Low pressure is beginning to affect the area, the met office said in its early morning weather bulletin.
Friday will be mainly fine with increased patchy cloud at intervals, particularly in the west. Temperatures will rise to 38 C inland, around 28 C on the west coast, around 32 C on the remaining coasts and around 30 C in the mountains.
Tonight will see increased local low cloud and later light fog, mainly near the coasts. Temperatures will fall to 22 C inland and on the south and east coasts, around 20 C on the remaining coasts and around 18 C in the mountains.
Saturday will be mainly fine but increased cloud will develop at midday and in the afternoon leading to isolated showers and thunderstorms in the mountains and inland.
Sunday will see the same pattern, starting off mainly fine but with scattered showers and isolated thunderstorms expected in the mountains, inland and possibly on the southwest and west coasts at midday and in the afternoon.
Monday will be mainly fine with increased local cloud in the afternoon, mainly in the mountains.
Temperatures will edge down over the weekend, remaining at the same levels on Monday at close to average for the time of year.