According to the National Centre for Hydro-Meteorological Forecasting, the Central Highlands and southern Vietnam are forecast to see scattered showers and thunderstorms on August 21, with rainfall of 15-35 mm and more than 70 mm in some areas.
Rain is expected mainly in the late afternoon and evening, with possible whirlwinds, lightning and strong gusts.
From the night of August 22, the northeast and Thanh Hoa Province are forecast to see moderate to heavy rain and thunderstorms, with very heavy rain in some areas. The spell could persist for several days.
Heavy rain may cause flooding in low-lying areas, urban centres and industrial zones, while mountainous areas face risks of flash floods on small rivers and streams and landslides on steep slopes.
The tropical depression was centred northwest of China's Hainan Island at 7 pm on August 20, with maximum sustained winds of 39-49 kph and gusts reaching level 8.
The system was moving west-northwest at about 5 kph.
By the evening of August 21, it is forecast to enter the Gulf of Tonkin and could strengthen into a storm, with winds reaching level 8 and gusts of level 10.
The storm is expected to be about 200 kilometres east-southeast of Hai Phong at that time.
It is forecast to continue moving slowly west-northwest at 3-5 kph by the evening of August 22, maintaining winds at level 8 and gusts at level 10.
Over the following 48-72 hours, the system is expected to continue west-northwest at 3-5 kph and gradually weaken.
As the tropical depression strengthens, winds in the Gulf of Tonkin are forecast to reach levels 6-7, rising to level 8 near the storm centre, with gusts of level 10. Waves could reach 2.5-3.5 metres, producing rough seas.
Authorities warned vessels in affected waters of thunderstorms, whirlwinds, strong winds and high waves.