The observatory said the Standby Signal No 1 may be issued as soon as Wednesday morning.

An area of low pressure is consolidating around 800 kilometres from Hong Kong and is expected to gradually develop into a tropical cyclone.

Acting senior scientific officer Shum Chi-tai told an RTHK radio programme on Tuesday that the forecaster will assess whether a higher signal has to be issued on Thursday.

"Generally speaking, the impact of a tropical cyclone on Hong Kong depends on a lot of factors," he said.

"Apart from its distance from Hong Kong, it will also depend on its strength at the time.

"The wind field structure of the cyclone will also affect the wind force and the amount of rain it brings to Hong Kong."

Shum said Hong Kong will continue to see hot weather on Wednesday and there will also be swells, which means the public should stay away from shorelines.

The tropical cyclone is expected to move towards Hainan and the vicinity of the coast of western Guangdong in the middle and latter parts of this week.

Shum added that during that time, there will be squally thunderstorms.

Meanwhile, a doctor has called on the public to stay hydrated and take rest regularly to prevent heatstroke and heat exhaustion amid soaring temperatures in the city.

Dr Lam Wing-wo made the remarks a day after the observatory recorded the highest temperature so far this year, with 34.2 degrees seen at its headquarters in Tsim Sha Tsui.

He told an RTHK radio programme that the human body takes time to adapt to hotter weather, and vulnerable groups such as those with chronic diseases, the elderly and children are more prone to heat-related illnesses.

For instance, Lam said, children can suffer heat exhaustion after being exposed to the sun for 30 minutes to an hour, and can get heatstroke in just half an hour.

"Before exercising, you have to drink more water. If it’s vigorous exercise, you have to drink one litre of water every 30 minutes. If you have drunk one litre of water, you also need to take electrolyte drinks because you are also losing sodium during exercising. Many athletes lose sodium when they are exercising in the hot weather, causing heat convulsion and heat syncope."

He said it is also crucial to wear sun protective clothing and stay under shelter regularly.

Simon Liu, president of the Hong Kong Construction Association, said the sector has set up misting fans and ice makers at construction sites.

Liu said older workers are encouraged to put on smart watches that will provide health alerts.

The industry will remind frontline staff to take sufficient rest when the Heat Stress at Work Warning is in place, he added.