UK Weather - Autumn 2023

[Simon Culling]

As we enter meteorological Spring, the weather is taking a big turn for the better. We will see at least a week of warm sunny weather, particularly in England + Wales, and the temperature may approach 30°C in the SE. Indeed we may see our warmest day of the year and it is rather unusual for this to occur in September - the current yearly high is 32.2°C at RAF Coningsby on 25th June and also at Chertsey Abbey Meads on 10th June.

As a large and deep low moves north towards Greenland (containing the remnants of Hurricane Franklin), we are losing the main impetus of the jet stream over the UK and high pressure is now building in from the SW - and it will move over the UK and take up residence over nearby Europe and give us a SE flow to bring the warm, dry + sunny weather for the coming week. This may well persist into and beyond next weekend.

[spwill]

Certainly looking very warm for September.
Looks warm for the start of the Rugby World Cup across the channel in Paris. I'm heading over to Europe this week but not for the rugby.

[Simon Culling]

I hope you enjoy your trip to Europe, spwill. It is going to very warm or hot over most of the continent and quite stormy + wet in Iberia - certainly for the next day or so.

Here in the UK it was warm and sunny on Sunday with a high of 27°C in southern coastal counties - Hampshire, Sussex + Kent. The forecast for the coming week is still warm, or even hot, and sunny with the high for Wednesday through to Friday now expected to be up to 32°C. We may well record our warmest day of the year. GFS has an 850mb temperature in excess of 20°C over SE England on Thursday from tonight's run and has the heat lasting to at least next Monday (11th Sept).

[spwill]

Thanks Simon, enjoy that very warm -hot weather.

[Simon Culling]

We have been enjoying/enduring a late hot spell here in the UK - it rather depends on your perspective. It has been predominantly sunny over much of the country since the weekend, with just a few thundery showers overnight in more western parts - these have all been elevated storms and not given a great deal of rain except to parts of Ireland yesterday. Hot again today (Fri 8th Sept), although there is more medium and high cloud around. These are the UK maximums for the first four days of this week:

Monday 4th September - 30.2°C at Whitechurch (Pembrokeshire)
Tuesday 5th September - 30.8°C at Otterbourne (Hampshire)
Wednesday 6th September - 32.0°C at Kew Gardens
Thursday 7th September - 32.6°C at RHS Wisley (just to the south of London - site of the Royal Horticultural Society)

The hot weather looks like continuing through the weekend with possibly a high of 33-34°C on both days if cloud does not build too much. There is the prospect of a few home grown thunderstorms over the weekend and this may limit the temperatures a bit.

The current record high temperature for September is 35.6°C at Hesley Hall near Bawtry in South Yorkshire on 2nd September 1906. This is the oldest of the monthly UK record maximums. Whilst the predictions for the weekend do not threaten this record, the current run of 5 days with somewhere recording a high of 30°C or more is a record for September and it looks like that run will extend to at least 7 days.

[Simon Culling]

The hot spell lasted through the weekend, but has relented today, Tuesday 12th September. These are the recent maximums which extended the run of days with 30°C or above to 7:

Friday 8th September - 30.9°C at Cavendish (Suffolk)
Saturday 9th September - 33.2°C at Kew Gardens
Sunday 10th September - 32.5°C at Cambridge NIAB
Monday 11th September - 26.9°C at Cavendish

Quite a few thunderstorms developed on Sunday, especially in southern and eastern England with a notable complex of cells developing to the SW of Milton Keynes around 1pm and moving NE to reach The Wash (on the North Sea coast) around 4pm. These cells developed quite a gust front, with a squall recorded to 77mph at the Met Office site at Holbeach on the coast (on The Wash in Lincolnshire). Attached is a graph (courtesy of Blitzortung) of the lightning strikes over the UK between noon and midnight on Sunday. 123K strikes is not bad for mid-September.

The CET for September was 20.4°C (+6.8°C) up to the 11th and the EWR was 6mm up to the 10th, which is 8% of the monthly average of 76mm. The remarkable anomaly of +7°C will fall away over the coming week as more normal temperatures for mid-September prevail.

