The Weather of September 2025 - A stormy and hot September

September 2025 was marked by the successive strikes of tropical cyclones Tapah, Mitag and Ragasa. The ferocious strike of Super Typhoon Ragasa necessitated the issuance of the Hurricane Signal No. 10 on 24 September once again after Typhoon Wipha, tying the record of issuing the No. 10 Signal twice in the same year since 1964. Despite the succession of tropical cyclones affecting Hong Kong, September 2025 was hotter than usual in Hong Kong, mainly attributing to the warmer than normal sea surface temperature and stronger than usual southerly flow in the lower atmosphere over the northern part of the South China Sea. The monthly mean minimum temperature of 27.3 was 1.2 degrees above the normal and one of the second highest on record for September. The monthly mean maximum temperature of 32.2 degrees and mean temperature of 29.3 degrees were respectively 1.7 degrees and 1.4 degrees above their corresponding normals and both the third highest on record for September. There were in total 13 hot nights, breaking the previous record set in September 2021. The 15 very hot days in the month was also one of the highest records for September. Mainly due to the passage of tropical cyclones, the monthly rainfall was 528.7 millimetres, about 64 percent above the normal of 321.4 millimetres. The accumulated rainfall this year up to September was 2514.0 millimetres, about 12 percent above the normal of 2242.8 millimetres for the same period.

Under the influence of an anticyclonic aloft, apart from isolated thunderstorms on 2 - 3 September, the local weather was generally fine with prolonged heat on the first six days of the month. The maximum temperature at the Observatory rose to 35.3 degrees on the afternoon of 5 September, the highest of the month. Besides, Tropical Depression Tapah formed near Luzon on 5 September. It tracked west-northwestwards across the central and northern parts of the South China Sea and intensified into a tropical storm on 6 September. Tapah turned north-northwestwards towards the western coast of Guangdong on 7 September and further intensified into a severe tropical storm that night. Tapah made landfall near Taishan of Guangdong and then moved into inland Guangxi the next day. Locally, the weather was mainly cloudy with occasional squally showers and thunderstorms on 7 September. The strike of Tapah necessitated the issuance of the No.8 Gale or Storm Signal that night. Gale force winds prevailed over many places, with winds reaching storm force offshore and on high ground at first on 8 September. The rainbands of Tapah also brought heavy squally showers to the territory. More than 100 millimetres of rainfall were recorded over many places on 8 September.

An active southerly airstream brought occasional showers and isolated thunderstorms to Hong Kong on 9 September. More than 30 millimetres of rainfall were recorded over New Territories West. Under the influence of the subtropical ridge, apart from a few showers on 10 September, local weather was generally fine and very hot on 10 - 14 September.

Affected by an upper-air disturbance, while it remained very hot on 15 - 17 September, it was mainly cloudy with a few showers and thunderstorms. Showers were heavier on the morning of 17 September with more than 30 millimetres of rainfall recorded over Hong Kong Island and Tseung Kwan O. Under the influence of an anticyclone aloft, apart from a few showers and isolated thunderstorms, it was very hot with sunny periods on 18 September.

Besides, Tropical Depression Mitag tracked northwestwards across the northeastern part of the South China Sea on 17 September and intensified into a tropical storm the next day. It further intensified into a severe tropical storm on 19 September and made landfall near Shanwei of Guangdong that afternoon. Under the influence of the northeast monsoon, Mitag then gradually turned to move westwards across the northern part of the Pearl River Estuary and weakened into an area of low pressure over inland Guangdong on 20 September. Locally, it was mainly cloudy with a few squally showers on 19 September. Mitag and its remnant brought heavy showers and squally thunderstorms to Hong Kong on the following two days. More than 250 millimetres were recorded over Hong Kong Island and the eastern part of the New Territories and rainfall even exceeded 300 millimetres over Lantau Island on these two days.

Furthermore, tropical cyclone Ragasa formed over the western North Pacific to the east of the Philippines on 18 September. It moved west-northwestwards on the next three days and intensified progressively into a super typhoon. Ragasa moved across Luzon Strait on 22 September and continued to track west-northwestwards across the northern part of the South China Sea the next day, edging closer to the coast of Guangdong. Ragasa skirted about 120 kilometers south of Hong Kong with super typhoon intensity on the morning of 24 September. It made landfall over Yangjiang of Guangdong that afternoon and weakened. Ragasa then moved across the coast of Guangxi and the vicinity of the northern part of Vietnam the next day, and dissipated gradually at night. Locally, under the influence of Ragasa's outer subsiding air, it was very hot with sunny periods on 22 September. As Ragasa came closer to Hong Kong, winds strengthened progressively the next day and squally showers set in later in the afternoon. Under the influence of Ragasa's extensive circulation with fierce winds, storm to hurricane force winds affected many places in Hong Kong on 24 September. The maximum 60-minute mean wind speeds recorded at Waglan Island and Cheung Chau were 133 km/h and 114 km/h respectively. There were also frequent heavy squally showers and more than 200 millimetres of rainfall were generally recorded over the territory on that day. As the approach of Ragasa coincided with the astronomical high tide, storm surge induced by Ragasa resulted in unusually high water level in many parts of the territory. The sea level at Quarry Bay rose to a maximum of 3.4 metres above Chart Datum, close to the level when Super Typhoon Hato hit Hong Kong in 2017. The fierce winds of Ragasa also triggered overtopping waves, causing flooding in many parts of the coastal areas of Hong Kong. According to preliminary reports, at least 101 people were injured during the passage of Ragasa. A woman and her son were swept away by swells at the waterfront of Chai Wan and were later rescued. There were at least 1,224 reports of fallen trees, 22 reports of flooding and four reports of landslides. Under the influence of the outer rainbands associated with Ragasa, there were still a few showers and isolated thunderstorms on 25 September.

Under the influence of a northeast monsoon and the subsequent anticyclone aloft, apart from a few showers, local weather was generally fine and hot on the last five days of the month.

Six tropical cyclones occurred over the South China Sea and the western North Pacific in September 2025.