The observatory says it will consider lowering the number eight typhoon signal between 4pm and 7pm on Wednesday, as Kompasu moves away from Hong Kong.
At 2pm, the typhoon was centred about 490 kilometres southwest of Hong Kong and was forecast to move west towards Hainan.
"As Kompasu departs from Hong Kong and is about to make landfall over Hainan Island, local winds are subsiding gradually," said senior scientific officer Lee Shuk-ming.
Lee said squally showers and thunderstorms were expected during the day and people should beware of flooding.
Much of Hong Kong was brought to a standstill by the typhoon, with the stock market and schools suspended.