The Observatory issued the Amber Rainstorm Warning on Thursday morning, before upgrading it to Red an hour later.
It said at 9:36am that heavy rain exceeding 50 millimetres in an hour "has fallen or is expected to fall generally" over Hong Kong.
The Education Bureau instructed schools to implement contingency measures to ensure students' safety. Classes already in session should continue until the end of school hours, it added, and students must only be released when conditions are safe.
The amber signal was also issued on Wednesday night, and was later replaced by a red signal before midnight before all warnings were cancelled at 1:40am on Thursday.
The forecaster warned of flooding in some low-lying and poorly drained areas, and urged those who are likely to be affected to take necessary precautions.
The warning came as the territory braced for the impact brought by the remnant of tropical cyclone Danas, which moved southwest across eastern Guangdong, possibly skirting past Guangzhou.
An active southwest monsoon to its south will generally affect the vicinity of the Pearl River Estuary on Thursday and Friday, the Observatory added, bringing torrential rain and squally thunderstorms over the region.
It will consider issuing the Strong Monsoon Signal depending on the local wind change.
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Last updated: 2025-07-10 HKT 09:40