The Met Office has issued a yellow alert for Friday saying temperatures will rise to 40 C inland.
Th extreme hot weather warning is in force from 11 am to 5 pm. It said vulnerable groups such as the elderly and very young should be exercise care.
The Met Office said Friday will be mainly fine, with temperatures of 40 C inland, 31 C on the west coast, 34 C on remaining coasts and 30 C in the mountains.
The weather will be mainly fine over the weekend and Monday, edging down gradually to average for the time of year.