The observatory has warned that the cold snap forecast for between Wednesday and Friday may feel even colder amid a chance of rain.

Lee Fung-ying, an acting senior scientific officer, told an RTHK programme that easterly winds currently affecting the region would turn into northerlies tonight, bringing a significant drop in temperatures.

That will see the mercury drop to 16 degrees before it further dips to 11 degrees over the next few days.

"When it's rainy or just damp and cold, water droplets or moist air can cling to your skin," Lee said.

"Since water is much better than air at conducting heat, it will speed up heat loss from the body."

A paediatrician told the same programme that there is a high chance of the winter flu season starting this month or in February, urging people to get vaccinated as soon as possible.

Patrick Ip, professor in paediatrics and adolescent medicine at the University of Hong Kong, said the elderly and children, particularly those with chronic illnesses, should head to a doctor at first blush if they catch the flu virus, as there are highly effective drugs that can be administered within 48 hours of the onset to reduce the possibility of complications developing.

"We expect the winter flu peak will very likely start some time between the end of January and February. We believe many children and the elderly will be susceptible, especially with the new subclade K variant," he said.

"They must take preventative measures such as getting sufficient rest, and consider putting on a mask if they're ill or in crowded places. They should also wash their hands thoroughly and get inoculated against the flu as soon as possible."

Ip noted that a 14-year-old boy who was infected with influenza A is now in a critical situation after developing complications, including serious pneumonia and septic shock.

The teenager, who also suffers from chronic illness, got the jab three days before influenza A symptoms showed, Ip said, adding that antibodies take about two weeks to develop.

Children aged nine or below, he stressed, should get two doses of flu vaccine administered four weeks apart to get sufficient protection.