Weak low pressure is affecting the area while a hot air mass is expected as from Monday, the met office said in its early morning forecast.
Sunday will be mainly fine but clouds that develop from midday on are expected to lead to scattered showers, mainly in the mountains. Temperatures will rise to 38 C inland, around 32 C on the coasts and 27 C in the mountains.
Tonight will be mainly fine. Locally there may be low cloud and light fog early in the morning.
Temperatures tonight will drop to 22 C inland and on the coasts and around 17 C in the mountains.
Temperatures are expected to edge up through to Tuesday, remaining at the same levels on Wednesday at noticeably above average for the time of year.