Typhoon Rammasun was drawing strength from warm seas as residents in the northern province of Quang Ninh braced for landfall on Friday.
The National Center for Hydrometeorological Forecasting expect the typhoon to enter the Gulf of Tonkin on Friday night and reach the border of Quang Ninh in the morning, with winds of between 118 to 149 kilometers per hour.
Authorities have warned of strong gales, floods and severe destruction of houses and trees. The typhoon is estimated to be the strongest to hit northern Vietnam after Son Tinh arrived in October 2012, killed at least eight people and leaving three others missing.
Quang Ninh, the neighboring city Hai Phong and nearby provinces began evacuating 120,000 residents of vulnerable areas and summoning thousands of fishing boats to the shore on Friday morning.
Le Thanh Hai, deputy director of Vietnam’s weather forecast center, said: “No boat will be able to bear it at sea. Trees will fall, houses will collapse--except for those with firm frames.”
Hai told Tuoi Tre newspaper that the typhoon will arrive with “rare and dangerous” wind power drawn from the usually warm sea temperatures.
“Water temperatures of 27C and above can allow a storm to form and grow. But the East Sea temperature is now 31 degrees. Evaporation from the sea surface will fuel the storm,” he said.
On Friday morning, Rammasun was 570 kilometers to the southeast of Quang Ninh, moving at 20 kph with winds of 150-183 kph.
The typhoon weakened as it passed over the Philippines (where it killed at least 38 people) but gained strength again at sea before hitting China's Hainan Island on Friday afternoon.
"No boat will be able to bear it at sea. Trees will fall, houses will collapse--except for those with firm frames." -- Le Thanh Hai, deputy director of Vietnam's weather forecast center, on Rammasun
China's Meteorological Administration categorized it a super-typhoon, a definition given to storms with sustained winds of 241 kph or higher.
Vietnam Airlines had to delay 22 domestic and international flights on Thursday by between 30 and 90 minutes, blaming strong winds and rains ahead of the typhoon. More than 3,000 passengers were affected.
The forecast center said more strong winds and downpours are likely to tear down the northern coast and islands from Friday night until Sunday, followed by massive flooding that's expected to last until next Tuesday.
The Red River alone is could rise by as much as 7.5 meters.
The typhoon is forecast to travel into mountainous provinces in the northwest, bringing an average rainfall of 200-300 mm, and 500 mm to some areas.
Such downpours carry a high threat of mudslides and flash floods.
Then the typhoon will slow to a tropical depression, the center said.