The Observatory on Tuesday said the Black Rainstorm Warning Signal will last at least until 3pm.
"Areas of intense thundery showers associated with active southwest monsoon and upper-air disturbances continue to affect Hong Kong and the vicinity of the Pearl River Estuary," it said.
It called on members the public to seek safe shelter and remain there until the rain eases.
Winds of up to 110km per hour have been registered at Green Island off Kennedy Town. The weather agency warned violent gusts may continue to affect the region.
Meanwhile, more than 10,800 strokes of lightning have been recorded in the SAR between midnight and 9:59am Tuesday. Nearly half of them were on Lantau Island.
The Drainage Services Department said as of 11am, it has received 21 reports of flooding.
A spokesman added the department has deployed 180 emergency response teams to monitor 240 locations.
The MTR says trains across its network are running normally, although certain entrances at Quarry Bay, Kowloon Tong and Wong Tai Sin stations were shut down due to flooding.
Some flood-prone entrances at Sham Shui Po, Choi Hung and Wan Chai stations were also sealed off as a precaution.
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Last updated: 2025-08-05 HKT 13:02