The Observatory said the Standby Signal No 1 will remain in effect at least until noon on Saturday.

The forecaster issued the storm alert at 12.20pm after a tropical depression formed over the northeastern part of the South China Sea.

Senior scientific officer Lee Shuk-ming said the observatory will assess this weekend whether a higher signal is needed.

“We can see that the tropical cyclone is expected to be closest to the vicinity of the Pearl River Estuary later [on Saturday to Sunday morning],” she said.

"Depending on the track of the tropical cyclone, the speed of its intensification, and the distance between its associated strong winds and the Pearl River Estuary, the observatory will assess the need to issue higher tropical cyclone warning signals later [on Saturday and first thing on Sunday].”

As of 6pm, the tropical depression was about 540 kilometres east-southeast of Hong Kong and forecast to move northwest or west-northwest slowly, lingering over the northeastern part of the South China Sea. The Observatory also said it was intensifying gradually.