The observatory says it will consider issuing the Standby Signal No 1 on Tuesday evening, as Severe Tropical Storm Ma-on is expected to come within 800 kilometres of Hong Kong.

The storm is expected to move from the northern Philippines towards the coast of Guangdong.

"The forecast track and intensity of Ma-on remains uncertain under the influence of the terrain of Luzon," said Senior Scientific Officer Lee Shuk-ming.

Forecasters said Wednesday will be very hot with haze and the weather will deteriorate significantly as winds strengthen later in the day.

They added that there will be heavy squally showers and even stronger winds on Thursday.