The met office issued a new yellow weather alert over extreme high temperatures, valid from 10pm Wednesday night to 6am Thursday.
This is the fourth such warning for July only, with the service also warning of the possible health risks associated especially among vulnerable groups such as the elderly and children.
In the evening, temperatures are expected to fall to 26C inland and on the southern and eastern coastal areas, to 23C in the northern and western coasts, and 19C in the mountains.
On Thursday the temperature is expected to rise to 40C inland, around 34C in the southern and eastern coasts, 31C in the northern and western coasts, and 30C in the mountains.
In the afternoon, winds are expected to gradually pick up, particularly in the southern and eastern coasts before settling again towards the evening.
Fair weather is expected over the weekend with a gradual rise in temperatures, which are expected to remain above average.