The Hong Kong Observatory issued the typhoon standby signal No 1 at 9.40pm on Sunday, and the signal will remain in force for some time, the Observatory said at 4.45am on Monday.

The No 1 signal is raised when a tropical cyclone is centred within 800km of Hong Kong and may affect the city.

A thunderstorm warning was issued earlier at 8.55pm on Sunday, and was expected to remain in effect until 8am today. Intense gusts reaching 70km/h (45mph) or above may affect Hong Kong, the Observatory said.

The forecast now calls for heavy showers and squalls.

At 5am, the tropical depression over the northern part of the South China Sea was estimated to be about 190km south-southwest of Hong Kong and is forecast to remain almost stationary, then move northwest or west-northwest slowly later, edging closer to the coast of western Guangdong.

In the past few hours, the tropical depression moved slowly with slight intensification. Unless it adopts a track closer to Hong Kong, the chance of local winds generally strengthening significantly in the morning is relatively low, the Observatory said.

Separately, tropical cyclone In-fa is forecast to move across the sea east of Taiwan in the next few days and intensify gradually.

The Observatory has issued two No 1 tropical cyclone warnings this year – one on June 12 and the other on July 7.