A September with Super Typhoon Saola and record-breaking rainstorm

     In terms of extreme weather, September 2023 was an eventful month in Hong Kong with the ferocious strike by Super Typhoon Saola on September 1 and 2, and the phenomenal rainstorm on September 7 and 8. With a maximum sustained wind of 230 kilometres per hour near its centre, Saola was the second most intense tropical cyclone affecting the South China Sea since 1950, and Hurricane Signal No. 10 was issued in Hong Kong during the passage of Saola, the first time since Super Typhoon Mangkhut hit Hong Kong in September 2018. A trough of low pressure associated with the remnant of Tropical Cyclone Haikui brought prolonged torrential rain to Hong Kong on September 7 and 8, and necessitated the issuance of the Black Rainstorm Warning for 16 hours and 35 minutes, setting the longest record since the introduction of the rainstorm warning system in 1992. Mainly attributed to the heavy rain associated with Saola and troughs of low pressure in the first half of the month, the Observatory recorded an all-time high September rainfall of 1 067.1 millimetres, more than three times of the September normal of 321.4 millimetres and easily breaking the previous record of 844.2 millimetres set way back in September 1952. Moreover, the rainfall deficit in the first eight months of this year was mostly compensated for by the record-breaking rainfall in September. The accumulated rainfall this year up to September was 2 224.3 millimetres, slightly less than the normal figure of 2 242.8 millimetres for the same period. Despite the stormy weather in the first part of the month, there was a long spell of sunny and very hot weather with 10 consecutive very hot days from September 21 to 30 in the later part of the month. It also set the longest record of consecutive very hot days for September. Overall, the month remained hotter than usual with a mean temperature of 28.5 degrees, 0.6 degrees above the normal of 27.9 degrees.
     Super Typhoon Saola moved generally westwards across the coastal waters of Guangdong on September 1 and skirted past within 40 kilometres to the south-southeast of Hong Kong that night. It continued to move across the coast of western Guangdong and weakened from a super typhoon into a tropical depression progressively the next day. Saola entered Beibu Wan on the morning of September 3 and weakened into an area of low pressure later that day.
     The storm to hurricane force winds of Saola impacted many places in Hong Kong on September 1 and 2. The maximum 60-minute mean wind speeds recorded at Waglan Island and Cheung Chau were 154 km/h and 116 km/h respectively. The storm surge induced by Saola also resulted in flooding in some low-lying coastal areas of Hong Kong, including Sha Tin, Tai Po, and Tai O. The water level at Sai Kung rose to about 4.5 metres above Chart Datum at midnight on September 1. Saola brought squally heavy showers to Hong Kong on September 1 and 2. More than 150 millimetres of rainfall were recorded over most parts of the territory and rainfall even exceeded 250 millimetres over Central and Western, Wan Chai and Tsuen Wan Districts on these two days. According to preliminary reports, there were over 3 000 reports of fallen trees, 21 reports of flooding and two reports of landslides in Hong Kong. There were also about 40 reports of damaged scaffolding, signboards and windows. The power supply was temporarily interrupted in some places. At the Hong Kong International Airport 460 flights were cancelled. While more than 80 people were injured, there were no fatalities in Hong Kong during the passage of Saola. As Saola departed from Hong Kong and weakened progressively, local winds moderated later on September 2 .
     Over the western North Pacific, Tropical Cyclone Haikui headed towards Taiwan on September 3. It made landfall over eastern Guangdong and weakened into an area of low pressure inland two days later. Under the influence of the outer subsiding air of Haikui, apart from isolated showers and squally thunderstorms, it was generally fine and very hot during the day in Hong Kong on September 3 and 4. Affected by the remnants of Haikui, it was mainly cloudy with a few showers on September 5 and 6.
     Under the influence of a trough of low pressure associated with the remnants of Haikui over the coast of Guangdong, the weather of Hong Kong started to deteriorate with heavy rain and squally thunderstorms on the night of September 7. The incessant downpour continued to affect the territory till the next day. During the torrential rain, the Hong Kong Observatory Headquarters registered a record-breaking hourly rainfall of 158.1 millimetres from 11pm to midnight on September 7, the highest since records began in 1884. The two-hour total rainfall of 201.0 millimetres and 12-hour total rainfall of 605.8 millimetres recorded at the Observatory Headquarters during this phenomenal rainstorm also broke their respective records. Moreover, the 24-hour rainfall from 4pm on September 7 to 4pm next day reached 638.5 millimetres, about a quarter of the normal annual total rainfall of Hong Kong, and close to the highest records kept by the historical rainstorm on May 30, 1889. More than 400 millimetres of rainfall were recorded over many parts of the territory and rainfall even exceeded 800 millimetres over the Eastern District and Southern District of Hong Kong Island on September 7 and 8. Flash floods and landslides affected many parts of the territory, causing widespread traffic disruption and damage to infrastructures. According to preliminary reports, there were 75 reports of landslides and 60 reports of flooding. Power and water supply were temporarily interrupted in some places. At least two people were killed and more than 140 were injured during the rainstorm.
     With a trough of low pressure lingering over the coastal areas of Guangdong, local weather remained unsettled with outbreaks of heavy showers and thunderstorms on September 9 to 16. The showers were particularly heavy in some areas. More than 100 millimetres of rainfall were recorded over most parts the territory and rainfall even exceeded 400 millimetres over parts of Sai Kung, Sha Tin and Tai Po Districts on these eight days. Under the rain, temperatures at the Observatory dropped to a minimum of 24.8 degrees on September 10, the lowest of the month. With the weakening of the trough of low pressure, the showers eased off with sunny periods during the day on September 17.
     Under the influence an anticyclone aloft, apart from isolated showers, it was generally fine and very hot from September 18 to the end of month. Under light wind conditions, the maximum temperature at the Observatory soared to 34.4 degrees on September 22, the highest of the month. Moreover, the maximum temperature at the Observatory reached 33.7 degrees on September 29, making it the hottest Mid-Autumn Festival on record. Thundery showers triggered by high temperatures also affected the northern part of the New Territories on the afternoon of September 30.
     Six tropical cyclones occurred over the South China Sea and the western North Pacific in September 2023.
     Details of issuance and cancellation of various warnings/signals in the month are summarised in Table 1. Monthly meteorological figures and departures from normal for September are tabulated in Table 2.

Ends/Wednesday, October 4, 2023
Issued at HKT 15:00