Deep Dive 03/09/2024 – Summer stats… but is it over?
This is an in-depth Met Office UK Weather forecast for the next week and beyond. This week we look back at the summer and why there’s uncertainty in the forecast. Bringing you this deep dive is Met Office meteorologist Alex Deakin.
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00:00How was your summer? Why have we had so many thunderstorms recently? What is it
00:06with the uncertainty with our weather later this week? And could some parts of
00:12the UK see a September heatwave? We'll try and answer all of those questions in
00:16this week's Met Office Deep Dive. Welcome along. My name's Alex Deakin and every
00:23Tuesday we do a Deep Dive. 20, 30 minutes sometimes of what's going on in the
00:29weather world and we are coming at you from our Exeter HQ. We're here to talk
00:36about anything that you want to so do put any questions, any suggestions in the
00:41comments. If you like what you see do hit that big heart button as well. Thumbs up.
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00:51because this is the only place you can see the Met Office Deep Dive here on
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01:01As always there is plenty to get through. Yes it's September which means
01:07meteorologically and climatologically it is now autumn. Doesn't mean to say
01:12there won't be some summer sunny weather around but for us here at the Met Office
01:17summer is done. You can measure summer however you want to but for us we are
01:22now in autumn. So we will be taking a deep dive into the summer stats because
01:28there was some big variations. First of all though let's take a look at the
01:32weather for the short to medium term. Satellite image over the past few days
01:37shows quite nicely what's been going on. All the clouds have been rushing up from
01:40the south. That's why it's been quite warm and quite humid and that is also
01:45while we've seen quite a few thunderstorms. If we zoom in a little bit
01:49put the radar in you can track the storms that have been coming up
01:53Saturday across the UK. Then on Sunday down here in Devon we saw some
01:58torrential downpours across parts of Wales as well. And then on Monday
02:01particularly northeast Scotland catching a whole barrel load of showers but also
02:05further south thunderstorms breaking out and we're not done yet. A few heavy
02:09showers around still on Tuesday at time of recording and there is more to come
02:14in the short term as well. So all our air is rushing up from the south
02:18bringing with it yes some warmth some humidity but also plenty of thundery
02:23showers. Why so? Well as always it's to do with what's going on high up in the
02:28sky where the jet planes fly and with the jet stream. This is the position of
02:34the jet stream at the moment coming out up over Greenland and then dipping its
02:38way southwards. It's been south shifted for most of the summer to be fair. We did
02:44have a bit of high pressure back in the last week didn't we but now the jet
02:47stream is back here and it's in these troughs that low-pressure systems form
02:51hence why we've seen quite a bit of wet weather recently. But what goes on over
02:57the next few days really quite interesting with the jet stream. Notice
03:00how it's dipping here rising here so going like this. This is a trough but as
03:05we play through the next few days if we zoom in a little bit we can actually see
03:11something really quite interesting that happens. Basically this southern arm and
03:17the northern arm get closer and closer together. The trough narrows and narrows
03:21all the while pretty much sitting across the UK until something quite
03:27interesting happens later this week as we go through Wednesday night more into
03:31Thursday. They're getting closer and closer and closer until we get actually
03:36a circulation developing in here and that is called, there it is, a cutoff low.
03:42So actually what happens the jet stream kind of turns back on itself and you've
03:47got this low high up in the sky just circulating around on itself and that is
03:54a bit of a headache for weather forecasters to say the least and it it
03:57means our weather patterns become very slow moving and we are going to be stuck
04:02with low pressure nearby to the south in particular that means more wet weather
04:06but it also promises some fine weather further north. So we've got this low here
04:11it's called a cutoff low, the circulation high up in the sky.
