Although September 1 gave us prolonged strong sunshine, the week that followed was arguably rather disappointing. Winds from a westerly quarter brought plenty of cloud but delivered almost no rain, maintaining the overall dry theme of the second half of August. Only two days offered much sunshine, and maximum temperatures were mostly about a degree either side of 19C, which is slightly below what we might have expected. Nights were relatively warm, however.

Much-needed rain arrived in the small hours of September 9, continuing on and off through the day and amounting to some 13mm (0.5in) at the Wellington recording site. With a maximum temperature of only 14.5C (58F), it would prove to be the month’s coolest day. On September 10 the remnants of ex-hurricane Dorian passed to the north of the British Isles, dragging some sub-tropical air in its wake. Even without much sunshine, daytime temperatures were higher, reaching 22C (72F) on September 12 – pretty good for the time of year.

A significant change in the weather pattern then occurred, with the arrival of a ridge of high pressure from the west, to hold sway with only minor wobbles for the next ten days. September 13-15 were blessed by almost unbroken sunshine, with a light breeze from the north-east. After a chilly night with a minimum of 5.5C (42F), September 15 saw a maximum in Wellington of 23.5C (74F), such a wide daily temperature range being highly unusual. A weak weather front made September 16 a dull, damp day, but the mercury nevertheless reached the 21C (70F) mark.

For the next five days there was scarcely a cloud in the sky, with more chilly nights but unseasonably warm days, the maximum in Wellington on September 21 reaching 24.6C (76.3F). Only in the freakish heatwave at the end of September 2011 have we had higher temperatures so late in the year.

September 22 marked the start of autumn in every sense, with a much more active jet stream in the upper atmosphere guiding a series of Atlantic depressions across the British Isles. There was some rain every day for the rest of the month, with 34mm (1.3in) falling in the 48-hour period commencing at noon on September 23. The heaviest ‘daily’ fall was 22mm (0.87in), starting late in the afternoon of Saturday September 28 (Carnival Night), and with some torrential bursts during the night, slowly fizzling out next morning.

There was further rain on the last day, helping to turn a month that had been very dry into a wet one, with 98mm (3.9in) in Wellington (127 per cent of the average). Paradoxically, sunshine amounts were also on the high side, amounting to more than 110 per cent of normal.

The overall mean temperature of 15.0 C (59F) in Wellington was more than a degree above the long-term average, placing it among the top ten warmest Septembers in the last 60 years. Interestingly, the rain-soaked final third was the warmest part of the month, thanks to a string of very mild nights. In this area, every month since March 2018 has been warmer than average, with 11 of those months at least a degree warmer. Statistically speaking that is highly improbable, and unprecedented in the local records.

Simon Ratsey

WWN weather correspondent