The UV index updated on Weatheronline Monday morning showed Hanoi had hit level 6 and it was expected to reach 9.1.
Meanwhile in Ha Long city, Quang Ninh province, the UV level is predicted to hit 8.4.
Hai Phong, Hue, Da Nang, Ca Mau and Hoi An cities are forecast to see the UV index at level 9.6 to 9.9, and Nha Trang and HCM City will reach level 10.
A UV index of 6 to 7 is considered “high,” 8 to 10 "very high", and 11 and more is "extreme," according to the World Health Organisation.
From August 17 to 19, the UV radiation index will rise to a level that poses risks to those exposed — a rating of 9-10 — across the nation.
UV radiation is strongest between 10am and 2pm.
Central provinces and cities will also experience temperatures of between 35 and 37 degrees Celsius on Monday. Localities have been warned of fire risks due to high demand for electricity and heat.
According to the National Centre for Hydro-Meteorological Forecasting, showers with rainfall of up to 40mm per hour are expected to hit the northern region on August 16 night.
Some northern mountain provinces will see showers with rainfall up to 60mm per hour.
Storms, lightning and strong winds could appear, with warnings of floods and landslides issued for many northern mountainous localities.
The Central Steering Committee for Natural Disaster Prevention and Control has requested northern mountain provinces to review and be ready to arrange forces to control traffic in flood areas, ensuring safety of mines as well as coordinate with communication agencies to inform people about heavy rain to prevent and minimise damage./.