The observatory said on Wednesday that a trough of low pressure lingering over the coast of Southern China will continue to affect the city's weather in the next few days, bringing occasional heavy showers and thunderstorms on Thursday and Friday.

But its acting director, Chan Pak-wai, said it's difficult to predict the rainfall in the coming days, or whether the observatory will have to issue rainstorm warning signals.

“We will use a forecast system to issue warnings to relevant government departments about the coming rainfall as soon as we can. But those predictions may only remain valid for about 15 minutes. This forecast system that we’re using in Hong Kong is already one of the best in the world. So the level of uncertainty is really high,” Chan said.

He said the rainfall between midnight and 1pm was about 40 to 50 millimetres, compared to over 100 millimetres in Macau and over 200 millimetres in Zhuhai.

He added that the distribution of rainfall in the city had varied, with areas like Sai Kung and Chek Lap Kok recording more than other areas.

Authorities in Macau and Guangzhou on Tuesday night ordered schools to suspend classes a day in advance. Asked whether the observatory could issue warnings about school closures in advance if needed, Chan said the observatory has been updating other government departments, including the Education Bureau, about the latest weather situation.

Chan said the observatory expects the weather will continue to be unstable over the weekend, but will gradually improve next week as the trough of low pressure departs the coastal areas.

Temperatures will fall slightly to below 20 degrees on Monday as a surge of the northeast monsoon is expected to reach the coast of Guangdong, he said.