The temperature will be 1 degree Celsius higher on average this summer than last year, according to the Meteorological Department.
Department director-general Chomparee Chompurat said the hottest weather this week was detected in the North, in Phetchabun's Muang district, where it peaked at 41.5°C on Thursday. Closer to Bangkok, the highest temperature was recorded in Pathum Thani's Khlong Luang district, at 38.5°C.
"This year's summer will be hotter than last year's by one degree Celsius," she said.
She said most areas in upper Thailand would experience hot weather in the afternoons, with an average temperature of 35.5°C.
The highest temperature at 40 to 43°C is expected in Sukhothai, Tak, Lampang and Mae Hong Son, while Bangkok and its surrounding provinces are predicted to experience a peak of 38 to 39°C, Ms Chomparee said. The summer will last until the middle of May.
She also noted the cool season this year lasted two weeks longer than usual. It normally ends in the middle of February, but this year's cool season wrapped up earlier this month, partly because cold snaps continued to move south from China.
According to the department's weather forecasts for the next 24 hours, a thermal low will cover upper Thailand, causing hot weather and haze during the daytime, with very hot weather likely in certain areas.
Southerly and southwesterly winds will prevail over the lower Northeast, lower Central and Eastern regions.
Isolated thundershowers and gusty winds will remain in the Northeast and the East.
Southeasterly and northeasterly winds will be blowing across the Gulf of Thailand and the Andaman Sea, with isolated thundershowers in the South.
From Sunday to Wednesday, a moderate high-pressure system from China will extend to the South China Sea and the upper Northeast of Thailand, with southerly and southeasterly winds prevailing over the upper parts of the country where very hot weather is expected in some areas.
As a result, summer storms, gusting winds and hail are predicted to hit parts of the Northeast first before touching other regions.
People in upper Thailand have been advised to watch out for severe conditions and avoid staying outdoors, under big trees or unsecured billboards. Farmers should take precautions to protect crops from damage.
From 6pm Friday until 6pm Saturday, Bangkok and its nearby provinces are predicted to experience hot and hazy weather in the afternoon, with maximum temperatures of 33 to 39°C.