The observatory mulls signal No. 3 for tropical cyclone Rai, as it is forecast to skirt Hong Kong 200 kilometers away.
The Standby Signal, No. 1 has been in force since Monday noon.
At 9 pm Monday, Rai has weakened to a tropical storm and was estimated to be about 420 kilometres south-southwest of Hong Kong. It is forecast to move northeast at about 22 kilometres per hour across the northern part of the South China Sea and weaken rapidly.
According to the present forecast, Rai will edge closer to the coast of Guangdong Monday night to Tuesday morning. It will be closest to Hong Kong around Tuesday noon, skirting within about 200 kilometres south of Hong Kong.
"If there are signs that local winds would strengthen further, the Observatory will consider issuing the Strong Wind Signal, No. 3," the observatory said.
The HKO said rainbands associated with Rai are expected to bring rainy weather to the northern part of the South China Sea and the coast of Guangdong on Monday and Tuesday.
As there will be swells, the public is advised to stay away from the shoreline and not to engage in water sports.
There will still be one or two rain patches in the following couple of days with temperatures rising gradually.
Further ahead, an intense winter monsoon is forecast to reach the coast of southern China on Christmas Day. It will become appreciably cold early next week.
According to the HKO, the temperature will dip to 12 degrees next Monday, the day after Boxing Day.