Last month was the third warmest September since 1795, Armagh Observatory said, and also a wetter month than average.
Several very wet days were recorded, but the wettest was associated with Storm Agnes on September 27, the first named storm of the 2023/24 season.
The average was recorded by the Observatory as 15.2C, about 1.7C warmer than the most recent (1991-2020) 30-year September average at Armagh (13.5C).
In a report the observatory said in its almost 230-year long daily temperature series, from about 1795, this year saw the third warmest September on record.
This follows a pattern recorded in Armagh, where the last few Septembers, now five in a row, have been warmer than the most recent (1991-2020) 30-year average.
The warmest Septembers at Armagh are now 2021 (15.5C), 1865 (15.4C), 2023 (15.2C), 2006 (15.0C), and 1846 and 2016 (both 14.8C).
The high average temperature is described as being due to an exceptional warm spell near the beginning of the month lasting for a week from September 4 to September 9.
During this period every day except September 9 (maximum temperature 24.8C) exceeded the official threshold, 25C, for a heatwave in Northern Ireland.
The observatory described heatwaves as relatively uncommon in Northern Ireland and heatwaves in September as very rare.
They said the September 2023 heatwave provided a respite from an otherwise wet, albeit warm and sunnier than average summer, and included the warmest September day on record at Armagh – 28.5C on September 8.
The observations were made at Armagh Observatory, which has been recording the weather at Armagh since 1795.
