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
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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 and more
03:31into Thursday. 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
04:44get most of our meteorological definitions honestly but I thought this
04:46was interesting 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
05:09models 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 we can see this 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
09:16with bear with this is what I wanted to show let's get a pop as bars because
09:24this is quite funky if we look at 24-hour totals accumulations across the
09:31south actually let's do 12-hour accumulations so this is a rainfall bars
09:35building up over 12-hour period you can see that heavy rain likely across the
09:38southeast on Thursday but if we go through the system over the next few
09:42days just look at how those total plenty across France as well but the rainfall
09:46totals really building up across the south 12-hour accumulations those spikes
09:51in there now the detail going to be hard to pin down because of that
09:55complication with the cutoff low and it's tricky for the for the models to
09:59handle but the general signal that across parts of the Midlands South Wales
10:03Southern England there is going to be a series of pulses of rain coming in
10:07between Thursday and Sunday so some heavy downpours are possible we've got
10:12that warning out for Wednesday night and into Thursday but even after that we are
10:17going to see further pulses of heavy rain when we know more details will be
10:21firming up on exactly where and when those pulses of rain are like to hit
10:25but generally across the south it does look pretty wet as we go through this
10:29weekend into the early part of next week there are some signs it should be
10:33drier in the south early next week but there's quite a bit of uncertainty tied
10:37into that it may also turn a little cooler because as well as turning wetter
10:41across the south again it's also actually going to turn a little warmer
10:44because if we look at the temperature profiles let's take the pressure off we
10:51don't need the jet stream anymore so it has turned a bit fresher over the past
10:55couple of days quite warm and humid over the weekend but now we've got the cooler
10:57air coming in from the Atlantic but if we go through the next few days notice
11:02how the warmth comes back because the lows in here and it's pushing the
11:05warmer weather back up from the south and east back across the UK so it is
11:09going to turn warmer and more humid once more over the next couple of days but of
11:12course in the south you're not really going to feel that warmth it'll be humid
11:16but you're not going to if you haven't got the sunshine you won't be we won't
11:19be feeling the benefit however further north you will potentially feel the
11:24warmth and the sunshine let's put the pressure pattern back on because as we
11:31talk about what's going on down here the cutoff low high up in the sky got
11:35the low pressure system at the surface you've got the wet weather across the
11:37south but further north actually it's this high pressure that's influencing
11:42things a little bit yes we've got an easterly wind so this is the wind at the
11:46surface coming in from the northeast but the winds coming in here but actually
11:51for much of the northern half of the UK under the influence of the high pressure
11:58we are going to see plenty of fine and bright weather so Scotland Northern
12:03Ireland particularly northwest England going to see some sunshine over the next
12:07few days little will change on Sunday there's a weak front trying to push in
12:10but it's all the rain is going to be across the south and actually for much
12:13of northern Britain the back end of this week looks pretty good dry and fine high
12:17pressure sinking air plenty of sunshine there is going to be easterly winds so
12:23let's put those put the cloud on because that is going to generate some windier
12:28conditions with those easterly winds it could be a bit claggy around some
12:32eastern coasts but with an easterly or northeasterly wind it's Northern Ireland
12:37Western Scotland Northwest England are going to see the lion's share of the
12:40sunshine so if you've got any plans that Saturday at the moment again detail of
12:46where the rain is a little uncertain but the general weather pattern is set so we
12:50can be 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 have to
13:33have three days over that threshold which across Scotland is 25 certainly
13:38possible Friday and Saturday whether we get three days or not a bit of a
13:42question mark about that but certainly in this part of the world temperatures
13:45are going to be above average significantly so during Friday so again
13:54let's zoom in it will be warm in the south the air is going to be warm but
13:56remember here we're not sure about where the rain is going to be so we can't rely
13:59too much on that because it could be quite cloudy in the south depending on
14:03where that rain is going to be but certainly across Western Scotland and
14:06parts of Northern Ireland temperatures like to be significantly above average
14:09and as I said in Western Scotland especially we could get over 25 degrees
14:14which would put us towards heatwave criteria so that could be quite
14:16interesting in Scotland that's Friday this is now Saturday so again
14:21temperatures significantly above average in this part of the world thanks to the
14:25fern effect now we've talked a little bit about the fern effect before but it's
14:30going to go through it one more time on here in case you missed it earlier let's
14:34draw I'm going to bring my drawing skills back imagine that is a mountain
14:42in Scotland we're a cross-section here so this is a cross-section of a Scottish
14:47mountain the winds remember are coming in from the east so we've got winds
14:56coming in this way going up the mountain as they go up towards the top of the
15:01mountain obviously it gets a little cooler and down the other side it gets
15:07warmer but it's significantly warmer on this side and there's a number of things
15:12going on as the air is coming in this way rising up there's a pretty cloud so
15:22the air rises up the mountain it cools it condenses it forms cloud and oh let's
15:28make the rain blue it rains so the air on this side is drier than the air on
15:35this side so the air coming in rises it rains it forms cloud the air coming down
15:40this side is drier now drier air as it descends warms up faster than wet air on
15:47this side so that's one reason why it's warmer on this side the other reason is
15:51that actually on this side because you've lost a lot of the cloud oh look
15:57at that's a thing of beauty it's often sunnier on this side of the mountain as
16:01well so that also heats things up another thing you're doing is blocking
16:04the air here so you're actually drawing air down from higher up and as the air