04:15Meanwhile the main jet is now pushing way up here to the north so this is
04:21what's called a cutoff low. It's a bit like an Oxbow Lake you may have studied that
04:25in your geography GCSE or O level depending on how old you are. So yeah an
04:30Oxbow Lake is a similar kind of feature geographically. The main jet is now up
04:35here and we have got this cutoff low sitting right over the UK. I just want to
04:40show you something a definition it's from Wikipedia which is not where we get
04:44most of our meteorological definitions honestly but I thought this was
04:47interesting because the Wikipedia definition of a cutoff low sometimes
04:51referred to as weather man's woe which really made me smile this morning
04:56weather person's woe perhaps in this day and age but yeah because it gives us
05:00grief it's hard to forecast what the weather is doing particularly with the
05:04detail when you've got one of these cutoff lows. They're tricky for computer
05:08models to analyze and it's really difficult to pin down the details why
05:13there's been so much uncertainty about the forecast this week. If you watched
05:16Aiden's week ahead forecast yesterday to talk about different scenarios and this
05:20was one of the main reasons for that because of this cutoff low forming
05:24where's it going to be and what's it going to do to the showers. Now thankfully
05:27since Aiden recorded the week ahead yesterday the computer models are now in
05:32more agreement so we're pretty happy that this is going to happen with this
05:35cutoff low the National Weather Service which is the American equivalent of the
05:40Met Office a closed upper level low which has become completely displaced or
05:46cut off hence the name from the main westerly flow usually between about 20
05:51and 45 degrees north so this is actually a little further north than that but
05:55yeah we can get them close by to the UK and that is what we are seeing with our
06:01jet stream by the time we get to let's just zoom out so there you have it
06:05that's the cutoff low there and there's the main flow going on so it's
06:09completely detached and what happens is that weather just becomes stagnant it
06:13kind of sticks around again and the definition here can remain stationary
06:18for many days and that's what we're going to see towards the second half of
06:22this week and indeed through the weekend it means we're going to see quite a bit
06:25of wet weather closest to that low across the south in particular let's
06:31look at the rainfall patterns for the next few days so if we go through well
06:34let's go through Wednesday first of all so tomorrow we're looking at quite a few
06:39showers breaking out they're going to be kind of hit and miss but there could be
06:42some quite heavy ones they could be almost anywhere but we're a bit concerned
06:45tomorrow and into Thursday about the showers developing across the south so
06:51heavy showers developing tomorrow night across the south and then lingering as
06:55we go into Thursday in this zone here so much so that we actually have issued
07:01a yellow warning well just missed it there for tomorrow evening so there's a
07:10rainfall warning out across parts of the south could be some thunderstorms mixed
07:14in with it but it's more just about the heavy rain developing later tomorrow in
07:19the form of heavy showers but then as we see things going on in the atmosphere
07:22that cutoff low developing bringing in some more humidity from the south likely
07:27to spark some heavy downpours that could last for much of Wednesday night
07:32and into Thursday across parts of the south we do have that yellow warning in
07:35place go and check out our app and our website for more details on that yellow
07:39Met Office warning which runs through Wednesday night but generally it just
07:44stays wet across the south because of that setup let's fast forward to
07:48Thursday now and if we play through you can see it's further pulses of rain
07:52coming in across the south through Thursday and if we just fast forward
07:58Friday Saturday bands of rain now and you can you can picture it now so the
08:04low pressures here you've got that cutoff low high up but the pressure
08:07patterns mean that the low pressures all the way through the atmosphere so you've
08:12got this low pressure here this weather front there's the cutoff low high ups the
08:15things are just not moving you've got low pressure down to the south generating
08:19pulses of wet weather coming up across the south generating an easterly wind
08:23further north more on that in a moment but in the south we're just keeping an
08:26eye on the rainfall totals because they're really going to build up and
08:30let's take a look at those totals and let's look at the total accumulation
08:36let's skip forward to Sunday so this is the rainfall accumulations through
08:44Sunday a little bit across the north that's the lowest in the scale we just
08:47want to highlight the where the most rainfall is going to be across the south
08:50because we're going to see those bands now often when you look at the rainfall
08:53accumulations for the next five days it's Western Scotland that's been wettest
08:56certainly has been through this summer again as we'll see in a minute but for
09:00the rest of this week it's in the south where we're going to see the heaviest
09:03pulses of rain let's break that down a little bit more by looking at the
09:07rainfall totals as those bar charts I'll get the right one in a second bear with
09:17bear with this is what I wanted to show let's get a poppers bars because this is
09:25quite funky if we look at 24-hour totals accumulations across the south actually
09:32let's do 12-hour accumulations so this is a rainfall bars building up over 12
09:36hour period you can see that heavy rain likely across the southeast on Thursday
09:39but if we go through the system over the next few days just look at how those
