There are limits to how far in advance weather forecasters can work out the path of rainstorms, a former observatory chief said on Friday, as he explained why Thursday night's massive deluge took Hong Kong by surprise.

At one point, record-breaking hourly rainfall of 158.1 millimetres was recorded at the observatory in Tsim Sha Tsui.

A Black Rainstorm Warning in effect for many hours was accompanied by widespread flooding, landslides, road subsidence and serious traffic and transport disruption.

A former assistant director of the observatory, Leung Wing-mo, said it is not surprising that forecasters were unable to give the public more of a heads up about what was in store for the city.

"One, scientists do not understand the mechanism of such rainstorms 100 percent. Second, it's very difficult to simulate and then forecast, even with high-speed computers, to pinpoint exactly where and when this kind of record-breaking rainfall will occur," he said.

"If you want the observatory to give you a long lead time, then inevitably it will give much more false alarms, so we have to strike a balance."

Leung, who is also a spokesman for the Hong Kong Meteorological Society, said Hong Kong can expect to see weather like this more often in future, due to global warming.

"In the past few decades, record-breaking events have been occurring much, much more frequently...This is a clear indication that climate change has a role to play. As a matter of fact, climate change is making extreme weather more extreme," he said.