The districts of Viana do Castelo, Braga, Porto, Aveiro, Coimbra, Leiria, Santarém, Lisbon, and Setúbal will be under a yellow warning between 9 am on 20 September and 8 am on 21 September because of persistently high maximum temperatures.

The yellow warning is the least severe of the three and is issued when there is a risk to certain activities depending on the weather situation.

The IPMA forecasts a mass of hot, dry air over the Iberian Peninsula from 19 September onwards, which should cause a "gradual rise in temperature" with maximum values ​​of 38 degrees Celsius in the Alentejo and Tagus Valley regions.

According to the Institute, the gradual rise in temperature from 19 September is due to "the influence of an anticyclone located north of the Azores, which extends in a ridge to the Bay of Biscay, promoting the transport of a mass of air of continental origin (hot and dry) over the Iberian Peninsula".

"The maximum temperature should reach values ​​above 30 degrees in most of the mainland territory, approaching 38 degrees in some places, namely in the Alentejo and Tagus Valley," predicts IPMA.

The minimum temperature, the institute adds, "will also register a slight increase, not falling below 20 degrees in some places in Alto Alentejo, Beira Baixa, Tagus Valley and the eastern Algarve".

According to IPMA, this episode of hot weather "will be characterised essentially" by its duration, "with a high probability of configuring a heat wave in a large part of the territory".

"Although the predicted maximum temperature values ​​are lower than those recorded in some episodes this summer, they will be substantially higher than the average values ​​for the second half of September," it points out.

The institute states that a heat wave is considered "when, over a period of six consecutive days, the maximum air temperature is 5°C higher than the average value of daily maximum temperatures for a given reference period."