September has been a crazy month with big temperature swings and historic rain/flood events for UK as well as Europe.

From record +NAO in August to record -NAO in September with unusually strong blocking over Greenland has had a lot to do with how extreme this month has been.

According to World Climate Service, based on MSLP (mean sea level pressure), this is the greatest month-to-month swing since Nov-Dec 1978.

Check out UK rainfall anomalies comparing August to September. Record wet August in Western Scotland.

Record wet September for a large swathe of Central England.

Record-breaking rainfall for some this September

The huge swing in pressure between Iceland and Azores and shift in jet position is to blame as well as warm, moist atmosphere and the continuation of abnormally warm Atlantic and Mediterranean waters!

While record heat spread across the continent twice, we also witnessed two waves of early season arctic air with the first bringing the coldest early September air since 1986 for some.

So what’s behind the switch in NAO?

Perhaps a shift in SOI (Southern Oscillation Index) from negative (El Nino) to positive (La Nina) coupled with other factors including eastward progress/amplification of the MJO (Madden Julian Oscillation) and possibly higher than normal levels of ozone within the stratosphere over the North Pole.

A rise in SOI could have weakened the jet perhaps allowing tremendous amounts of heat trapped within the mid latitudes & far north land masses to lift towards the pole forcing strong blocking to develop.

While another record warm month for much of central and eastern Europe, September turned out to be another sub-par month. According to the Met Office, the month ended up 0.3C below normal.

This makes for a quite interesting ‘recent trend’ of 3 out of the past 4 months running below average for the UK as a whole.

June -0.4C

July -0.5C (-2C by mid month)

August +0.3C

Check out the temperature anomaly for the last 120 days.

Some Summer & Sept Highlights For UK, Ireland & Europe

Since the very warm May (particular night-time), a cool pattern has repeatedly returned to NW Europe.

Fresh snow on the Cairngorm Plateau Sept 12th. I last photographed fresh fresh snow cover back on June 12th!

Quite the west-east 2-day temp anomaly contrast!

Tweet from Neil Kaye (Met Office)

If it feels like it is a bit cold at the moment here in the UK, this #dataviz shows that yesterday was the coldest day for the first half of September (1st to 15th) since 1998. I don’t think today is going to be much warmer!

Unusually widespread coverage of below 0C for early September.

On Sept 11th, With a CET of 9.4°C, it was the coolest day in the 1st half of September since 1986, and the coolest for the date since 1908 (!)

Dudince, Slovakia – 87 days above 25 °C in a row, the longest series in Slovakia (until this year 57 days in 2013 at Dolné Plachtince)

Mariazell in Austrian Styria, 864 m first time since beginning of measurments (1948) with snow cover in September.

Mount Dillon, Ireland had an air frost Sept 13th (-1C rounded) which makes it the earliest air frost in the Republic of Ireland since 2010 which occurred on August 30th at the same station.

Exceptionally low temperatures also in #Italy ! -24.6°C Col Major, Mt. Blanc [4,750 m], new September EUROPEAN record! [but short period of record] -21.6°C Capanna Margherita [4,560 m], only 0.4°C from its monthly record and never been so cold so early.

Record low maximum temperatures for the 1st half of September in 5 of 9 federal states [Länder] of #Austria ! 7.5°C Salzburg 8.2°C Linz 8.4°C Vienna [34.6°C 10 days ago] 8.6°C St. Pölten 8.9°C Eisenstadt 7.3°C Innsbruck, only 0.1°C from the record set in 2007.

Remarkable low temperatures in #France Sept 14th. -0.7°C Aurillac lowest so early since 2001 1.8°C Alençon […] since 1986 3.6°C Gourdon […] since 2001 4.1°C Albi […] since 1996 4.2°C Auch […] since 2001 5.4°C Toulouse […] since 1984

-0.3C at Santon Downham, Suffolk on Sept 14th made it the earliest September frost for E Anglia since mid September 1986, when the same location reached -1.1C

On Sept 14th, 0.8°C in Hochfilzen (#Tyrol, 962 m) This is the lowest *maximum* temperature ever recorded in the country in September below 1,000 m. The previous record was 2.4°C in Karlstift (Lower Austria, 946 m) in 1977.

2024 Central European floods

Tweet via Nick’s Weather Eye

Record rainfall has fallen in northern Austria, below are 72 hour totals on the left against previous records on the right for those stations. No wonder Lower Austria has been declared a disaster area with that volume of rain working through into river systems.

The station of Jesenik, Czech Republic recorded 450mm or half a year’s worth of rain in 96 hrs, leading to disastrous flooding.

321 mm in 4 days in St. Pölten (Lower Austria)making it the wettest month at the station since records began.

3 day rain totals.

