Hoang Phuc Lam, deputy director of the National Centre for Hydro-Meteorological Forecasting, said rain would increase across the north from the night of October 3, with the heaviest rainfall expected on October 4.
After the rain, the region is forecast to turn drier, with sunshine during the day and cooler conditions at night. Hanoi could see temperatures fall to around 20 degrees Celsius.
From October 6, much of the north is expected to remain dry at night and sunny during the day, although showers and thunderstorms could still occur in mountainous areas.
Thunderstorms may bring tornadoes, lightning, hail and strong winds.
Rain is also forecast to increase across central Vietnam from October 4, peaking during the day and night of October 5. Some areas could receive more than 100 mm of rain within three hours.
From Thanh Hoa to Ha Tinh, scattered showers and thunderstorms are forecast from around the night of October 4 through October 6, with intense rain possible in some areas.
Parts of Quang Tri, Hue and the south-central coast could see scattered showers and thunderstorms from around October 5-6, with locally heavy rain. Rain is expected to continue along the south-central coast from October 7 to 10.
In the Gulf of Tonkin, winds are forecast to shift northeast and strengthen to Force 6 from early October 5, prompting warnings for vessels operating at sea.
Meanwhile, rain across the Central Highlands and southern Vietnam is expected to ease from October 1 before increasing again from around October 5, mainly in the afternoon and evening.
Forecasters warned that thunderstorms during the period could bring tornadoes, lightning, hail and strong winds, while intense rainfall in central Vietnam could cause localised flooding, flash floods and landslides.
