Building to a Category-5 super typhoon, Mankhut is storming towards China with a possible Hong Kong strike on Sunday.

As the United States braces for Hurricane Florence, people in southern China are closely watching the forecast track of Super Typhoon Mangkhut.

Over open ocean, the storm strengthened rapidly and reached the equivalent of Category-4 on the Saffir-Simpson scale on Tuesday. That means winds of 250km/h with gusts in excess of 300km/h.

Category-5 is the top of the scale and will be reached on Wednesday.

The storm already made its first landfall over the Pacific island chain of the Marianas, just north of Guam. The rain gauge collected 167mm in 12 hours from this cyclone.

Such strength of winds over water whipped up waves as high as 11 metres. The water is warm, at 28 to 30 degrees Celsius, and Super Typhoon Mangkhut is in virtually ideal conditions for maintaining its strength until it hits the Luzon Strait off the Philippines.

From Saturday, that strength will start to ebb but the outer rainfall bands will produce torrential rain over Taiwan and much of Luzon.

The forecast track is likely to be accurate and brings Mangkhut towards Hong Kong with 160km/h winds.

Landfall along the coast off of China’s Guangdong, near Hong Kong, is likely on Sunday. The usual risks will be present: wind damage, flooding rain, and storm surges along the coast.