The National Centre for Hydro-Meteorological Forecasting said that on Friday afternoon, the depression was over the northern Gulf of Tonkin, about 110 kilometres east-northeast of Bach Long Vi Special Zone in Haiphong.
By 1 pm on August 22, the depression is forecast to move slowly north at around three kilometres per hour, about 190 kilometres east of Haiphong and 120 kilometres northeast of Bach Long Vi.
It could strengthen into Storm No. 4 by then, with winds at level 8 and gusts at level 10. It is forecast to turn south-southwest at around three kilometres per hour by 1 pm on August 23, remaining over the northern Gulf of Tonkin, about 150 kilometres east of Haiphong and 50 kilometres north of Bach Long Vi.
Over the following 48-72 hours, it could change direction again and move southeast at around three kilometres per hour.
The Gulf of Tonkin, including Bach Long Vi, Van Don, Co To and Cat Hai special zones and Hon Dau Island, is forecast to see winds of level 6-7, rising to level 8 with gusts at level 10 near the storm centre.
Waves are expected to reach 2.5-3.5 metres, with rough seas. Vessels in affected waters face risks from thunderstorms, tornadoes, strong winds and high waves.
From Friday evening, heavy rain is forecast across northeastern Vietnam, southern Phu Tho Province and Thanh Hoa Province, with some areas at risk of exceptionally heavy downpours.
Rainfall of 70-150mm is expected across these areas, with local totals exceeding 300mm.
Lang Son, Bac Ninh, Quang Ninh and Haiphong are forecast to be the hardest hit, with widespread rainfall of 150-250mm and local totals above 400mm.
Rainfall intensity could exceed 120mm within three hours, raising the risk of urban and low-lying flooding, flash floods and landslides in mountainous areas.
Nghe An, the Central Highlands and southern Vietnam are also forecast to see showers and thunderstorms on Friday afternoon and night, with rainfall of 15-30mm and local totals above 60mm.
Moderate to intense rain is expected to continue across northeastern Vietnam, southern Phu Tho and Thanh Hoa overnight on August 23 and throughout August 24, with 30-60mm forecast and more than 120mm in some places.
Total rainfall from Friday evening through August 24 is forecast at 100-200mm across affected areas, locally exceeding 400mm.
Lang Son, Bac Ninh, Quang Ninh and Haiphong could receive 200-300mm over the period, with local totals above 500mm.
The violent rain could continue beyond August 24.