The Observatory issued the Red Rainstorm Warning Signal at 7.40pm on Friday, about half an hour after it put out an Amber Rainstorm Warning Signal.

The forecaster said heavy showers and severe squally thunderstorms are affecting Hong Kong, and that the rain is especially heavy in Sai Kung, with more than 70 millimetres of hourly rainfall.

There will be flooding in some low-lying and poorly-drained areas, with the Observatory urging people to stay away from water.

It also warned of possible hail hitting the city and urged residents to seek shelter immediately.