A spokesman for the Hong Kong Meteorological Society, Leung Wing-mo, said on Monday that it would not be possible to tailor rainstorm warnings for particular districts, because rain is extremely unpredictable.

On Saturday, the Red Rainstorm Warning was in place for nearly seven hours and flooding and landslides were reported in Sai Kung and Tseung Kwan O.

The observatory had at one point raised the possibility of a Black Rainstorm Warning and some people questioned why the alert wasn't issued, even though rain was particularly heavy where they were.

Leung, a former assistant director at the observatory, said rainstorm warnings are generally issued when rainfall at a certain amount of monitoring stations around the city meet the relevant criteria.

He noted that on Saturday, government forecasters provided supplementary information regarding the amount of rain recorded in different places.

"Really, the downpours are part of nature, they may happen wherever. So forecasting where exactly the rain will be is very difficult," he said on an RTHK programme.

"We can only issue warnings based on rainfall for the whole of Hong Kong, that the amount recorded or the expected amount, triggers the corresponding rainstorm warning."

Leung described the heavy rain on Saturday as rare, although he noted that April is regarded as belonging to the "flooding season" in South China.

He said the climate has changed because of global warming and people can expect more extreme weather conditions and heavier rain from now on.

Meanwhile, Sai Kung district councillor Christine Fong said hundreds of vehicles were trapped by floods on Saturday after the heavy rain caught drivers by surprise.

She suggested officials build stormwater storage facilities or an artificial lake in Hebe Haven and Ho Chung.