09:43total plenty across France as well but the rainfall total is really building up
09:47across the south 12-hour accumulations those spikes in there now the detail
09:53going to be hard to pin down because of that complication with a cutoff low and
09:57it's tricky for the for the models to handle but the general signal that
10:01across parts of the Midlands South Wales Southern England there is going to be a
10:05series of pulses of rain coming in between Thursday and Sunday so some
10:10heavy downpours are possible we've got that warning out for Wednesday night and
10:14into Thursday but even after that we are going to see further pulses of heavy
10:18rain when we know more details will be firming up on exactly where and when
10:22those pulses of rain are like to hit but generally across the south it does look
10:26pretty wet as we go through this weekend into the early part of next week there
10:31are some signs it should be drier in the south early next week but there's quite
10:35a bit of uncertainty tied into that it may also turn a little cooler because as
10:40well as turning wetter across the south again it's also actually going to turn a
10:44little warmer because if we look at the temperature profiles
10:49let's take the pressure off we don't need the jet stream anymore so it has
10:53turned a bit fresher over the past couple of days quite warm and humid over
10:56the weekend but now we've got the cooler air coming in from the Atlantic but if
10:59we go through the next few days notice how the warmth comes back as the lows in
11:04here and it's pushing the warmer weather back up from the south and east back
11:07across the UK so it is going to turn warmer and more humid once more over the
11:12next couple of days but of course in the south you're not really going to feel
11:15that warmth it'll be humid but you're not going to if you haven't got the
11:18sunshine you won't be feeling the benefit however further north you will
11:23potentially feel the warmth and the sunshine let's put the pressure pattern
11:27back on because as we talk about what's going on down here the cutoff low high
11:34up in the sky got the low pressure system at the surface you've got the
11:36wet weather across the south but further north actually it's this high
11:41pressure that's influencing things a little bit yes we've got an easterly
11:44wind so this is the wind at the surface coming in from the northeast but the
11:50winds coming in here but actually for much of the northern half of the UK
11:55under the influence of the high pressure we are going to see plenty of fine and
12:01bright weather so Scotland Northern Ireland particularly northwest England
12:05going to see some sunshine over the next few days little will change on
12:09Sunday there's a weak front trying to push in but it's all the rains going to
12:11be across the south and actually for much of northern Britain the back end of
12:15this week looks pretty good dry and fine high pressure sinking air plenty of
12:20sunshine there is going to be easterly winds so let's put those put the cloud
12:25on because that is going to generate some windier conditions with those
12:30easterly winds it could be a bit claggy around some eastern coasts but with an
12:35easterly or northeasterly wind it's Northern Ireland Western Scotland
12:38Northwest England are going to see the lion's share of the sunshine so if
12:43you've got any plans that's Saturday at the moment again detail of where the
12:46rain is a little uncertain but the general weather pattern is set so we can
12:50be pretty confident that Western Scotland Northern Ireland and Northwest
12:53England are going to see some fine weather through the back end of this
12:55week into the early part of next week and we'll see some very high
12:59temperatures as well along with that so it's quite cool today across the north
13:0615 16 degrees a lot of cloud and outbreaks of rain still reasonably warm
13:10across the southeast but if we fast forward to Friday notice how Western
13:15Scotland is is really warming up here could get into the low 20s in Glasgow
13:19and in Western Scotland on the west coast in particular something called the
13:23fern effect may lift the temperatures higher potentially getting into the mid
13:2720s which would take us towards heatwave criteria now for a heatwave you
13:33have to have three days over that threshold which across Scotland is 25
13:37certainly possible Friday and Saturday whether we get three days or not bit of
13:42a question mark about that but certainly in this part of the world temperatures
13:45are going to be above average significantly so during
13:52that Friday so again let's zoom in it will be warm in the south the air is
13:56going to be warm but remember here we're not sure about where the rain is
13:58going to be so we can't rely too much on that because it could be quite cloudy in
14:01the south depending on where that rain is going to be but certainly across
14:05Western Scotland and parts of Northern Ireland temperatures like to be
14:08significantly above average and as I said in Western Scotland especially we
14:12could get over 25 degrees which would put us towards heatwave criteria so that
14:16could be quite interesting in Scotland that's Friday this is now Saturday so
14:21again temperatures significantly above average in this part of the world thanks
14:25to the fern effect now we've talked a little bit about the fern effect before
14:29but it's going to go through it one more time on here in case you missed it
14:33earlier let's draw I'm going to bring my drawing skills back imagine that is a
14:41mountain in Scotland we're a cross-section here so this is a
14:46cross-section of a Scottish mountain the winds remember are coming in from the
14:50east so we've got winds coming in this way going up the mountain as they go up
15:00towards the top of the mountain obviously it gets a little cooler and
