September began with a weak ridge of high pressure which brought dry sunny weather to many parts, but also some mist and fog to the south, and some heavy, thundery showers to the east. The remainder of the month was unsettled, with an above average number of rain days, particularly across England. North-westerly winds brought rain bands interspersed with bright showery weather until the 19th. It was also windy at times, notably during the passage of Storm Aileen - the first storm of the 2017/18 season - on the 12th and 13th. The end of the month was generally warmer, with southerly winds from the 20th. This also bought cloudy weather and rain and fog patches at times.
On the whole, September was slightly below the long-term average for temperature and sunshine, and generally above the long-term average for rainfall, especially in the south west, with south Cornwall experiencing over double the average rainfall amount.
UK Weather Averages: Sept 2017 (Source: Met Office)