The National Centre for Hydro-Meteorological Forecasting said Hanoi would see moderate to heavy rain on September 17, with very heavy downpours in some areas and temperatures of 23-29 degrees Celsius.
The weather agency warned of thunderstorms, tornadoes, lightning and hail in Hanoi, with convective clouds developing and moving northwestwards across the capital.
Rain and thunderstorms are expected to spread to surrounding areas, bringing possible tornadoes, lightning and strong gusts.
In the northwest, scattered showers and thunderstorms are forecast, with moderate to heavy rain in southern Lao Cai and Son La and heavy to very heavy rain in southern Phu Tho.
The northeast will also see scattered showers and thunderstorms, with moderate to heavy rain forecast for southern Tuyen Quang and Thai Nguyen, northern Phu Tho, Bac Ninh, Hanoi and Ninh Binh.
From Thanh Hoa to Nghe An, moderate to heavy rain is forecast, with very heavy rain possible in some areas. Other central regions will see scattered showers and thunderstorms, mainly in the late afternoon and evening.
Southern Vietnam, including HCM City, is forecast to see scattered showers and thunderstorms in the late afternoon and evening, with locally heavy rain.
From early September 17 through the night, Ninh Binh, Thanh Hoa, Nghe An and southern Phu Tho are forecast to receive 40-80mm of rain, with localised totals exceeding 130mm.
Southern Lao Cai, Son La, Tuyen Quang and Thai Nguyen, along with northern Phu Tho, Bac Ninh and Hanoi, are forecast to receive 30-60mm, with more than 90mm possible in some areas. Rainfall intensity could exceed 100mm within three hours.
Flash floods, landslides and ground subsidence are also possible in Son La, Phu Tho, Lao Cai, Tuyen Quang, Thai Nguyen, Quang Ninh, Thanh Hoa and Nghe An.
Other parts of northern Vietnam and areas from Ha Tinh to Quang Tri are forecast to receive 15-30mm of rain, with localised totals exceeding 70mm.
From Hue to Lam Dong and across southern Vietnam, rainfall of 10-30mm is forecast for the late afternoon and evening, with more than 60mm possible in some areas. Thunderstorms could bring lightning, hail and strong gusts.
Heavy rain across northern Vietnam, Thanh Hoa and Nghe An is expected to gradually ease from September 18.
At sea, thunderstorms are forecast across the Gulf of Tonkin, waters from Gia Lai to Ca Mau and from Ca Mau to An Giang, the Gulf of Thailand, and the central and southern East Sea.
Thunderstorms could bring tornadoes, force 7-8 gusts and waves exceeding two metres, posing risks to vessels in affected waters.