The Hong Kong Observatory issued a Red Rainstorm Warning Signal at 11.45pm on Wednesday before downgrading it to an amber warning an hour later, as heavy downpours hit the city.

The forecaster warned that there could be further heavy rain, which may cause serious flooding and traffic congestion.

It also said that normal school hours could be disrupted.

“Heavy rain will bring flash floods, and flooding is occurring or is expected to occur in watercourses,” the Observatory said.

“People should stay away from watercourses. Residents living in close proximity to rivers should stay alert to weather conditions and should consider evacuation if their homes may become flooded.”

The Observatory also issued a special announcement on flooding in the Northern New Territories at 11.55pm, as rainfall exceeded 70 millimeters in an hour in the Pat Heung and Kam Tin areas.

The Transport Department said at midnight that due to flooding, all Yuen Long-bound lanes of San Tin Highway near Mo Fan Heung have been closed.