Following a very interesting summer, September will go down as a quite remarkable month. We’ve seen it all with a full blown heatwave delivering the ‘warmest weather of the year’ to very unsettled conditions during the 2nd half with a named storm too.

The month has also seen a dramatic tale of two halves. From firm positive through first half to quite deep negative during the 2nd, hence the flip from summer (warm, dry) to autumn (closer to avg and very wet).

The main contributing factor I believe has been the active tropics and their northward progress/re-curve combined with warm ocean.

September 2023 tied 2006 for UK’s warmest on record.

While VERY warm, it was also quite wet.

It was Europe’s warmest.

MET OFFICE: Joint-warmest September on record for UK

https://twitter.com/RoostWeather/status/1700259854911516971

UK Notables

September 2023 joined the elite club for producing the YEAR’S WARMEST DAY (33.5C at Faversham, Kent) for only the 2nd time since 1954 (other being 2016). Both 2023’s 33.5C and 2016’s 34.4C both fell short of long standing UK all-time September record of 35.6C recorded at Bawtry, South Yorkshire set back on 2nd Sept, 1906. Interestingly the 2016 high was the UK’s warmest Sept maximum since then.

What set September 2023 aside from the rest however is the number of consecutive days at or above 30C. With 7 consecutive days (between 4th and , comfortably surpassed the previous record of 5 set back in 1929 and 1911.

On the 8th, Northern Ireland witnessed their warmest September day on record with a maximum of 28.0C recorded at Castlederg, Co Tyrone. This edges past NI’s previous monthly record of 27.6C set in Armagh back in 1906.

Excerpt from Met Office: Heatwave, September 2023 

The September 2023 heatwave saw three consecutive days from 7th to 9th having a UK daily
mean temperature exceeding 20°C. The following chart shows that these were the UK’s three
warmest September days on record, by a significant margin, based on a daily series from 1960.
Note however this statistic must be interpreted cautiously because it is highly probable that one or
more September days prior to 1960 may have been warmer. In particular, on 1 September 1906
the temperature exceeded 32°C as far as northern Scotland with (32.2°C at Gordon Castle,
Moray) and 32 to 33°C widely across England. In contrast to September 2023, the Met Office
Monthly Weather Report for September 1906 states ‘It is not a little remarkable that the abrupt
change from the very hot to much cooler conditions was not accompanied by a thunderstorm in
any part of the country.’

This heatwave set a significant number of new September daily maximum temperature station
records, as listed in the table below for stations with 100+ year record lengths. In addition,
Stornoway Airport, Western Isles and Durham each recorded their highest September temperature
since 1 September 1906 (respectively 22.9°C on 7 September 2023 and 28.4°C on 9 September
2023), while Sheffield and Oxford each recorded their highest September temperature since 8
September 1911 (30.5°C on 9 September and 30.8°C on 7 September 2023). Bradford (27.6°C on
9 September), and Morpeth, Cockle Park, Northumberland (26.7°C on 9 September) also equalled
their previous station records.
Several long running stations also set September daily minimum temperature records as listed in
the table below. In addition, Sheffield recorded its highest daily minimum temperature since 8
September 1898 (18.1°C on 9 September 2023).

https://twitter.com/RoostWeather/status/1700704723236532528

https://twitter.com/TheSnowDreamer/status/1700531397662797890

https://twitter.com/TheSnowDreamer/status/1700579735049253038

https://twitter.com/danholley_/status/1700575731799986687

https://twitter.com/MarkVogan/status/1700464423544946883

https://twitter.com/SnowbieWx/status/1708824153313194391

We ended the month with Storm Agnes.

https://twitter.com/CIRA_CSU/status/1706888197589135761

https://twitter.com/RoostWeather/status/1707676078557319349

Meanwhile on the continent…

https://twitter.com/bbcweather/status/1704759929746096218

https://twitter.com/sebastianp_dk/status/1703658950556516559

https://twitter.com/extremetemps/status/1705405120685425098

https://twitter.com/extremetemps/status/1705647557815197768

https://twitter.com/bbcweather/status/1704759929746096218

Storm Daniel wrecked havoc on parts of Greece

https://twitter.com/WxNB_/status/1704531241402306583

 

Following the record rains produced by Storm Daniel, more misery for the same part of Greece.

https://twitter.com/insiderscorner/status/1707369565032677633

Heavy rains also affected parts of the UK too courtesy of Ex-Hurricane Lee’s sideswipe.

https://twitter.com/MarkVogan/status/1704389019029676384

https://twitter.com/metoffice/status/1704503201838309704

https://twitter.com/AdrianHarding99/status/1703717707852472436

https://twitter.com/bbcweather/status/1703676092920332333

Big contrasts both by day and night.

https://twitter.com/metoffice/status/1703309842721525859

https://twitter.com/metoffice/status/1703042413911032101

While a tie for UK, and warmest across Europe, it was also a record globally.

https://twitter.com/RyanMaue/status/1708968413803032922

Meteorology Behind the Heat wave

While a warmer than average Atlantic and Mediterranean will have played a role, Hurricane Franklin’s due north track over the central N Atlantic during late Aug, first few days of September pumped the ridge to it’s N and E over the NE Atlantic and over UK and Ireland leading to the exceptional heat wave.

