The region saw scattered showers and thunderstorms on Sunday, with daytime temperatures reaching 30-33 degrees Celsius.
Heavy rain is expected as a tropical convergence zone linked to a tropical depression shifts northwards, combined with the weakening southwestern edge of a continental high-pressure system.
From Sunday night, the northern midlands and delta and southern parts of Son La and Phu Tho are forecast to receive 20-50mm of rain, with isolated totals above 100mm.
On Monday, rainfall of 40-100mm is forecast across the northern midlands and delta, Phu Tho and southern parts of Son La, Tuyen Quang and Lao Cai, with some areas receiving more than 200mm.
From Tuesday through Wednesday night, the same areas could receive 60-120mm, with isolated totals exceeding 200mm.
Total rainfall through Wednesday is forecast at 100-250mm across the affected areas, with some places receiving more than 300mm.
Authorities have issued a level-one disaster risk warning for heavy rain, thunderstorms, lightning and hail. The downpours could cause flooding in low-lying and urban areas, as well as flash floods and landslides.