Why have severe storms hit Hong Kong despite lower cyclone intensity?

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Hong Kong’s recent severe rainstorms are the result of a monsoon providing rich moisture that amplified the residual impact of storm Danas, despite it weakening after making landfall, the local forecaster has said.

The Observatory shared the explanation after heavy downpours and thunderstorms struck the city on Thursday, resulting in six reported cases of flooding and prompting the government to cancel all day school classes on Friday.

Danas landed in Zhejiang in eastern China on Tuesday, with the wind speed at its centre significantly reduced. But the remnants of the then tropical cyclone remained active, which brought adverse weather to Hong Kong, the forecaster explained.

“Besides Danas, there was also an active southwest monsoon, which was rich in humidity,” Shum Chi-tai, an acting senior scientific officer at the Observatory, told a radio programme on Friday.

“In the presence of such atmospheric factors, despite Danas reducing to a low pressure area with lower wind speed at its centre, the accompanying rains and thunderstorms still brought a very significant impact.”