Tuesday will be slightly less warm than previous days with temperatures below 40 C inland, but a new heat wave is expected from Thursday with a particularly hot weekend ahead.
In its early morning weather bulletin, the Met Office said seasonal low pressure and a warm air mass are affecting the area.
Increased local low cloud and patchy fog in the morning will soon clear and the weather will become mainly fine.
In the afternoon, increased cloud may lead to isolated showers, with the possibility of thunderstorms, mainly in the mountains.
Temperatures will rise to 39 C inland, 28 C on the west coast, around 31 C on the remaining coasts and 27 C in the mountains.
Tonight will be mainly fine but at dawn there will local low cloud and patchy fog. Temperatures will fall to 21 C inland, the south and east coasts, 19 C on the remaining coasts and 18 C in the mountains.
Wednesday, Thursday and Friday will be mainly fine. Temperatures will remain at the same levels on Wednesday but will rise of Thursday and Friday to above the seasonal average.