Then we had more northward tracking ‘canes’ but further west over the Atlantic which yes pumped heights to their N and E but the high was further west over Atlantic rather than UK and so we were actually under the trough with multiple lows sweeping across our shores.

Warmth persisted despite the often wet and windy conditions, in large part due to unusually warm surrounding SST’s.

Through the final 10 days of Sept, high pressure has been strong over Europe bringing yep, unusual warmth to much of the continent, the story of 2023 of course.

My Weather Station Stats for September 2023

MAX TEMP: 28.1C MIN TEMP: 1.5C WETTEST DAY: 00 mm PEAK GUST: 53.3 kph WET DAYS: 19 DRY DAYS: 11

DAYS ABOVE 20C: 8 MAX PRESS: 1024 mb MIN PRESS: 976 mb TOTAL RAIN: 124.07 mm AVG Temp: 13.1C

Record warm, wet October to follow in September’s footsteps?

It would appear October opens with the continuation of low pressure dominance, however, the high over Europe is attempting to slide westwards through week 1 of Oct. This westward shift should do 2 things, 1) ‘Eventually’ reduce rainfall for the SE UK and 2) increase temperature.

A weak active MJO pulse appears to have departed the maritime continent and heading across the Pacific, this ‘should’ reduce the activity within the tropical Atlantic and aid the westward progress of the Europe HP. Unfortunately for Ireland, NI and Scotland, systems will ride the northern flank of the high which looks to be centred too far south for the northern UK to not be affecting by spells of wind and rain.

Some ‘October heat’ perspective

29.9C at Gravesend, Kent (1, Oct, 2011)

Scotland 27.4C (3, Oct, 1908 at Tillypronie (Aberdeenshire)

Wales 28.2C (1, Oct, 2011 at Hawarden Airport (Flintshire)

Northern Ireland 24.1 (10, Oct, 1969 at Strabane (Tyrone)

https://twitter.com/SnowbieWx/status/1707734396051001792

So we have two ‘heat surges’ on the way thanks to a deepening trough to the west while high pressure remains anchored over Europe, with the aid of lows tracking NNE, stiff southerly winds will tap the unusual heat over Iberia northward and draw it north.

Surge 1 only makes to to southern UK with fresher conditions across much of Wales, northern England, Scotland, NI and Ire.

The 2nd surge scheduled for late next week looks (currently) to be more widespread with rare 15C isotherm at 850mb (5000ft up) reaching the central Highlands.

I recon, IF we can remotely achieve those predicted 850s seen by the ECMWF and some GFS outliers then we may see 22 to 24C in Scotland, 21 to 23C NI & Ireland, 25 to 27C England and Wales. Keep in mind 15 to 20C at 850 is quite different in autumn’s 2nd month compared to July or August!

Latest comparison between GFS operational and ECMWF 850mb temperature solution for surge 1

ECMWF

Surge 2

ECMWF

As you can see, there’s is a massive difference in solution between GFS (much less bullish) than ECMWF with any real warmth only reaching SE England. Time will tell regarding that 2nd surge. The first is only England, possibly Wales and not Ireland, NI or Scotland.

Beyond next week, well there’s a large degree of uncertainty and I have low confidence in the outlook.

That being said, I believe we have another warmer, possibly wetter than average month. Like we observed in September, IF we get a real surge of warmth (though nowhere near as extreme) through the first week to 10 days of October, then anything cooler during the 2nd half of the month is unlikely to make the month even average never mind cooler than average overall. The warm surrounding seas will also have influence.

To make some sort of call on October (which I’m struggling with), heights rise across the southern UK, leaving northern areas vulnerable to more frequent spells of wind and rain, less wet further south in the longer range. Wet in the short term.

The tropics should ease in activity while we have an often strong zonal (west-east) jet stream mid to late month. This would keep NI, Scotland, W/NW Ireland and NW England wetter than average with a chance of further seasonal ‘wet & windy’ days.

If the high happened to re-position west of the UK and Ireland, we could see opportunity for something colder and there are hints of that later week 2.

CFSv2 weeklies.

CFSv2 monthly

Further storms is possible given the seasonal shift and warm ocean fuel. The combination of increase in cold vs warm, strengthening jet, very warm SST’s in the Atlantic, El Nino and +IOD is likely to support the continuation of wetter than average as well as warm in W & N.

As for the continent, further record warmth is expected.

Warmer, Wetter than average month overall for UK & Ireland

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