In a statement, the IPMA said that the districts of Porto, Setúbal, Viana do Castelo, Lisbon, Leiria, Aveiro, Coimbra and Braga are under alert due to the "persistence of high maximum temperature values".

The yellow warning will be in force at least until 6:00 pm today in the districts of Porto, Viana do Castelo, Aveiro, Coimbra and Braga.

In the case of the districts of Setúbal, Lisbon and Leiria, the alert will be in force at least until 6:00pm on Tuesday.

On Friday, the IPMA had already warned that mainland Portugal was expected to experience temperatures over the weekend and the beginning of the week that was “above normal” for the month of September.

“Air temperatures will gradually increase from today, Friday the 13th, until the 16th, and the most significant increase will be felt on the northern and central coasts,” the institute said in a statement published on its website.

According to the IPMA, “maximum temperatures are expected to be above 30 degrees Celsius, across the entire territory, with some areas in the southern region reaching temperatures above 35°C.”

The minimum temperature “will also increase, especially in the Tagus Valley region and in Alto Alentejo, where minimum temperatures are expected to be above 20°C (tropical nights).”

“The forecast temperatures are clearly above the usual for the month of September, and a heat wave cannot be ruled out in some places, but not across the mainland”, warns the IPMA.

“Due to this situation, the IPMA will issue a hot weather warning for some coastal districts”, the note also states.