Weak low pressure is beginning to affect the area, the Met Office said in its early morning weather bulletin. There may be light dust in the atmosphere at intervals, it added.
Monday will be mainly fine although there will be increased cloud at intervals. Temperatures will rise to 30 C inland, around 26 on the south and east coasts, around 24 C on the west and north coasts and 22 C in the mountains.
Tonight will be mainly fine but locally there will be increased clouds. Temperatures will fall to 15 C inland and the west and north coasts, around 17 C on the remaining coasts and 10 C in the mountains.
Tuesday, Wednesday and Thursday will start off mainly fine but in the afternoon there will be increased cloud that may lead to isolated showers with the possibility also of thunderstorms, mainly in the mountains and inland.
Temperatures will remain at the same levels, which are close to average for the time of year, over the three days.