Mainly attributing to weaker than normal northeast monsoon over southern China, September 2024 was much hotter than usual in Hong Kong.  The mean temperature of 29.2 degrees, mean maximum temperature of 32.0 degrees and mean minimum temperature of 26.8 degrees were 1.3 degrees, 1.5 degrees and 0.7 degrees above the respective normals and were respectively the third, one of the fourth and one of the seventh highest for September on record.  With stronger than normal troughing flow in the lower atmosphere over the coast of southern China and the northern part of the South China Sea, the month was also cloudier and wetter than usual.  The mean amount of cloud in the month was 74 percent, 8 percent above the normal of 66 percent and one of the ninth highest on record for September.  The monthly total rainfall was 520.9 millimetres, about 62 percent above the normal figure of 321.4 millimetres.  The accumulated rainfall this year up to September was 2104.3 millimetres, about 6 percent lower than the normal figure of 2242.8 millimetres for the same period. 

After a quiet period of tropical cyclone activities over the South China Sea in August, an area of low pressure over the seas east of the Philippines intensified into a tropical depression on 1 September and was named Yagi.  Yagi moved across Luzon the next day and entered the northern part of the South China Sea on 3 September.  Moving generally westward across the northern part of the South China Sea, Yagi rapidly intensified from a severe tropical storm to a super typhoon in just 24 hours.  Yagi attained its peak intensity with an estimated maximum sustained wind of 230 kilometres per hour near its centre on 6 September, making it the second strongest tropical cyclone in the South China Sea since the Observatory’s records began in 1950.  After moving across Hainan Island and Beibu Wan, Yagi made landfall over the northern part of Vietnam later on 7 September and progressively weakened into an area of low pressure over the inland areas on 8 September. 

With the weakening of the trough of low pressure, the showers eased off with sunny periods during the day on 25 September.  An anticyclone aloft brought generally fine weather on 26 – 27 September.  Affected by an upper-air disturbance, the weather turned cloudier with more showers on 28 September with severe squally thunderstorms in the afternoon.  Waterspout was also reported over the sea area off Hung Hom, the first occurrence in the Victoria Harbour according to reports received by the Observatory since 1959.  Under the influence of the outer subsiding air of Super Typhoon Krathon, apart from isolated showers, it was generally fine in Hong Kong on the last two days of the month.