Tweet from World Climate Service

Coldest summer (June-September) in Iceland since 1998 (coldest in Reykjavik since 1992). Pretty remarkable, considering the extraordinary warmth of the Atlantic basin overall.

 

Tweet from World Climate Service

According to

GHCN-Monthly data, there were zero climate observing sites worldwide with a record cool/cold June-August (min 30 years of data). It’s the first time this has happened in at least a couple of decades, although some other years came close (e.g. 2005, 2016).

Tweet via Dan Harris (Met Office)

At 14.03ºC, the September Central England #Temperature was almost 3ºC below that of last year, and distinctly average (-0.2ºC compared with the 1991-2020 mean)

Poor summer in the west end of Glasgow/Paisley area. Via Jeffrey Blackshaw

MJO phases

Throughout August the MJO was stuck in phases 3-4-5 likely promoting the strong +NAO but then strengthened in phases 5 into 6.

This possibly helped build pressure northwards over Europe while heights lowered over the Mediterranean then slight west-east movement of troughs opened the door to arctic air sweeping initially into the west before pushing east presenting the extreme rainfall event for central and east Europe. A record warm Med also played a role along with the level of warmth contained throughout the atmosphere.

The MJO has then shifted east from maritime continent to west and central Pacific (phases 7-8-1 eventually making it into the Atlantic likely playing a key role in strengthening high latitude blocking (-AO/NAO).

Written Sept 30th (below)

We end September on the cool, soggy side with a strong block over Greenland and low over Scandinavia providing a northerly flow into the UK while a strong south displaced jet is driving a train of lows to sweep into southern Ireland and UK.

The second half of September has witnessed quite incredible amounts of rain for the south and central UK. 2 to 3 times the September average has fallen with some spots witnessing their wettest 24 hour periods in history. Oxford and Woburn, Bedfordshire are among these places with records going back well over 100 years.

Even the past 72 hours have been quite wet in the south while largely dry in the north. That’s been the story much of this month and there’s more to come through today and tomorrow courtesy of the latest low sweeping through.

September 2024 Monthly Weather Report

UK Met Office September 2024 Stats

Highest Maximum 30.1°C on 1st at Cambridge, Botanic Garden (Cambridgeshire, 13mAMSL)
Lowest Maximum 7.3°C on 27th at Balmoral (Aberdeenshire, 283mAMSL)
Highest Minimum 18.8°C on 2nd at London, St James’s Park (Greater London, 5mAMSL)
Lowest Minimum -3.0°C on 25th at Tyndrum No 3 (Perthshire (in Central Region), 168mAMSL) and
Braemar No 2 (Aberdeenshire, 327mAMSL)
Lowest Grass
Minimum -6.8°C on 25th at Tyndrum No 3 (Perthshire (in Central Region), 168mAMSL)
Most Rainfall 119.9mm on 29th at White Barrow (Devon, 445mAMSL)
Most Sunshine 12.7hr on 5th at Stornoway Airport (Western Isles, 15mAMSL)
Highest Gust 60Kt 69mph on 29th at Berry Head (Devon, 58mAMSL)
Highest Gust
(mountain*) 88Kt 101mph on 14th at Aonach Mor (Inverness-shire, 1130mAMSL)
Greatest Snow
Depth at 0900
UTC
No non-zero values.

My September Stats

 

Chilly, Soggy October As Blocking Pattern Lingers?

October opens cool with that south shift jet bringing wetter south than north.

The cool theme looks likely to remain through at least week 2, even 3 of October due to the blocking high remaining anchored over Greenland.

Despite the MJO continuing it’s eastward journey back into the Indian Ocean (less favorable blocking cold phases), there’s often a lag from it’s previous position which was 7-8-1.

There’s also an ongoing record weak polar vortex which could make October quite a cool month before it strengthens in a few weeks. By the time it more than likely becomes ‘stronger than average’ as seen by various models, we could even have entered November.

The strong blocking is quite visible in the lower stratosphere at 50mb.

Even 10 days from now, it’s still there. Suggesting the blocking doesn’t go away.

The strong blocking within the troposphere is likely the cause of the weakened, displaced vortex within the stratosphere. Bottom up forcing causing the slowing zonal winds.

Here’s the 30 day ECMWF extended.

GFS extended

Even the CFSv2 monthly is on board.

Looking rather soggy too.

The tropics will most certainly need watching. Systems lifting heat north will affect the jet strength, position etc. A couple of named storms isn’t out of the question, especially mid and late month and based on the above model solutions, even a rare early snow event for parts of the UK or Ireland cannot be ruled out either.

The Verdict

Colder & Wetter than average Western Europe, Warmer, Drier East

Tags: featured, monthly-outlook