The observatory says it will issue the Standby Signal No 1 between 10pm on Thursday and 4am on Friday as Tropical Cyclone Trami edges closer to the SAR.

It said Trami is expected to gradually move towards the vicinity of Hainan and come within 800 kilometres of Hong Kong on Thursday night or Friday morning.

The observatory said a higher typhoon signal could be possible on Friday or Saturday, depending on the storm's movement and any change in local wind conditions.

The observatory said Trami could linger in the vicinity of Hainan or move eastwards across central and northern parts of the South China Sea early next week.