Seasonal low pressure is affecting the area the met office said in its early morning weather bulletin.
Thursday will be mainly fine with increased local cloud in the mountains in the afternoon.
Temperatures will rise to 38 C inland, around 31 C on the west coast, around 34 C on the remaining coasts and around 30 C in the mountains.
Tonight will be mainly fine with patchy fog and low cloud in places, mainly in the early hours.
Temperatures will fall to 21 C inland and on the coasts and around 17 C in the mountains.
Friday, Saturday and Sunday will be mainly fine, with increased localised cloud in the afternoons in the mountains and inland. Patchy fog and low cloud is forecast in the early hours of the morning.
Temperatures will remain at the same levels on Friday, a little above average for the time of year, rising slightly on Saturday and remaining at higher than average for the time of year on Sunday.