September 2018 continued the cooler trend that began in August after a long run of unusually warm to hot conditions from mid-April to the start of August. It was a relatively cool month but not exceptionally so as it was close to the 1981-2010 averages whilst for the 1961-90 averages, it was actually a fairly mild month in comparison.

Temperatures

The map below from the UK Met Office shows the temperature anomalies over the UK during September 2018 and you can see the blue colours over the large majority of Northern Ireland, parts of central and western Scotland, northwestern England and western Wales whilst most of the UK was under the white colours (close to average).

UK mean temperature anomaly map (against 1981-2010 averages) for September 2018. Credit: UK Met Office.

The UK mean temperature for September 2018 was 12.4°C which comes to only -0.2°C below the 1981-2010 UK average for September. This made it the coolest September since 2015 and was cooler than a similarly cool September in 2017. Meanwhile, the Central England Temperature (CET) for September 2018 was 13.7°C which comes to only -0.3°C below the 1981-2010 CET average for September. In comparison to the UK overall, September 2017 was relatively cooler than 2018 for the CET with a mean temperature of 13.5°C but September 2015 at a CET of 12.6°C was colder than both.

September 2018 felt like a much colder month than what the stats suggest here to a lot of people. There was likely some relativity involved here in that Summer 2018 was so warm that a close to average September was bound to feel quite cold in comparison.

The month started off quite warm with a maximum temperature of 26.1°C at Topcliffe, North Yorkshire on the 2nd, warmest September day since 13 September 2016. As the winds turned to the north by the end of the first week, it turned much colder over the UK with some notable chilly nights especially for so early in the season. This was to become a rinse and repeat scenario for September 2018. There was a lot of high pressure especially during the first few days of the month to the south of the UK and then again for the final week of the month with a rather more unsettled period mid-month including a particularly windy spell during the third week with the arrival of Storms Ali and Bronagh.

The remnants of ex-hurricane Helene on the 17th brought up warm and humid air into the UK with a new maximum temperature for the month being recorded of 26.5°C at Cambridge Botanic Gardens, Cambridgeshire.

As high pressure built for the final week of the season, the chilly nights returned bringing minimum temperatures as low as -3.6°C at Katesbridge, Northern Ireland on the 29th; the lowest September minimum temperature for the UK since 2012. The data below is the UK minimum temperatures for every September from 2009 to 2017.

  • 2017; -1.2°C (Altnaharra, Sutherland on the 22nd)
  • 2016; 0.1°C (Katesbridge, Northern Ireland on the 22nd and Aboyne, Aberdeenshire on the 26th)
  • 2015; -1.3°C (Braemar, Aberdeenshire on the 30th)
  • 2014; -0.1°C (Katesbridge, Northern Ireland on the 18th)
  • 2013; -2.4°C (Kinbrace, Sutherland on the 6th and Braemar, Aberdeenshire on the 26th)
  • 2012; -4.1°C (Braemar, Aberdeenshire on the 23rd)
  • 2011; -0.4°C (Tyndrum, Stirling on the 15th)
  • 2010; -4.4°C (Tyndrum, Stirling on the 26th and Kinbrace, Sutherland on the 27th)
  • 2009; -1.2°C (Kinbrace, Sutherland on the 19th and Katesbridge, Northern Ireland on the 20th)

Rainfall

September 2018 was a very mixed month for the UK in terms of rainfall as can be seen by the rainfall anomaly map below of the UK. Northwestern Scotland, Wales and northwestern England tended to be wetter than average during the month. As a result, the UK overall had 108% of its average September rainfall but the devil is in the detail as this figure masks significant regional variations. A lot of the rainfall over the midlands and west of the UK occurred during Storm Bronagh on the 20th/21st including a daily rainfall of 78.6mm at Capel Curig, Gwynedd in the 24-hour period ending at 0900 GMT on the 21st. Sheffield in North Yorkshire recorded a daily rainfall total of 61.4mm during the same period which made it the wettest September day on record at the station whilst 66.2mm fell at Weston Park (also in Sheffield), North Yorkshire making it the wettest September day on record here too since records began in 1882.

Northern Ireland had its driest September since 2014 with only 62% of its average September rainfall.

Despite some regions being wetter than average, September 2017 was wetter at the vast majority of places especially over England.

UK rainfall anomaly map (against 1981-2010 averages) for September 2018. Credit: UK Met Office.

Sunshine

It was a sunnier than average September for the UK with 108% of the average September sunshine but like the rainfall, this masks the regional variation over the country. It was sunnier in eastern regions than the west this month including London having over 130% of its average September sunshine. It was the sunniest September over the UK since 2015 and whilst quite significantly sunny over the southeast of England, September 2018 was not a record breaker.

UK sunshine anomaly map (against 1981-2010 averages) for September 2018. Credit: UK Met Office.

What is remarkable though is the fact that the period May-September 2018 was the sunniest such period on record for the UK back to 1929 with a total of 1005.0 hours of sunshine beating the previous record of 1001.3 hours of sunshine in 1989. There has only been a few years that achieved 900 hrs or more of sunshine for the UK back to 1929 for the May-September period. The other years being 1940, 1949, 1955, 1959, 1975, 1976, 1995 and 2006 whilst only 1959, 1989, 1995 and 2018 achieved at least 950 hours for the May-September period.

 

UK sunshine totals for May-Sep from 1929 to 2018

Storm Ali

One of the highlights of September 2018 for the northwest of the UK with a particular emphasis on Northern Ireland was the arrival of Storm Ali on the 19th September. I’ll let the UK Met Office explain the situation:

On the 19th, Storm Ali brought widespread disruption to Northern Ireland with a maximum gust of 79 knots (91 mph) recorded at Killowen, County Down. Power shortages were quite widespread with 90,000 customers reportedly without power in Northern Ireland, 30,000 in Scotland, 15,000 in north-east England, 2000 in Cumbria and 500 in Wales. There were reports of numerous roads and rail lines blocked and closed, especially in Northern Ireland. Ferry services were cancelled or delayed and flights were delayed and diverted. The Foyle Bridge was closed for a time, as was the M1 when a power cable came down across the motorway. One man died and there were reports of multiple injuries from flying debris and from trees falling onto cars. In Scotland, the Tay Bridge was closed. An elderly couple were attended to by emergency services in Llandudno after they were blown over in the strong wind, the man later died of his injuries. 

The maximum wind gust of 91 mph during Ali at Killowen made it the windiest September day in Northern Ireland since September 1961 on the 16th with the remnants of ex-hurricane Debbie. Ballykelly, Co. Derry at this time recorded a maximum wind gust of 106 mph. Ali was certainly a shock to the system for early Autumn as such a stormy depression wouldn’t be too out of place in mid-Winter rather than mid-September.

Featured photography

To conclude this analysis, I would like to share the following photography from September 2018 given by the credited people. Thank you for capturing these amazing pictures and may you prosper to even greater photography opportunities in the near future.

Our featured image of this article. Credit: Liam Ball.

 

Sean Bruen is a forecaster for Metcast (and Snow Watch). His main interests are historical and long range weather. He LOVES snow (his Twitter account is @SnowbieWx, go figure!) and his favourite season is Winter.