A December with tropical cyclone warning signal
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December 2021 was warmer than usual with a mean temperature of 18.9 degrees, 0.7 degrees above the normal figure of 18.2 degrees (or 1.0 degree above the 1981-2010 normal figure). It was also drier than usual with a total rainfall of 19.5 millimetres, about 32 per cent below the normal of 28.8mm (or about 27 per cent below the 1981-2010 normal of 26.8mm). The annual total rainfall of 2 307.1mm in 2021 was about 5 per cent below the annual normal of 2 431.2mm (or about 4 per cent below the 1981-2010 normal of 2 398.5mm).
Dominated by a dry northeast monsoon and the subsequent replenishment, the weather was generally fine in the first two weeks of December. The weather was very dry from December 1 to 4. With the moderation of the northeast monsoon and a cloud band covering the coast of Guangdong and the northern part of the South China Sea, local weather became mainly cloudy with a few light rain patches on the night of December 14 and the following day. Apart from a few rain patches in the morning, it was rather warm with sunny periods during the day on December 16. The maximum temperature recorded at the HKO that day was 25.8 degrees which was the highest of the month. Under the influence of an intense northeast monsoon, the weather became appreciably cooler and dry on December 17 and the next two days.
Meanwhile, after wreaking havoc in the Philippines, Super Typhoon Rai weakened into a severe typhoon and continued to move westwards entering the southern part of the South China Sea on December 17. It re-intensified into a super typhoon on December 18, making it the first super typhoon occurring in the South China Sea in December since 1961. Rai gradually turned to move north towards the northern part of the South China Sea on December 19 and weakened progressively. With Rai weakening into a severe tropical storm and tracking northeastwards towards the coast of Guangdong, the HKO issued the Standby Signal No.1 on the morning of December 20. Breaking the record of Irma in 1974, Rai became the storm which necessitated the issuance of the latest tropical cyclone warning signal in a year since 1946. Rai continued to move northeastwards across the northern part of the South China Sea and progressively weakened into an area of low pressure on December 21.
Affected by the rain-bearing cloud band associated with Rai, it was rainy and cool from December 20 to 21. More than 20mm of rainfall were recorded over Lantau Island and the northwestern part of the New Territories on these two days. Local weather remained mainly cloudy with a few rain patches from December 22 to 24.
An intense winter monsoon reached the coast of Guangdong on the morning of December 25. Locally, the weather was mainly cloudy with sunny periods during the day and became cool that night. Under the influence of the winter monsoon and an upper air disturbance, it was cloudy and cold with a few rain patches from December 26 to 28. The minimum temperature recorded at the HKO plummeted to 9.9 degrees on the morning of December 27, the lowest of the month. With the prevalence of the dry northeast monsoon and thinning out of clouds covering the coast of Guangdong, the weather turned generally fine and dry with cool mornings for the rest of the month.
Two tropical cyclones occurred over the South China Sea and the western North Pacific in December 2021.
Details of issuance and cancellation of various warnings/signals in December are summarised in Table 1. Monthly meteorological figures and departures from normal for December are tabulated in Table 2.
Ends/Tuesday, January 4, 2022
Issued at HKT 16:00
Issued at HKT 16:00
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