The National Centre for Hydro-Meteorological Forecasting said hot weather had returned to northern Vietnam over the past two days, while central localities have endured a prolonged heatwave since June 12.
The western low-pressure system is expected to strengthen and expand eastward on June 22, bringing widespread heat and severe heat to most northern provinces, except Lai Chau and Dien Bien.
Daytime temperatures are forecast to range between 36 and 38 degrees Celsius, with some locations exceeding 39 degrees.
Forecasters said the heatwave would reach its peak on June 23-24, when temperatures across northern Vietnam are expected to rise to 37-39 degrees Celsius, with isolated areas recording higher levels.
Showers and thunderstorms are forecast to return from June 25, bringing cooler conditions. The heatwave is expected to end on June 26.
For central Vietnam, severe and exceptionally severe heat is forecast between June 22 and 25.
Temperatures from Thanh Hoa to Hue are expected to reach 38-40 degrees Celsius, while some western mountainous areas could record temperatures above 40 degrees.
South-central coastal provinces are forecast to see highs of 36-38 degrees Celsius, with some locations exceeding 38 degrees.
Although temperatures in central Vietnam are expected to ease slightly from June 26, hot weather is likely to remain widespread and may persist through the end of June.
Elsewhere, daytime conditions will remain generally sunny, with isolated hot weather. The Central Highlands and southern Vietnam are likely to experience scattered showers and thunderstorms in the late afternoon and evening, with locally heavy rainfall possible.