Heavy rain caused flooding in Chai Wan and other areas.

Heavy rain caused flooding in Chai Wan and other areas.
Torrential downpours suddenly hit Hong Kong on Thursday night, causing severe flooding in parts of the city that left commuters trapped in their vehicles in traffic jams and pedestrians stranded on streets turned into rivers.

The Observatory raised its highest-level black rainstorm warning, while Shenzhen authorities released a statement cautioning that water from the reservoir across the border would be discharged at midnight, which might cause flooding in the New Territories.

Videos being shared on social media showed scenes of Hong Kong streets turning into rushing rivers and people being caught up in flooding around the city, while one clip showed a swamped underground railway station.

The black warning, signifying rain exceeding 70mm an hour has fallen or is expected to fall generally over Hong Kong, was issued at 11.05pm for the first time since October 2021, replacing the red alert raised at 9.50pm. The amber warning, the lowest in the three-tier system, was raised just 25 minutes earlier.

The Observatory said heavy rain was especially affecting the northern part of the New Territories, including Sheung Shui, Ta Kwu Ling and the Sha Tau Kok area. More than 150mm of rainfall was recorded in just two hours.

The sudden downpours caused flash flooding across the city and Eastern district on Hong Kong Island also among of the worst-hit areas. In one incident, water gushed into a bus near Chai Wan Road.

The water was ankle-deep inside the double-decker, but it managed to navigate through the flooded roads without breaking down.

Roads in Causeway Bay and Tin Hau were also submerged, with rainwater also flooding into a double-decker near Happy Valley.

Rain also gushed into the Cross-Harbour Tunnel, one of the main road arteries between Hong Kong Island and Kowloon, forcing vehicles to greatly reduce their speed as they travelled through the centimetres-deep water.

Train stations were also hit by the downpours. At about 12.30am, rail operator the MTR Corporation announced that stations between Whampoa and Kwun Tong had to be closed with passengers told to leave for their own safety.

At Wong Tai Sin station, rainwater flooded a platform, with the water reaching ankle height.

"The entire escalator is full of water ... We are like sailing in a vessel," said a woman on board a train at Wong Tai sin Station in a social media post, where she uploaded a video clip showing the river-like platform. "Even Wong Tai Sin can't save you," she joked, referring to the temple god in the district.

A flooded platform in Wong Tai Sin MTR station.

A flooded platform in Wong Tai Sin MTR station.
A shopping centre in the district was also affected, with chairs floating in the water.

In Wan Chui Estate in Chai Wan, water gushed into a shopping centre, filling the ground floor with ankle-deep water.

A minibus in Sheung Shui was also trapped in the downpour, with muddy floodwater gushing into the vehicle, forcing passengers to raise their legs to avoid getting wet.

In the Sha Tau Kok area near the border with mainland China, rainfall exceeding 150mm in two hours was recorded.

Sha Tau Kok Road was waterlogged, causing vehicles to travel slowly. Residents reported being stranded on buses.

At 11.40pm, the government said it had been informed by Shenzhen authorities about the reservoir discharge.

It said various departments, including the District Office (North), Drainage Services, Police, Fire Services, Water Supplies and Social Welfare had been informed and could take any necessary measures.

The District Office was also making arrangements to tell local residents about the appropriate actions to take.

The Observatory said that under the influence of a trough of low pressure, there would be showers and thunderstorms over the coast of Guangdong in the next few days.