A Strong Wind Signal No. 3 is in force in Hong Kong on Saturday evening, but the weather is expected to deteriorate rapidly at around midnight, warned the Hong Kong Observatory.
The International Federation of Red Cross and Red Crescent Societies said it expects "substantial damage" in the Philippines.
"There was plywood and shards of glass flying through the corridors", Reynolds told CNN. "Flooding is expected in the usual low-lying areas", said Posadas.
Mangkhut had been a category 5 storm for days since wreaking havoc in U.S. Pacific territories of Micronesia before edging towards the Philippines, where it is the 15th and strongest storm this year. Electricity posts are down and landlines aren't working.
Ricardo Jalad, the country's civil defence chief, told an emergency meeting led by President Rodrigo Duterte that about 4.2 million people were vulnerable to the most destructive effects near the typhoon's 78 mile eye.
"We are bracing for the worst here", said Lanelyn Carrillo, a spokeswoman for World Vision, a humanitarian organization.
As of Friday morning, 2,298 families had been preemptively evacuated from their homes in Ilocos, Cagayan Valley and Cordillera administrative regions in northern Luzon.
In all, almost 37 million people are estimated to be in Mangkhut's path. GDACS estimates that the typhoon will have a "high humanitarian impact" based on the high wind speeds and the exposed population located in vulnerable areas. NPR's Jason Beaubien wrote of his experience covering typhoon Haiyan from the Philippines, and the devastation the storm wrought there, "The municipal water wasn't working".
The massive storm cut a swathe of destruction when it struck the northern tip of Luzon island, threatening the lives and homes of roughly four million people. "We are preparing our emergency assets and relief items".
More than 9,000 people have been moved to temporary shelters as Mangkhut, locally known as Ompong, makes its way towards the rice and corn producing provinces of Cagayan and Isabela where it is forecast to make landfall overnight.
Super Typhoon Mangkhut is hurtling through the Pacific, boasting maximum wind speeds of 180 miles per hour that threaten millions of people across Asia in what is estimated to be one of the strongest systems on record.
Hong Kong is also in Mangkhut's sights and preparations there were already underway yesterday, though the storm was not expected to hit until Sunday.
The gambling enclave of Macau, next door to Hong Kong, suffered catastrophic flooding during Typhoon Hato last August that left 10 dead and led to accusations of corruption and incompetence at its meteorological office.
Wong said officials hoped to send elderly people living alone in Tai O to shelters on Saturday afternoon while buses would also be arranged to help residents in parts of Tuen Mun and Yuen Long to move away.
Super Typhoon Mangkhut is hurtling through the Pacific toward Asia boasting winds as fast as 180 miles per hour.
Continued storm surges and torrential rainfall could trigger floods and more landslides.
Ferry services in the Qiongzhou Strait in southern China were halted on Saturday and helicopters and tugboats dispatched to Guangdong to transfer offshore workers to safety and warn ships about the typhoon, state-run Xinhua News Agency reported.