The National Centre for Hydro-Meteorological Forecasting said on Sunday morning that the depression was over waters off Quang Tri and Hue, about 100 kilometres north-northeast of Hue and 170 kilometres east-southeast of Quang Tri.
Packing winds of level 6 and gusts of level 8 on Vietnam's 12-level scale, the system was moving northwest at about 15 kilometres per hour and was expected to reach the coast between Ha Tinh and Quang Tri overnight or early on Monday.
Its circulation has already brought level-6 winds, gusting to level 8, to Con Co special zone in Quang Tri, while moderate to heavy rain hit areas from Quang Tri to Danang overnight and early on Sunday.
Waters from southern Nghe An to Danang, including Con Co, are forecast to see level-6 winds, gusting to level 8, and waves of two to four metres. Coastal areas from Ha Tinh to Quang Tri are expected to experience similarly strong winds from Sunday evening.
Ha Tinh to Hue is forecast to receive 100-200mm of rain through Sunday night, with some areas exceeding 350mm. Thanh Hoa, Nghe An and Danang could receive 40-70mm, with isolated totals above 150mm.
Heavy rain is also forecast for northern Vietnam. The northern midlands and Red River Delta, as well as southern parts of Son La and Phu Tho provinces, could receive 30-60mm overnight, with some areas exceeding 100mm.
On Monday, rainfall of 100-200mm is forecast from Thanh Hoa to Quang Tri, with isolated totals above 350mm. The northern midlands and delta and southern parts of Son La, Phu Tho, Tuyen Quang and Lao Cai could see 40-70mm, locally exceeding 150mm.
Heavy rain is expected to persist on Tuesday, with 30-60mm forecast across those northern areas and from Thanh Hoa to Quang Tri, and isolated totals above 100mm. The widespread downpours could last until Wednesday morning.
Flooding is also expected on rivers from Thanh Hoa to Danang through Wednesday. Water levels on the Ngan Sau and Ngan Pho rivers could reach alert levels 2-3, while other rivers are forecast to rise mostly to levels 1-2, with some exceeding level 2.
Authorities have warned of a high to very high risk of flash floods and landslides in parts of Ha Tinh, Quang Tri, Hue, Danang and Quang Ngai.