August 2017 was hotter than normal and the prolonged heat was relieved by the successive strikes of tropical cyclones Hato and Pakhar within a five-day period during the latter part of the month. Both cyclones led to the issuance of the No. 8 Gale or Storm Signal, with Hato even necessitating the issuance of Hurricane Signal No. 10 on August 23, the first time since July 2012. The mean temperature recorded in the month was 29.3 degrees, the seventh highest for August on record and 0.7 degrees above the August normal of 28.6 degrees. Due to the rain brought by Hato and in particular Pakhar, the monthly total rainfall amounted to 489.1 millimetres, about 13 per cent more than the normal figure of 432.2 millimetres. The accumulated rainfall this year up to August was 2,248.9 millimetres, a surplus of 18 per cent compared to the normal figure of 1,905.5 millimetres for the same period.
     Under the influence of an active southwest monsoon, the weather in Hong Kong was generally cloudy with morning showers that affected mostly the southeastern part of the territory on the first two days of the month. The showers got heavier and became more widespread in the morning on August 3, leading to the issuance of the Red Rainstorm Warning Signal. After another showery morning on August 4, particularly over the southern and southeastern parts of the territory, the weather turned sunny and very hot on August 5 as a ridge of high pressure extended over southeastern China. Generally fine and very hot conditions then persisted for a further three days.
     With a freshening of the southwest monsoon, the weather turned cloudy on August 9 and showery activities increased. A southwest-to-northeast corridor of heavier showers extended from Tsuen Wan to Tolo Harbour on August 10, and then shifted eastwards the next day running from Hong Kong Island to Sai Kung. As the monsoon winds subsided, convective development became less active on August 12 despite some localised showers over Lantau Island. With a ridge of high pressure extending westwards from the Pacific to cover southeastern China from August 16 to 18, a spell of fine weather prevailed from August 13 to 22. Despite an outbreak of thundery showers due to intense day heating at Tai Po on August 18, showers were mostly isolated during the period and there were no showers on August 20 and 21.
     With prolonged sunshine, daytime temperatures at the Hong Kong Observatory reached 33 degrees or higher during the latter part of the fine spell. As Hato headed towards Hong Kong, the subsidence effect ahead of its circulation brought hazy skies and oppressive heat on August 22. The maximum temperature at the Hong Kong Observatory that afternoon soared to an all-time record-breaking high of 36.6 degrees. Squally showers associated with the outer rainbands of Hato then started to affect the territory later that day. The weather deteriorated further overnight as stormy weather battered the city during the passage of Hato in the morning on August 23. Hurricane force winds affected Cheung Chau and the southeastern part of the territory, and a maximum gust of 193 kilometres per hour was recorded at Waglan Island. As the approach of Hato coincided with the astronomical high tide, a storm surge induced by Hato resulted in unusually high water level and serious flooding in many parts of the territory, including Tai O, Heng Fa Chuen, Lei Yue Mun, Sha Tin and Lau Fau Shan. The water level at Quarry Bay rose to a maximum of 3.57 metres that morning, the second highest after the record high of 3.96 metres set by Super Typhoon Wanda in 1962. With Hato making landfall over Zhuhai to the west of Macau in the afternoon and weakening further inland, local winds subsided significantly later in the day.
     The weather was a mixture of sunshine and scattered showers over the next two days, with some heavy showers affecting Tuen Mun and Shek Kong on August 25. Ahead of the visit of Pakhar, oppressively hot and hazy conditions affected Hong Kong again on August 26. Rain and squalls associated with the intense rainbands north of Pakhar started to affect the territory later that evening. Stormy weather persisted for most of the day on August 27 as Pakhar skirted past just to the southwest of Hong Kong, with winds persistently reaching storm force over the northeastern and southern parts of the territory and occasionally attaining hurricane force on high ground. The temperature at the Hong Kong Observatory fell to the month's lowest of 24.0 degrees in rain that day. Heavy showers and squally thunderstorms continued to affect the territory the next day as winds gradually subsided. As the lingering rainbands associated with Pakhar finally cleared away, the weather turned sunny on August 29. Hot conditions with a mixture of sunshine, haze and thundery evening showers then persisted towards the end of the month.
     Seven tropical cyclones occurred over the South China Sea and the western North Pacific in August 2017.
     Details of issuance and cancellation of various warnings/signals in August are summarised in Table 1. Monthly meteorological figures and departures from normal for August are tabulated in Table 2.
Ends/Monday, September 4, 2017
Issued at HKT 16:28
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