The reading at Hanoi's Lang station was one of six monitoring sites across northern Vietnam that climbed above 40 degrees that day. Vinh Yen in Phu Tho Province topped the country at 40.8, followed by Lang at 40.7, Bac Ninh at 40.5, Hoi Xuan in Thanh Hoa at 40.5, Do Luong in Nghe An at 40.3 and Huong Khe in Ha Tinh at 40.2.
The National Center for Hydro-Meteorological Forecasting said the severe heat will persist across the north through May 27 and across central Vietnam until around May 28.
In the Red River Delta and Phu Tho, highs are expected to range from 38 to 40 degrees, with some sites exceeding 40.
From Thanh Hoa south to Hue, similar readings are forecast, with western mountainous belts potentially the hottest.
Da Nang and the eastern strip of provinces from Quang Ngai down to Lam Dong, home to Da Lat, are forecast to see 36-38 degrees, with isolated spots above 40.
