IPMA Sends the Mainland Back Above 30 Degrees From Midweek, With 40 Inland and Tropical Nights on the South Coast

The week of rain that closed August is over. In its weekly outlook for 31 August to 6 September, the Portuguese Institute for the Sea and the Atmosphere says that "after a week of precipitation, which may still be weak on the northern and central coast during this weekend, the hot, dry weather typical of this time of year will return next week", and it puts the return in two stages rather than one.

The mechanism is an anticyclonic region building gradually over the Bay of Biscay. That produces a first rise in temperature over the weekend, then a second and larger rise in the middle of next week.

The numbers

Minimum temperatures start the week between 10 and 16 degrees, then climb to between 16 and 22 degrees by midweek. At that point IPMA flags the possibility of tropical nights, meaning the temperature does not fall below 20 degrees, on the southern coast of the Algarve, in the southern region along the Spanish border, and in the Lisbon area.

Maximums start the week between 20 and 35 degrees. From midweek they move to between 30 and 40 degrees, with the exception of the coastal strip, which stays between 25 and 30. The highest values will occur in the south throughout.

Wind will be weak to moderate, running as nortada along the western coastal strip and turning predominantly easterly from midweek. An easterly flow in early September is the component that matters most for anyone watching fire risk, because it carries dry continental air off the Spanish interior rather than damp Atlantic air off the sea.

Why a 40-degree week now is not the same as a 40-degree week in July

The ground is different. The Atlantic depression that turned August into autumn brought a week of rain across much of the mainland, which raises fuel moisture in the short term. What an easterly flow and a 40-degree midweek do is dry that surface fuel out again, quickly.

Portugal has already had a hard fire season. INE's cross-referencing of six years of fire history against the hazard chart found that four fifths of everything burnt since 2020 sits in under a third of the mainland, most of it more than fifteen minutes from a fire station. Earlier heat spells this summer pushed about seventy interior councils to maximum fire danger.

Heat carries its own toll independent of fire. INSA estimated 680 excess deaths across three summer heat spells this year. And the water is not automatically the safe option: ninety-two drownings by 31 July set a decade record, with rivers accounting for more than a third.

What This Means for Expats

• Plan the week in two halves: Monday and Tuesday are the mild end. The heat arrives midweek, so move outdoor work, moves and long drives to the front of the week where you can.
• If you are on the coast: maximums stay between 25 and 30, so a coastal address will feel very different from one 30 kilometres inland, where 40 is on the table.
• Tropical nights in Lisbon and the Algarve: nights above 20 degrees are what make heat cumulative. If your flat has no cooling, ventilating overnight stops working when the outside air is the problem.
• Burning is the wrong instinct now: a dry easterly after rain is the classic set-up for a garden fire escaping. Check the daily ICNF and IPMA fire-risk index for your council before lighting anything.
• Fire danger is likely to climb by Wednesday or Thursday: if you are inland, this is the week to clear the ground around a house rather than the week after.

IPMA's outlook is a forecast, not a warning, and no heatwave has been declared. But two upward steps inside seven days, an easterly flow and 40 degrees inland in the first week of September is a combination worth reading before the weekend plans are made.