The National Center for Hydro - Meteorological Forecasting said a hot low pressure area would again develop in the west starting Monday, and raise temperatures in the two regions.
It forecast maximum temperatures during the day to rise to more than 35 degrees Celsius from Wednesday.
American site AccuWeather said the highest temperatures in Hanoi would be 37 degrees in the day and 25 degrees at night on Tuesday and 38 degrees and 27 degrees on Wednesday.
Sa Pa Town in Lao Cai, situated 1,500m above sea level, will have temperatures of 17-23 degrees on Tuesday, and 17-25 degrees on Wednesday.
The central region should expect daytime temperatures of 36-39 degrees, and even over 40 degrees in some places, from Tuesday.
But meteorologists said this heat wave would not be as intense as the previous one in early May when temperature records fell like nine pins, rising to 43 degrees on the 5th, 44.1 degrees on the 6th and 44.2 degrees on the 7th.
Last week cold winds from the north cooled the north and center.
Meanwhile the monsoon has set over the south and Central Highlands.
The weather office said last month that summer would be warmer than normal but have fewer storms due to the influence of the El Nino phenomenon, a recurring climate pattern caused by changes in water temperature in the central and eastern tropical Pacific Ocean.