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An exceptionally warm and stormy October
     October 2016 was marked by record-breaking high mean temperatures, despite a succession of tropical cyclones passing in the vicinity of Hong Kong and the duration of sunshine falling under 80 per cent of normal figure. The monthly mean maximum temperature of 29.1 degrees, monthly mean temperature of 26.8 degrees and monthly mean minimum temperature of 25.0 degrees were all 1.3 degrees above their respective normal figures and were the highest ever recorded for October. The monthly total rainfall was 624.4 millimetres, about six times higher than the October normal figure of 100.9 millimetres and the second highest on record for October. The accumulated rainfall of 2888.9 millimetres for the first 10 months was about 24 per cent above the normal figure of 2334.0 millimetres for the same period.
     After the passage of the remnant circulation of Severe Typhoon Megi over southern China in late September, unsettled weather prevailed over the coastal waters of Guangdong, with heavy showers and thunderstorms affecting Hong Kong in the early morning on October 1. The Red Rainstorm Warning Signal was issued as more than 70 millimetres of rainfall was recorded generally over the urban areas and the eastern part of the New Territories. Local weather was a combination of increasing sunshine and decreasing showers over the following four days as a weak low pressure system drifted southwestward off the coast of Guangdong and passed to the south of Hong Kong. Meanwhile, Tropical Storm Aere followed the weak low pressure system into the northeastern part of the South China Sea on October 6 and lingered for days to the southeast of Hong Kong before weakening into an area of low pressure on the night of October 10. Locally, it was generally fine and hot during the day on October 6 and the temperature at the Hong Kong Observatory rose to a maximum of 32.4 degrees, the highest of the month. After some showers that night, the weather gradually turned sunnier over the next four days. 
     As Aere weakened and drifted away towards Xisha, cooler air associated with the northeast monsoon gradually spread south towards the coastal areas of Guangdong. Temperatures at the Hong Kong Observatory fell to a minimum of 22.0 degrees on October 11, the lowest of the month. Local weather turned windier with some rain patches under a strong easterly airstream on October 11 and 12, and it became mainly fine over the next four days as the winds gradually subsided. 
     Meanwhile, Severe Typhoon Sarika moved across Luzon and entered the South China Sea on the morning of October 16. Tracking west-northwestwards, Sarika made landfall over Hainan Island on the morning of October 18 and moved across Beibu Wan on October 19. Under the combined effect of Sarika and the northeast monsoon over southern China, local weather became windy with squally showers from October 17 to 19. The convergence of the northeast monsoon and the southerly airstream associated with Sarika triggered prolonged periods of heavy rain and thunderstorms on October 18 and 19. The rain was most intense on the afternoon of October 19, with more than 100 millimetres of rainfall falling generally over Hong Kong, and rainfall even exceeded 200 millimetres over the urban areas, Sha Tin and Tai Po, necessitating the issuance of the first ever Black Rainstorm Warning Signal in October since the Rainstorm Warning System commenced operation in 1992. The hourly rainfall of 78.7 millimetres recorded at the Hong Kong Observatory between 3pm to 4pm that day was also the highest in October since records began in 1884. 
     As Sarika dissipated over inland Guangxi, Super Typhoon Haima moved across Luzon and entered the northern part of the South China Sea on the morning of October 20. Affected by the subsiding air ahead of Haima, it was mainly fine and hazy in Hong Kong that day. Haima weakened gradually into a typhoon as it edged towards the coast of eastern Guangdong and made landfall near Shanwei around noon on October 21. With the approach of Haima, local winds strengthened significantly and reached gale force in many places during the day. The rainbands of Haima also brought squalls and heavy rain to Hong Kong. More than 100 millimetres of rainfall was recorded over Kowloon City, Wong Tai Sin and the eastern part of the New Territories. As Haima moved further inland, local weather improved with sunny periods and a few isolated showers over the next three days.  
     With the establishment of an upper-air anticyclone over southern China, local weather became fine and hot from October 25 to 28. The passage of a cold front on October 29 brought cooler weather and some showers. The weather remained windy with sunny intervals the next day before mainly fine weather returned on the last day of the month as the monsoon winds subsided.
     Five 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 summarised in Table 1. Monthly meteorological figures and departures from the normal for October are tabulated in Table 2.
Ends/Wednesday, November 2, 2016
Issued at HKT 17:26
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