Temperatures are expected to hit 40C inland on Thursday, but some minor respite from the blistering heat is expected on Friday and Saturday before temperatures rise to above average again on Sunday.
A yellow alert for high temperatures is in force from 11.00 am to 4.00 pm, the second such warning in as many days.
In its early morning weather forecast, the met office said that Thursday will be mainly fine although local increased cloud is expected to form after midday, mainly in the mountains.
Temperatures will rise to 40C inland, around 35C on the southeast, east and north coasts, around 32C on the southwest coast and around 30C on the west coast and the mountains.
Thursday night will be mainly fine, although there may be low cloud at intervals on the west coast. Temperatures will fall to 23C inland and on the north coast, around 24C on the southeast and east coasts, around 22C on the remaining coasts and around 20C in the mountains.
Friday and Saturday will be mainly fine. At night and early in the morning, there may be local low cloud and patchy fog.
Temperatures will edge down on both days.
Sunday will be mainly fine with low cloud and patchy fog possible at night and early in the morning.
Temperatures will be rise to above the seasonal average.