According to the National Center for Hydro - Meteorological Forecasting, the storm lay centered 450 km southeast of Hong Kong at 10 a.m. on Wednesday after passing through the Philippines on Tuesday and injuring at least two people.
In the next 24 hours, the storm will move towards the west-northwest direction at the speed of 20-25 kph.
At 10 a.m. on Thursday, the storm will be in the southwest of China's Guangdong Province, 270 km east of China's Leizhou Peninsula, with maximum wind speeds of 133 kph.
Both Japan and Hong Kong's weather forecast station said the storm will enter Guangdong on Thursday.
Under the storm's impacts, Vietnam's northern localities should expect heavy rains from Thursday afternoon to late Friday night, with rainfall at 100-200 mm in most areas and more than 300 mm in some places. The north central province of Thanh Hoa will also receive rainfall between 50-150 mm, the center said.
Maon made the third storm to enter Vietnamese waters this year.
Vietnam should expect 10-12 storms and low-pressure areas in the East Sea this year, with around half making landfall.