2016 An extraordinary month. Overall very warm, with a CET of 14.6, making it the second equal warmest September from 1910. The first five days were unsettled, and then southerly winds brought hot humid air to the south. 29.3C was recorded at Gravesend on the 7th. It was slightly cooler from the 8th to the 12th, and then 34.4C was recorded at Gravesend on Tuesday 13th, a real record breaker, in many ways: it is the hottest day of the year; it is remarkably late (you have to go back to 1926 for one later); it is the first time 90F (32.3) is exceeded in September since 1949; it was the first time since 1962 that the warmest day of the year in Britain was in September; and it was the hottest September day since 1911.
There was also some very heavy thunderstorms to the north and west: Prestbury in Cheshire saw 32.4 mm of rain in just one hour; Manchester City’s Champions League clash with Borussia Monchengladbach had to be postponed. There were some high minima that night, with 20.8C recorded at locations in Kent. The next day was warm too, with Marham reaching 31.1C. The second half of the month was much more changeable. Rainfall was about average, but it was dry in the east and wetter in the west. Sunshine was close to average.
2017 An unsettled, westerly month. There were few warm days; the highest temperature of the month was 24.0C at Hawarden on the 4th, and the lowest -1.2C at Altnharra on the 22nd. The CET was a little beneath average, at 13.8C. It was a wet month across most of the country, particularly England (130%) and especially Wales and Northern Ireland.
It was a dull month, England and Wales averaging 112 hours (just 77%), and Northern Ireland only saw 97 hours. Storm Aileen 12-13th, was the first named storm of the season, and its effects were increased by its earliness with leaves still being on the branches.
2018 The month started fine in the south, but soon became unsettled everywhere. There was a notable gale midmonth. The end of the month was more settled in the south. Temperatures were very close to average; rainfall 108% (being particularly wet in the NW). It was sunny in the east, with London having 130% of average, and 108% of sunshine country-wide.
The highest temperature of the month was 26.5 C at Cambridge on the 17th, the lowest -3.6 C at Katesbridge (County Down) on the 29th. 105 mph was reported omn the Tay Bridge, Dundee, during the storm on the 19th.
2019 A fairly average month, with CET slightly above average at 14.3C, but wet (127%), particularly in the south. The month was mostly quiet, becoming warm, but then very unsettled and wet for the final third of the month.
It was a sunny month (115%). The highest temperature of the month was 27.7C at Weybourne (Norfolk) on the 22nd.
2020 Overall very slightly warmer than average, but maxima were above average in the SE. A westerly start to the month. Then was another short heatwave: after a warm start to the month, it was 29.6C at Charlwood (Surrey) on the 14th, and then 31.3C at Frittenden (near Tunbridge Wells, Kent) on the 15th. It didn’t seem that long ago that 30C+ was rare in September; there were none between 1973 and 1999.
It wasn’t hot everywhere: here in East Scotland for example it was 18-19C, and in the west of Scotland it was only 13.2C at Baltasound (Shetland). Like all of this summer’s heatwaves, the heat was short-lived: the next day it was much cooler, but still warm, with a high of 26.8C at Hurn near Bournemouth on the 16th.
There were some unusually early frosts late in the month. On average it most mosty dry (77% rainfall) and sunny (117%). The temperature fell to -5.0C at Altnaharra on the 24th and Braemar on the 27th.
Source : https://www.trevorharley.com/weather-september.html