The Met Office has issued an extreme high temperature warning for Friday saying that the maximum temperature inland is expected to reach around 40 C.
The new yellow alert is in force from 11 am to 4 pm.
In its weather forecast, the Met Office said a warm air mass is affecting the area and Friday will be mainly fine although in the afternoon increased local cloud may lead to scattered showers, mainly in the mountains.
Temperatures will rise to 40 C inland, 31 C on the west coast, around 35 C on the remaining coasts and 30 C in the mountains.
Tonight will be mainly fine but patchy fog and low cloud is possible early in the morning.
Temperatures will fall to 24 C inland and on the south and east coasts, around 21 C on the west and north coasts and 18 C in the mountains.
The weekend and Monday will be mainly fine but in the afternoon increased cloud may lead to scattered showers, mainly in the mountains.
Temperatures will be at the same levels on Saturday which are a little above average for the time of year, and will edge down on Sunday and Monday.