Children, the elderly and other vulnerable people should remain indoors as much as possible.
Temperature is expected to remain at 41 C inland and 32 C in the mountains.
This is the second yellow alert in as many days and applies from 11 am to 5 pm.
According to the met office, Monday will be mainly sunny, with maximum temperatures of 41 C inland, around 34 C on the south and east coasts and 32 C on the remaining coasts and in the mountains.
Tuesday, Wednesday and Thursday will remain mainly fair with temperatures dropping gradually to close to average for the time of year.