September 2026 Shatters Mainland Portugal's Heat Record, With Daytime Highs 4.21 °C Above Normal
IPMA says September 2026 was mainland Portugal's hottest since 1931: a mean of 23.41 °C, maximums 4.21 °C above normal, a national heatwave at 72 percent of stations and only 12.7 mm of rain. What it means for drought, reservoirs and the weeks ahead.
September 2026 was the hottest September in mainland Portugal since records began in 1931, according to the preliminary monthly bulletin published on Wednesday by the Instituto Português do Mar e da Atmosfera (IPMA, the national weather and sea institute). The institute classes the month as "extremely hot and very dry". It follows a summer that IPMA ranked the third hottest since 1931.
The numbers
The mean air temperature across the mainland was 23.41 °C, 2.91 °C above the 1991 to 2020 normal. The previous warmest September, in 2018, averaged 23.00 °C.
The daytime figure is the striking one. The mean maximum temperature reached 31.03 °C, 4.21 °C above normal and also the highest since 1931, against 30.21 °C in 2018. Nights were warm but not record-breaking: the mean minimum of 15.79 °C was 1.60 °C above normal, the fourth highest since 1931 and the highest since 2000. The warmest September nights on record remain those of 1987, at 16.78 °C.
The highest temperature of the month was 44.2 °C at Alcácer do Sal on 3 September. Across IPMA's network, stations set 15 new September records for maximum temperature and 18 for minimum temperature.
Two heatwaves, one of them national
IPMA identifies two heatwaves. The first, from 2 to 7 September, was confined to the interior North. The second, from 18 to 27 September, was the larger event: it lasted 6 to 10 days depending on the station and was recorded at 72 percent of IPMA's stations, covering most of the mainland.
The islands ran hot too. In Madeira, Funchal Observatory (25.6 °C) and Chão do Areeiro (17.1 °C) had their warmest September mean temperatures since 1961, and the Azores had their second warmest September since 2000, IPMA said.
Rain, soil and reservoirs
Only 12.7 mm of rain fell on average, about 30 percent of normal, making it the fifth driest September since 2000. The driest remains September 2017, with 2.0 mm. Soil moisture fell slightly by the end of the month, mainly in the Centre and West.
The dry month closed a hydrological year (October to September) that had been wet overall: IPMA's August bulletin put rainfall from October 2025 to the end of August at 1,083.5 mm, 140 percent of normal. Its drought map for the end of August still showed the Alentejo and Algarve in meteorological drought, with severe drought spreading in the south-east. The end-of-September drought classification will come with the full bulletin, which IPMA normally publishes in mid-month.
Storage held up but slipped. The weekly reservoir bulletin from the Agência Portuguesa do Ambiente (APA, the environment agency) put total volume at 9,673 cubic hectometres on 28 September, 73 percent of capacity, down 120 cubic hectometres in a week, with every basin falling. Most basins were still above their own September averages; the exceptions were in the north and centre: the Lima, the Cávado, the Vouga and the Mondego.
What comes next
The heat has not fully broken. IPMA's five-day city forecasts issued on Wednesday afternoon show highs of 26 °C to 29 °C in Lisbon, Évora and Beja from Thursday to Sunday, with little or no rain expected there. Its extended outlook for 5 October to 1 November expects above-normal temperatures this week, especially in the Centre, South and Algarve, but no clear temperature signal for the week of 12 October, and somewhat wetter than normal weeks later in the month.
