According to HKO, the storm is expected to skirt around 100 to 200 kilometers (62 to 124 miles) south of Hong Kong on Wednesday afternoon, bringing strong winds and rain.
The Philippines is bracing for Typhoon Mangkhut, the most powerful storm this year, with millions living in its path and fears its rains could combine with monsoonal storms that could spark landslides and flash flooding.
After leaving the Philippines, the fast-moving storm is expected to blow toward Hong Kong and southern China on Sunday if it maintains its course, forecasters said.
Mangkhut, the 22nd typhoon this year, will pass through the Philippines with estimated maximum sustained winds of 250 kilometres per hour (155 miles per hour), reported The Philippine Star.
Officials of the state weather bureau - Philippine Atmospheric Geophysical and Astronomical Services Administration (PAGASA) - said in a livestreamed press conference on September 12 that the typhoon is seen to reach a peak intensity of 220 to 270 kilometers per hour (kph).
"We're anxious for the 10 million people in the Philippines living in the path of this destructive storm, including those who have been displaced several times due to the monsoon rains last July and August", Senator Richard Gordon, the chairman of the Philippines Red Cross, said on Wednesday. It can affect as many as 43.4 million people, according to the United Nation's Global Disaster Alert and Coordination System.
More than three million people live in northern Luzon, which is sparsely populated compared to other parts of the island.
Mangkhut is forecast to be the strongest typhoon to hit the Philippines this year and is threatening farmlands in Northern Luzon just before the rice and corn harvest.
Typhoon Mangkhut has already hit Guam resulting in flooding of streets, uprooting of trees as well as widespread power outages.
Typhoon Haiyan left more than 7,300 people dead or missing and displaced more than 5 million in the central Philippines in 2013.
The state weather service said heavy rains and strong winds are expected from Friday over the north and centre of Luzon, along with rough seas on the coasts. We are preparing our emergency assets and relief items.
The International Federation of Red Cross and Red Crescent Societies said it expects "substantial damage" on the Philippine path of Mangkhut.
The storm's current path has it passing through the Luzon Strait between the Philippines and southern Taiwan, before continuing on to Hong Kong and Macau.