The Observatory says it will issue the Standby Signal, No. 1 on Friday evening, if a tropical cyclone developing near the Philippines comes within 800km of Hong Kong.

"The area of low pressure over Luzon will enter the South China Sea and intensify into a tropical cyclone," it said.

"The tropical cyclone will move across the central and northern parts of the South China Sea and intensify gradually over the weekend, maintaining a distance of about 500km or more from Hong Kong."

The weather agency expects Saturday to start off mainly fine and very hot, but showers and squally thunderstorms could follow later in the day.

Conditions will deteriorate on Sunday with winds to pick up. Members of the public were urged to stay away from the shoreline and not engage in water sports.