The storm is expected to see some weakening as it enters Philippines but rainfall will be very heavy over these areas.
Even so, Guy said will come "dangerously close to clipping the country", causing heavy rain and flooding in the island's north, where local authorities are preparing relief goods and security forces have been put on alert.
Mangkhut is now barrelling across the Pacific with gusts of 255 kilometres (160 miles) per hour.
The charity estimates three million people in the country live in the direct path of Mangkhut.
It is due to hit Hong Kong - a densely packed skyscraper city with more than seven million residents.
A typhoon may bring widespread damage such as uprooted trees and damage to infrastructure, according to PAGASA's forecast. It was expected to leave the Philippine area of responsibility (PAR) by Tuesday morning.
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.
Cathay Pacific Airways Ltd. encouraged passengers travelling this weekend to and from Hong Kong to re-book and the airline will waive the fees.
While Mangkhut is now stronger than Hato -- also known as Tropical Storm Isang -- or Haima it is likely to decrease in strength as it nears southern China, particularly if it crosses land over northern Luzon, which will slow the storm, according to Guy, the meteorologist.
The current track of Mangkhut is that the typhoon is likely to brush northern Luzon, a less populated part of the largest island in the Philippines, which is home to the capital, Manila.
Floods, landslides and huge waves could put millions at risk as Typhoon Mangkhut hurtles towards the Philippines and Hong Kong.
"As Mangkhut has a large circulation with intense winds, it will pose considerable threat to the coast of Guangdong", according to the Hong Kong Observatory.
It could be felt strongest in the provinces of Cagayan and Batanes with an intensity of 205 to 255 kph.