It issued the Strong Wind Signal Number 3 at 12.40am, warning that Kompasu had intensified into a severe tropical storm.
The observatory said that as of 8am on Tuesday, it was centred about 620 kilometres southeast of Hong Kong and is forecast to move towards the northeastern part of the South China Sea at about 25 kilometres per hour.
It warned that Hong Kong’s weather would deteriorate significantly later on Tuesday, with heavy rain and squalls and storm surges.
The station said heavy rain might cause flooding in low-lying areas, and warned the public to take appropriate precautions.
Meanwhile, the authorities have announced that a number of services will be closed on Tuesday.
The Education Bureau said kindergartens and schools for those with special needs will be suspended, while the Social Welfare Department had advised people not to take children or any relatives to facilities such as child care centres, elderly services centres, rehabilitation centres or day activity centres.
The Food and Health Bureau said outdoor mobile specimen collection stations will not open, and anyone who needs to have a Covid test must go to a community testing centre or indoor testing station instead.
The Labour Department has reminded employers to make the appropriate arrangements for staff prior, during and after tropical cyclone and rainstorm warnings. It also reminded contractors of construction sites to be aware of safety risks during inclement weather, after a worker died after being caught up in a scaffolding collapse in Happy Valley on Friday brought about by strong winds.
Earlier, the observatory said it would deploy additional staff to monitor the approaching storm amid criticism over its forecasts as Tropical Storm Lionrock passed by the city.
Director Cheng Cho-ming said its public messages for Lionrock had not been ideal, and hoped the forecaster would do a better job this time around.