But the Observatory also said unless Fengshen adopts a track closer to the Pearl River Estuary or intensifies significantly, the chance of a higher signal is relatively low.
It said the T3 signal would be in force until at least 6pm on Tuesday.
The Education Bureau announced that kindergartens, along with schools for children with physical disability and intellectual disability, will be suspended on Tuesday.
The tropical cyclone was expected to move west or west-northwest at about 18 kilometres per hour across the northern part of the South China Sea, and is forecast to be closest to Hong Kong on Monday night to Tuesday morning, skirting at around 400 kilometres or more to the south of the territory.
But the Observatory expects Fengshen will turn southwestwards and weaken on Tuesday, and move away from the coast of Guangdong.
The forecaster said rain would be more frequent on Tuesday and early Wednesday, under the combined effect of the storm and the northeast monsoon.
The weather would also be persistently windy in the next couple of days, forecasters said.
Temperatures in Hong Kong will fall to around 19 degrees in the urban areas, and a couple degrees lower in the New Territories, it added.
The forecaster said flooding may also occur over some low-lying coastal areas during high tides in the next few consecutive nights.
Members of the public are advised to stay away from the shoreline.
The weather will then turn brighter in the latter part of this week.
