The observatory said it will issue the Standby Signal No 1 on Tuesday night at the earliest, with an area of low pressure over the central part of the South China Sea expected to develop into a tropical cyclone.

"Depending on its rate of development of entering within 800 kilometres of Hong Kong, the observatory will issue the Standby Signal No 1 between Tuesday night and early Wednesday at the earliest," forecasters said.

It's expected to move towards the vicinity of the coast of Western Guangdong and Hainan in the middle and latter parts of the week.

The observatory also said it will look at whether to issue a higher signal later on Wednesday or Thursday.

It added that the latter part of the week will see windy weather with occasional squally showers and swells.