The number eight signal was issued at 5.20pm on Tuesday, but it was widely expected, and the Observatory said earlier on Wednesday that the signal would stay “before noon”.
The Education Bureau said classes of all day schools would be suspended.
The Social Welfare Department said all child care centres, elderly services centres and day rehabilitation units, including sheltered workshops and integrated vocational training centres, would not be open.
It said the units would resume normal operation in two hours after the number eight signal is cancelled, if the weather and conditions permit.
The Immigration Department said services at its headquarters and all branch offices would be suspended.
It said people who had made an appointment to replace their smart identity cards on Wednesday could do it on any subsequent working days without making a new appointment.
The judiciary said all court and tribunal hearings would remain adjourned on Wednesday morning.
It said hearings would remain adjourned on Wednesday afternoon if the number eight storm signal remains in force at 11am.
The MTR said it is providing limited services, with trains on most of its rail lines running at 10-minute intervals. The Airport Express is running at 30 minute intervals.
The rail operator said it had deployed additional staff to check overhead lines and track conditions and carry out any urgent repair work when it is safe to do so.
Most bus services have been suspended, and ferry services are also suspended.