The Observatory issued the Standby Signal Number 1 at 9:10pm on Wednesday, as a tropical depression edged closer to Hong Kong.
Forecasters said there will be a significant shift in weather conditions and urged the public to prepare for stormy weather on Friday.
The tropical depression was estimated to be about 740km south-southeast of Hong Kong at 9pm.
It is forecast to move north-northwest at about 10km per hour in the general direction of the coast of western Guangdong.
The Observatory said the standby signal will remain in force before noon on Thursday.
It added that the latest forecast track showed that the tropical depression will keep a distance of more than 500km from Hong Kong through till Thursday morning and local winds were not expected to strengthen much by then.