The Strong Wind Signal No 3 will be issued between 4pm and 5pm, the observatory said on Monday afternoon, as Severe Tropical Storm Nalgae moved closer to Hong Kong.

The observatory forecast that Nalgae would edge "rather close" to the vicinity of the Peal River Estuary on Wednesday and Thursday, and while the storm was expected to weaken gradually, how much it would do so was "rather uncertain".

"Members of the public are advised to stay away from the shoreline and not to engage in water sports. Those engaging in outdoor work or hiking should also take note of the changes in weather," the observatory said.

At 2pm, Nalgae was estimated to be about 640 kilometres south-southeast of Hong Kong and was forecast to move north-northwest at about 10 kilometres per hour towards the northern part of the South China Sea.