• Sryan Bruen

Warmest day of the year in September?

Cover pic: Komorebi through the trees during a misty September sunrise in Shropshire by Liam Ball.

August 2021 had a north south divide in terms of mean temperature anomaly to average with the north of the UK tending to be warmer relatively speaking compared to the south where it tended to be the coolest August since at least 2017. The absolute max temperature for the month was only 27.2°C which was the lowest in the UK during the month of August since 2010 and was lower than the 32.2°C at Heathrow back in July which stands as the highest temperature of the year to date but also the lowest annual absolute max temperature since 2014 which was the most recent year to not achieve at least 33°C (in fact 2015 to 2020 all achieved 34°C or greater at some point somewhere in the UK). This begs the question if the July 2021 max temperature will be beaten. The latest 32°C on record in the UK was on 19 September 1926 whilst 34.4°C was recorded as late as 13 September back in 2016. I mention these figures or stats because they give us the indication that there are a couple of weeks to go when writing this blog post (1 September 2021) of potential for the July maximum temperature of 32.2°C to be beaten through the month of September and be the warmest day of 2021 in the UK as the media would put it; the correct terminology would be "highest recorded temperature of the year".

So how likely is it, based on history, that September will record the highest temperature of the year for the UK? The most recent example of this happening was back in 2016 which I briefly mentioned above when 34.4°C was achieved on 13 September at Gravesend (Kent). The previous highest temperature had been 34.1°C on 24 August at Faversham (Kent) so it was a very close run thing and really showed just how exceptional the September 2016 heat was.

Before 2016, we have to go all the way back to 1954 to find the last time September had the highest temperature of the year in the UK with 30.4°C at Camden Square (Greater London) on 1 September following a notably dull, cool and wet summer; Summer 1954 is widely regarded to be one of the worst summers of the 20th century for warm, dry and sunny weather. Wide parts of England and Wales failed to achieve 25°C during the months of July and August 1954.

I am not going to mention in detail every single time September has had the highest temperature of the year for the UK as the table below outlines each occasion it occurred in the 20th century. Pre-20th century years also include but not limited to 1898, 1895, 1891, 1890 and 1880. All of the years give us an average or return period of 1 in 15 years but this is not a very helpful figure to indicate how common or uncommon such a stat is as for example, it happened consecutively between 1890 and 1891 but there was a 62 year gap between 1954 and 2016. It also seemed to happen a lot more commonly through the late 19th century into the early 20th century.

Credit: Met Office.
Occasions where September recorded the highest temperature of a given year in the UK in the 20th century with the second trilogy of columns also showing the 2nd or previous highest temperature in that same year.

One more question that one must answer before seeing the possibilities for September 2021 is, how common is 90F (32.2°C) or greater in the UK during September as that was the maximum temperature achieved in July 2021?

The table below shows all the times that 90F has been achieved during the month of September in the UK since 1895. As with September recording the highest temperature of the year, 90F is uncommon in September. 2016 was yet again the last time this was achieved but before that, you have to go all the way back to 1949! 90F used to be more common during September through the early 20th century as compared to the rest of the 20th century.

Occasions where 90F (32.2°C) was achieved in the UK during the month of September.

With all that out of the way and now understanding how unlikely it is that September 2021 will record the highest temperature of the year and beat July 2021, let's look at some of the latest modelling.

The charts below are from the latest ECMWF operational run initialised at midday on 1 September for t144 or 144 hours away which gets us to 7 September. It is going for an area of high pressure to be centred around Belarus ridging back into the UK and Ireland with warm air being advected northward on a southeasterly airflow from the Mediterranean linking back to a large residue of hot air over Africa. Pressure becomes lower in the west the following days but relatively warm air stays put over the UK up to the end of the run, particularly over England.

ECMWF operational run 500mb height synoptic for Europe on 7 September 2021 initialised on 1 Sep 2021 at 12z.

ECMWF operational run forecasted 850 hPa temperatures for Europe on 7 September 2021 initialised on 1 Sep 2021 at 12z.

ECMWF operational run forecasted 850 hPa temperatures for the UK and Ireland on 7 September 2021 initialised on 1 Sep 2021 at 12z.

Going by the raw forecasted temperatures on this ECMWF operational run, the peak of the warmth seems to be the 9 September with 29°C projected around the Bristol area. Typically, models fall somewhat short of actual temperature values and can usually add a degree or so onto forecasted figures which would give possible maximum highs of 30-31°C which would not be enough to be for September to beat the July maximum of 32.2°C. Sometimes, the difference between verified and projected values can be even larger however. These kinds of temperatures are favoured by their corresponding 850 hPa values of 15-17°C - usually can double these to get surface values although that heavily depends on other things such as sunshine, wind direction and fetch, wind strength, time of year and soil moisture deficits etc.

ECMWF operational run forecasted max temperatures for southern UK and Ireland on 9 September 2021 initialised on 1 Sep 2021 at 12z.

Other models such as the GFS and UKMO show similar to the ECM with a southeasterly airflow and warm air being advected northwestward from the south.

GFS operational run 500mb height synoptic for Europe on 7 September 2021 initialised on 1 Sep 2021 at 12z.

UKMO operational 500mb height synoptic for Europe on 7 September 2021 initialised on 1 Sep 2021 at 12z.

So going by the above modelling, it looks like there will be some very warm to possibly locally hot weather on the way through next week in the UK. The fetch and direction of the southerly to southeasterly wind will be critical in where gets the highest temperatures and how high it could possibly get too. The airmass at the moment being projected on the modelling looks warm enough to achieve an isolated 30°C somewhere but as mentioned, that will be determined by other factors and seeing how this scenario develops as we get closer to next week.

It likely won't be all dry and sunny too as the anticyclone is centred a good bit to the east of the UK and there will be showers around. The warm air could prove volatile too with a chance of these showers being thundery in nature. All to be revealed as they say.

Thank you for reading.

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