[Simon Culling]

There has been a general cool down over the UK in the last few days, but the much cooler weather has been restricted to Scotland and the south east is still very warm - today (Sat 16th Sept) it has reached 27-28°C in and around London. In contrast, here in the SW we have had rain off and on all day today in Tiverton and a maximum of just 17°C. The rest of the weekend will be warm + muggy over the southern half of the UK and the conditions are marginal for more thunderstorms tomorrow, some of which could be strong.

As mentioned above, much cooler weather is over Scotland and to emphasise this there was a sharp frost in the far north on the morning of Wednesday 13th September - the temperature fell to -3.5°C at Kinbrace Harchery and this is a new date record low for this date.

Below I have attached a picture of the shelf cloud associated with the strong thunderstorm mentioned in my last post (on the 9th Sept) - the picture was taken at the end of its land track and near to the east coast - in the village of Fosdyke, which is near to The Wash.

https://twitter.com/simon_culling/statu ... 5318997026

[Simon Culling]

Conditions were rather more than 'marginal' on Sunday, with a large area of thunderstorms affecting most of the SE + S half of England during the afternoon and especially the evening. It was a very wet day across the whole country and this was indicated by very high pecipitable water contents in the ascents made during the day. It was particularly wet in Devon, Somerset + Wiltshire during Sunday - these areas caught the early thunderstorms and a number of gauges had around 100mm of rain which resulted in some flash flooding. Here in Tiverton we had around 40mm of rain for Sunday (17th Sept) but thankfully no flooding. We also had a long lightning display in the early hours of Sunday morning with a period of torrential rain making all my gutters overflow!

https://www.bbc.co.uk/news/uk-england-somerset-66840766

Later in the evening on Sunday, another wave of storms moved NE from France and gave a very good lightning display to much of SE England + East Anglia and there was a small tornado on the south coast at Littlehampton (W Sussex) just before midnight.

Since the weekend it has become cooler and we have a low pressure system approaching the UK today (Weds 20th Sept) which contains the remnants of Hurricane Lee and this is giving us a wet and windy autumnal day - the low is just to he west of Scotland at 975mb at noon. The low will be followed by and even cooler NW flow for Thurs/Fri before it warms up a bit at the weekend.

[Richard]

How long did it take for the 40mm of rain to fall Simon

[Simon Culling]

Hello Richard - with regard to the rainfall last Sunday, here in Tiverton it was thankfully spread over the day and hence we avoided the flooding. The more prolonged heavy rain affected other parts of Devon and particularly parts of Somerset. Looking at the trace of a nearby PWS (about 400m to my south), peak rainfall rates in the town were around 50mm/hr, but in the worst affected areas, rates were up to 100mm/hr.

It has turned much cooler in the last few days and become unsettled with rain or showers on most days, and this pattern will continue through the week. A rather deep low is expected to affect the UK on Wednesday, although there is some uncertainty on its likely track. It will contain the remnants of Tropical Storm Ophelia, so there is going to be lots of moisture and rain. There are some tentative signs of very warm or even hot weather returning to southern Europe as we go into October, but it is not clear yet if this will extend north to the UK.

The CET for September was 18.1°C (+4.5°C) up to the 21st and the EWR was 73mm up to the 22nd which is 96% of the monthly average. Unfortunately the much cooler weather expected for the rest of the month will probably mean that September will not end up being warmer than all of the summer months. The average CET for the three summer months was just 16.5°C.

[Simon Culling]

Storm Agnes passed by yesterday with rather less impact than had been expected by the forecasters. It had deepened very quickly on Tuesday when it was way to the SW of Eire and had already began to fill as it approached the British Isles. Whilst it was a windy day for Eire and UK coasts bordering the Irish Sea, most of us were left wondering what all the fuss and warnings were about. However, it would have been rather different if the depression had arrived 12-24 hours earlier when in its deepest phase as there was plenty of evidence of a sting jet on its SW flank. See this tweet and the interesting discussion in its thread - the water vapour imagery is great:

https://twitter.com/RoostWeather/status ... 8837146892

The picture attached is taken from this tweet - the sting jet is at the tip of the curl of cloud on the left side of the picture. The models suggested top wind speeds (in gusts) in the sting jet as it reached the surface were around 120mph - thankfully that happened over the Atlantic.

Looking forward, the weather looks set to calm down as high pressure becomes the dominant factor for the weather of NW Europe. For us in the UK it looks to be dry and a little warmer than average.