The heat index will be above 40 degrees Celsius in many places on Sunday, with the Meteorological Department predicting highs of 54C in Krabi and 50.2C in Bangkok's Bang Na district.
The heat index will be 44.3C in Phetchabun, 43.2C in Chon Buri and 41.3C in Chok Chai district of Nakhon Ratchasima, respectively.
The department said this heat index, also known as the apparent temperature, is at dangerous to very dangerous levels. Health officials on Saturday issued a health alert as a heatwave swept across many provinces, with peak "feels like" temperature.
The "feels like" temperature, or apparent temperature, refers to how the air feels to humans, resulting from the combined factors of relative humidity, air temperature and wind speed. In warm climates, it is known as the heat index, and in cold climates, it is known as wind chill or chill factor.
The department also warned that upper Thailand will have summer storms over the weekend due to a high-pressure front moving in from the South China Sea.
It said the storms could take place across the North, the Northeast, the East and the Central Plains on Saturday and Sunday.
Moisture from the Gulf of Thailand and the South China Sea is being pushed by the high-pressure system onto the mainland. As it mixes with hot air, rainstorms and hail can be expected.
Summer storms on Saturday were expected to affect six provinces in the North; 18 provinces in the Northeast; 14 provinces in the Central Plains; and four provinces in the East.
Areas expected to be affected Sunday include 11 provinces in the North, such as Mae Hong Son and Chiang Mai; 14 provinces in the Central Plains, including Greater Bangkok; and four provinces in the East, including Nakhon Nayok, Prachin Buri, Chachoengsao and Chon Buri.
