It issued the signal at 12.40am, warning that Kompasu had intensified into a severe tropical storm.
The HKO said that as of 6am on Tuesday, it was centred about 680 kilometres away from 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 HKO said heavy rain might cause flooding in low-lying areas, and warned the public to take appropriate precautions.
Kompasu is forecast to skirt within 400 kilometres to the south of Hong Kong early on Wednesday.
Earlier, the HKO 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.
He said a review would be carried out to see how the observatory could improve its forecasting and communication.
Last updated: 2021-10-12 HKT 06:08