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A rainy May
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     May 2017 was wetter than usual. The total rainfall recorded in the month was 399.3 millimetres, about 31 per cent above the normal figure of 304.7 millimetres. The accumulated rainfall recorded in the first five months of the year was 533.8 millimetres, about 17 per cent below the normal figure of 640.8 millimetres for the same period.
 
     Under the influence of a maritime airstream, the weather in Hong Kong was hot with sunny periods on the first three days of the month. There was coastal fog on the morning of May 2. Upon the passage of a trough of low pressure, local weather deteriorated with heavy showers and squally thunderstorms on May 4. More than 30 millimetres of rainfall were recorded over widespread areas. With the weakening of the trough of low pressure, the weather became mainly fine and hot in the next two days, with some mist and haze. 
 
     A fresh easterly airstream brought cloudier weather and a few showers to the territory on May 7 and 8. With the passage of a trough of low pressure, there were thundery showers in the early morning on May 9. The visibility was rather low the next day under light wind conditions. With the setting up of a ridge of high pressure over the coast of Guangdong and the northern part of the South China Sea, the weather became mainly fine and hot on May 11. Temperatures at the Hong Kong Observatory rose to a maximum of 31.6 degrees, the highest of the month.
 
     With a trough of low pressure lingering over the coastal areas of Guangdong and the northern part of the South China Sea, there were more showers in mid-May. The showers were heavy at times on May 15, bringing over 50 millimetres of rainfall to Kowloon and Sha Tin. With the trough of low pressure moving to the northern part of the South China Sea and the setting in of a relatively dry easterly airstream, the weather turned mainly fine on May 17. Under the influence of a maritime airstream, the weather became generally cloudy with a few showers over the next six days. 
   
     With the passage of a trough of low pressure across the coast of Guangdong, local weather deteriorated with heavy showers and squally thunderstorms on May 24. More than 70 millimetres of rainfall was recorded over widespread areas, with rainfall exceeding 300 millimetres in Kwai Tsing and Sham Shui Po, necessitating the first issuance of a Black Rainstorm Warning Signal this year. Serious flooding was reported in many areas over the territory including Lai Chi Kok, Tseung Kwan O, Ho Man Tin and central and western parts of Hong Kong Island.
 
     With the trough of low pressure moving towards the northern part of the South China Sea, local weather improved with sunny intervals on May 25. An easterly airstream brought mainly cloudy weather with one or two isolated showers to the territory the next day. Under the influence of a dry continental airstream, it was mainly fine and dry from May 27 to 29. With the continental airstream weakening and being gradually replaced by a southwesterly airstream, the weather was hot with a few showers towards the end of the month.
 
     There was no tropical cyclone over the South China Sea and the western North Pacific in May 2017.
 
     Details of issuance and cancellation of various warnings/signals in May are summarised in Table 1. Monthly meteorological figures and departures from normal for May are tabulated in Table 2.
 
Ends/Friday, June 2, 2017
Issued at HKT 16:53
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