Temperatures could soar as high as 44 degrees Celsius in the North, Northeast and Central Plains throughout the Songkran festival, nearing a record set in 1960, the Meteorological Department said.
Dangerously hot weather with haze were expected daytimes in parts of three regions until Monday, with the mercury hitting 40-44 degrees Celsius, the department predicted on Tuesday.
Thundershowers with gusty winds are likely in 10% of the total region on April 12-14. Thundershowers with gusts and hail are possible over 30% of the region April 15-18.
Officials warned residents to avoid big trees, advertising signboards and unstable structures during severe weather.
Department director-general Wanchai Sakudomchai said Tuesday's maximum temperatures were recorded at 42-43C. The mercury in Sukhothai province on Monday already touched 44C, almost matching the highest recorded temperature set 56 years ago.
The record for Thailand is 44.5C, set on April 27, 1960 in Uttaradit province.
Meanwhile, deaths blamed on the heatwave were reported in Uthai Thani and Pathum Thani on Sunday, with the latest occurring in Phitsanulok, where a man was found dead in a dried-up canal in the middle of a rice field on Tuesday morning.
Denchai Thongpark, 47, left home in tambon Tha Nang Ngarm of Bang Rakam district at 11am Monday to fish and never returned, according to his wife.
Police initially assumed long exposure to the sun may be the cause of Denchai's death, as the mercury reached 41C on Monday while he was out for fishing.