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

The forecaster said the alert would be cancelled when Tropical Storm Danas moves further away from Hong Kong.

"Danas is expected to skirt around 400 kilometres from the east to southeast of Hong Kong [Saturday night to Sunday morning]," senior scientific officer Choy Chun-wing said.

"Danas will then accelerate northeastwards and move in the general direction of the western coast of Taiwan to the vicinity of Taiwan Strait, departing from Hong Kong gradually. When Dennis no longer poses a threat to Hong Kong, the Observatory will cancel all tropical cyclone warning signals."

At 5pm, Danas is about 410 kilometres east-southeast of Hong Kong and is forecast to move north or north-northeast slowly.