Northern Vietnam regions have been enjoying a mild reprieve from scorching heat over the past week thanks to the impacts of cold air, which brought downpours. But starting Monday, with the rise of a low-pressure area from the west, the heat will return to Lai Chau, Dien Bien, Son La, Hoa Binh provinces and certain areas in the northeast, including Hanoi, according to the National Center for Hydro-Meteorological Forecasting.

Temperatures will rise to 35-37 degrees at the highest. The heat will persist until the end of the week, with Wednesday and Thursday forecast to be the hottest days, the center said.

U.S. weather forecast service AccuWeather said Hanoi temperatures will be at 27-36 degrees on Monday and Tuesday, before rising to a maximum of 37 degrees starting Wednesday. Temperatures in high-altitude locations like Lao Cai's Sa Pa will be highest in the middle of the week, at 28 degrees, before gradually decreasing to 23 degrees towards the end of the week. Nighttime temperatures will be between 17-19 degrees.

Central Vietnam regions will also be hotter starting Monday, with the highest temperatures during the day at 35-38 degrees. The heat wave is likely to persist in central Vietnam until the end of Friday.

Southern Vietnam and the Central Highlands regions will see rains due to the effects of monsoon winds, the national forecast center said, warning of floods in lower areas. The highest temperatures in the Central Highlands will be around 32 degrees, and 34 degrees for southern Vietnam, it said.

Vietnam's average temperatures this month is about 0.5-1 degree higher than the national average over the past years, meteorologists said. The El Nino effect, which may happen in July-August, may cause higher heat and fewer rains and storms.