Chief Secretary Eric Chan has called the government's decision to suspend day schools on Friday "difficult", saying he understood that the move might bring inconvenience to people.

The senior official, who heads the steering committee on handling extreme weather, said officials would "rather be safe than sorry" by issuing warnings ahead of time in their approach towards ensuring public safety.

Writing on social media, Chan said he was "very worried" upon learning from the observatory that the SAR could be hit by heavy rain and thunderstorms on Thursday and Friday under the influence of remnants of storm Danas and a southwest monsoon.

The forecaster predicted that the city could have faced its fifth-largest rainfall on record, or 390 millimetres a day, in an "extreme" scenario, he said.

"Therefore, we chose to issue early warnings and make advance preparations, even if it meant 'doing too much', even if the safety margin was too large, even if the rapidly changing weather led to forecast deviations," Chan said in a post.

"We would rather be safe than sorry."

He said decisions made to suspend day-school classes and postpone the secondary school registration for primary six pupils might bring inconvenience for some residents.

However, Chan noted that making early announcements could give residents more time to prepare for the severe weather than if they were notified on the morning of the day of a school suspension.

"The safety of young pupils must be our most important consideration. Through early announcements, we also hope that it'll be easier for parents and caregivers to make preparations," he wrote.

Chan added it would be more challenging to forecast rainfall than typhoons even with existing technology, citing the low predictability of rainstorms.