15:04down the other side it gets warmer but it's significantly warmer on this side
15:10and there's a number of things going on as the air is coming in this way and
15:14rising up there's a pretty cloud so the air rises up the mountain it cools it
15:25condenses it forms cloud and oh let's make the rain blue it rains so the air
15:32on this side is drier than the air on this side so the air coming in rises it
15:37rains it forms cloud the air coming down this side is drier now drier air as it
15:43descends warms up faster than wet air on this side so that's one reason why
15:49it's warmer on this side the other reason is that actually on this side
15:53because you lost a lot of the cloud oh look at that's a thing of beauty it's
15:58often sunnier on this side of the mountain as well so that also heats
16:02things up another thing you're doing is blocking the air here so you're actually
16:05drawing air down from higher up and as the air gets pulled down it gets
16:11compressed as well and that also helps to warm things up because the pressure
16:16is higher at the bottom and you get the turbulent mixing as well so there's
16:20kind of four factors going on here that means warmer conditions on the lee side
16:26of the mountains compared to the windward side of that is the fern
16:31effect there's more on that on the Met Office website if you're interested in
16:34that but that is essentially what the fern effect is and it means that to the
16:39lee of mountains it's often warmer and particularly if you have just the right
16:43conditions the air is just in the right state just stable enough enough of a
16:47breeze coming in then you can get really big jumps in temperatures happens quite
16:51a lot in the Rockies where of course the mountains are that much bigger but it
16:54happens pretty frequently across Scotland as well and that is what we're
16:57likely to see later this week with those temperatures rising in western parts of
17:04Scotland because the winds are going to be coming in from the east or the
17:06northeast of the East Coast will be quite cool particularly if it stays cloudy but in the
17:12West you can have that fern effect lifting the temperatures say
17:15potentially into the mid-twenties across Western Scotland later this week and boy
17:20could Scotland do with a bit of warm and sunny weather because in Western
17:25Scotland in particular you have had a pretty grim summer time now for a look
17:30at some of those summer stats that we talked about at the start these are the
17:34headlines we issued a press release earlier this week summing up summer and
17:39August weather it was cooler than average
17:43coolest summer since 2015 so the coolest summer for nine years overall rainfall
17:51was pretty close to average slightly below think about ninety-five percent of
17:54the average but this is the key huge regional variation I say Western
17:59Scotland as we will see took the brunt of the wet weather over the summer and
18:04again sunshine amounts close to average but with some significant regional
18:09variations as we will also look at shortly but let's deal with temperatures
18:13first of all that's the main headline I guess this is the map showing the
18:21temperature anomaly so different from the average the average and this is the
18:26long-term 1991 to 2020 average so compared to the most recent 30-year
18:33climatological period white is pretty much close to average blue is cooler
18:37than average you can see that Western Scotland the west of Northern Ireland
18:40parts of southwest Scotland were cooler than average many of the places were
18:44pretty close to average so although it was slightly cooler than that average
18:49from 91 to 2020 it was only generally a little bit cooler than average
18:55interestingly if you've taken the average from 1961 to 1990 then the summer
19:01just gone would have been above average because our summers are getting warmer
19:05there's been a bit of chatter around this on social media people not really
19:08maybe grasping climate and how averages and graphs work but this graph shows
19:17quite nicely what has been going on so this is the variation we know that every
19:22summer is different right you're going to get variations up and down so this is
19:25a plot of the summers over the past century or more in fact and how they
19:32have varied this pink line here is the average for the most recent 20-year
19:38period from 1991 to 2020 that's what we're using our climate average as at
19:44the moment you can see that this one here that's the the latest summer the
19:47summer just gone which is slightly below that average but most of the recent
19:51ones have been above and four in the past 20 years have been right at the top
19:55there in the most extreme go back over time and you can see that trend this
19:59black dotted line that is plotting the averages waving around for the first
20:04half of last century but a significant and definite trend there in the past 50
20:10years that our summers are getting warmer 76 there also stands out again
20:16within that variability the cooler ones are down here notice the cooler ones now
20:21I was just below that average line but there's way more of them up here so we
20:26still got that trend so one summer that's slightly cooler than the most
20:30recent climate average does not mean climate change isn't happening far from
20:35it that summer this summer just gone if you'd taken it by the average of 1960 to
20:401990 would have been above average so that's how we know that our climate is
20:44warming you're still going to get variations that's normal but the overall
20:50trend that's climate change you can see it quite clearly in this graph so yes
20:55this summer was slightly cooler than average but recent summers have been
20:59much much hotter than average as well okay that's enough on temperatures let's
