In the afternoon, increased cloudiness is also expected in the north and center of the country, with light rain possible.
On Tuesday, hot weather will spread to all regions of the country. The forecast points to 37 degrees in Beja, a record temperature on the continent.
Highs will continue to rise gradually through Thursday, with winds not threatening the days of those already on holiday.
Water temperatures will be around 20 degrees in the Algarve and 17 on the coast facing the Atlantic Ocean.
Hot weather may trigger a warning from the Portuguese Institute of the Sea and Atmosphere (IPMA).