Temperatures will exceed 40 degrees Celsius in some areas of the central region on May 23, the National Hydro Meteorological Forecasting Center has warned.
Temperatures across the region, and parts of the north, will average 36-39 degrees.
The mercury will dip by 2-3 degrees at night but the weather will remain hot and muggy.
The heat wave is forecast to last until May 24.
The weather is better in the southern region with temperatures at 34-36 degrees and rain in the afternoons.