21:05look at rainfall now because as I promised there's been some big big
21:08variations with across the country with our rainfall this is summer and this is
21:15August so I just want to put next to each other because actually August just
21:19is a is a continuation really of what we've seen through through much of the
21:23summer certainly July was very similar so August kind of mirroring what we saw
21:27throughout the entire summer just a reminder summer is June July and August
21:31the whole months that's the way we do summer but yeah highlighting there the
21:36parts of Western Scotland significantly wetter than average whereas actually
21:40quite a chunk of England was quite a bit drier than average over the whole of
21:45summer some places a little pockets of white here and there which is close to
21:50average but a good chunk of England was was drier than average throughout the
21:54whole summer and eastern areas in particular quite a bit drier through
21:57August East Anglia Lincolnshire parts of Yorkshire whereas Western Scotland
22:03again really bearing the brunt that's that dark blue there is 200% so twice
22:07the August average rainfall falling across quite a chunk of Western Scotland
22:12so yeah you have had a very wet August and quite a wet summer in general in
22:16western parts of Scotland and the sunshine maps as you might imagine kind
22:21of show the same pattern with these western areas the grays duller than
22:26average the yellows sunnier than average little pocket there across parts of East
22:32Anglia and Whiz Beach I was there not long ago significantly sunnier than
22:37average throughout the summer and yeah reflected also in August sunnier than
22:43average here northeast England eastern parts of Scotland Western Scotland as
22:48you expect it's going to be wetter it's also likely to be duller than average
22:52too again overall sunshine figures not far off average across the whole of the
22:57UK but again this is just breaking it down a little bit yeah there's a lot of
23:03details in here but yeah thank you angels going full screen on that so you
23:07can pick it up the anomalies there 126 125 that's the percentage of the average
23:16so a quarter more than the average if you like across northern Scotland and
23:21western Scotland in terms of rainfall down at 69 71% for the Midlands and East
23:26Anglia and the sunshine hours again here East Anglia 111% to 10% more than
23:31average whereas western northern Scotland 85% compared to their usual
23:38average so yeah all of that information is available on the Met Office press
23:42release that was out this week one more one to show you on this because I like
23:45the map it's nice it's purple I like this colors on here but it was a windier
23:50than average August as well another reason I want to show you this one of
23:54the reasons why it was windier than average we had a storm the only storm of
23:57the summer months storm Lillian on the 22nd I think it was so yeah almost
24:03right across the board it was slightly windier than average and again with the
24:08wettest and windiest weather generally across Scotland through this month that's
24:11where we've seen the strongest winds but also notice unusually windy across
24:14northern parts of Wales and even southern Wales they're down across the
24:17across the Gower especially windy through this August so lots to pick out
24:24in the stats there just thought I'd show you some of the highlights do like
24:28talking about those as we go through so yeah it has been an interesting month
24:34and an interesting season with that storm through the 22nd some windy spells
24:40and the wettest weather across the north and the west a couple more things I just
24:45wanted to highlight before we finish up climate conversations new video available
24:52on our YouTube channel so check that out it's about sport sport was the
24:56climate theme through last month we've had the euros have had the Olympics
25:00still going on now the Paralympics so how is climate change impacting sport
25:05really interesting episode of climate conversations you can check that out
25:09we'll put the link in the chat as well and just want to highlight something
25:12else as well it's cloud appreciation day a week on Friday Friday this 13th of
25:20September so search that up find out more information about that because yeah
25:24a day where you can look up and just appreciate the clouds good for your
25:28mental health as well as learning a little bit about meteorology so just
25:32wanting to flag that as well as always would love your comments your questions
25:38I will get in the chat about 4 o'clock live answer any questions you've got
25:42there but we do read all the comments as well so if you've got any suggestions
25:45about how we could do this better what you'd like us to cover in future deep
25:49dives then do let us know in there please do hit the like button it does
25:53as a big big favor and share the love let everyone know that we're on here
25:58because not everyone does a couple of shout outs next door you don't get to
26:02see them but angel has been running the show in terms of the cameras and
26:06everything and she's quite new so a big shout out to angel for doing that today
26:09Simon's helping her out in there as well so yeah it's a team effort this Met
26:14Office deep dive it's not just the presenters doing it so yeah big shout
26:19out to them for helping out next door as well and to you for watching do share
26:22the love do hit the like button and it's I don't know if it's Aiden or Annie
26:28doing the 10-day trend tomorrow I should have looked that option I either Aiden
26:32or Annie will be doing the 10-day trends tomorrow so we'll find out a bit more
26:35about the wet weather through the course of the weekend and the outlook into next
26:40week as well so make sure you tune in for that okay I'll stop rambling now
26:43thanks very much for watching see you again soon