16:10gets pulled down it gets compressed as well and that also helps to warm things
16:15up because the pressure is higher at the bottom and you get the turbulent mixing
16:19as well so there's kind of four factors going on here that means warmer
16:23conditions on the lee side of the mountains compared to the windward side
16:30of that is the fern effect there's more on that on the Met Office website if
16:33you're interested in that but that is essentially what the fern effect is and
16:37it means that to the lee of mountains it's often warmer and particularly if
16:42you have just the right conditions the air is just in the right state just
16:45stable enough enough of a breeze coming in then you can get really big jumps in
16:49temperatures happens quite a lot in the Rockies where of course the mountains
16:53are that much bigger but it happens pretty frequently across Scotland as
16:56well and that is what we're likely to see later this week with those
17:00temperatures rising in western parts of Scotland because the winds are going to
17:05be coming in from the east or the northeast so the east coast will be
17:08quite cool particularly if it stays cloudy but in the west you can have that
17:13fern effect lifting the temperatures say potentially into the mid-twenties across
17:17western Scotland later this week and boy could Scotland do with a bit of warm and
17:23sunny weather because in western Scotland in particular you have had a
17:27pretty grim summer time now for a look at some of those summer stats that we
17:32talked about at the start these are the headlines we issued a press release
17:36earlier this week summing up summer and August weather it was cooler than
17:42average coolest summer since 2015 so the coolest summer for nine years overall
17:49rainfall was pretty close to average slightly below think about ninety five
17:54percent of the average but this is the key huge regional variation I say
17:59western Scotland as we will see took the brunt of the wet weather over the summer
18:03and again sunshine amounts close to average but with some significant
18:08regional variations as we will also look at shortly but let's deal with
18:13temperatures first of all that's the main headline I guess this is the map
18:20showing the temperature anomaly so different from the average the average
18:24and this is the long-term 1991 to 2020 average so compared to the most recent
18:3230-year climatological period white is pretty much close to average blue is
18:37cooler than average you can see that western Scotland the west of Northern
18:40Ireland parts of southwest Scotland were cooler than average many of the places
18:44were pretty close to average so although it was slightly cooler than that
18:49average from 91 to 2020 it was only generally a little bit cooler than
18:55average and interestingly if you've taken the average from 1961 to 1990 then
19:00the summer just gone would have been above average because our summers are
19:04getting warmer there's been a bit of chatter around this on social media
19:07people not really maybe grasping climate and how averages and graphs work but this
19:15graph shows quite nicely what has been going on so this is the variation we
19:21know that every summer is different right you're going to get variations up
19:24and down so this is a plot of the summers over the past century or more in
19:30fact and how they have varied this pink line here is the average for the most
19:37recent 20-year period from 1991 to 2020 that's what we're using our climate
19:43average as at the moment you can see that this one here that's the latest
19:46summer the summer just gone which is slightly below that average but most of
19:50the recent ones have been above and for in the past 20 years have been right at
19:55the top there in the most extreme go back over time and you can see that
19:59trend this black dotted line that is plotting the averages waving around for
20:04the first half of last century but a significant and definite trend there in
20:08the past 50 years that our summers are getting warmer 76 there also stands out
20:16but again within that variability the cooler ones are down here notice the
20:20cooler ones now I was just below that average line but there's way more of
20:25them up here so we still got that trend so one summer that's slightly cooler
20:30than the most recent climate average does not mean climate change isn't
20:34happening far from it that summer this summer just gone if you'd taken it by
20:38the average of 1960 to 1990 would have been above average so that's how we know
20:44that our climate is warming you're still going to get variations that's normal
20:48but the overall trend that's climate change you can see it quite clearly in
20:53this graph so yes this summer was slightly cooler than average but recent
20:58summers have been much much hotter than average as well okay that's enough on
21:03temperatures let's look at rainfall now because as I promised there's been some
21:08big big variations with across the country with our rainfall this is summer
21:14and this is August so I just want to put next to each other because actually
21:17August just is a is a continuation really of what we've seen through through
21:22much of the summer certainly July was very similar so August kind of mirroring
21:27what we saw throughout the entire summer just a reminder summer is June July and
21:31August the whole months that's the way we do summer but yeah highlighting there
21:36that parts of Western Scotland significantly wetter than average
21:39whereas actually quite a chunk of England was quite a bit drier than
21:44average over the whole of summer some places a little pockets of white here
21:49and there which is close to average but a good chunk of England was was drier
21:53than average throughout the whole summer and eastern areas in particular quite a
21:56bit drier through August East Anglia Lincolnshire parts of Yorkshire whereas
22:02Western Scotland again really bearing the brunt that's that dark blue there is
22:05200% so twice the August average rainfall falling across quite a chunk of
22:11Western Scotland so yeah you have had a very wet August and quite a wet summer
22:15in general in western parts of Scotland and the sunshine maps as you might
22:20imagine kind of show the same pattern with these western areas the greys
22:25duller than average the yellows sunnier than average little pocket there across
22:31parts of East Anglia Whiz Beach I was there not long ago significantly sunnier
22:37than average throughout the summer and yeah reflected also in August sunnier
22:43than average here northeast England eastern parts of Scotland Western
22:48Scotland as you expect it's going to be wetter it's also likely to be duller
22:51than average too again overall sunshine figures not far off average across the
22:57whole of the UK but again this is just breaking it down a little bit yeah
23:03there's a lot of details in here but yeah thank you
23:05angels going full screen on that so you can pick it up the anomalies there 126
23:12125 that's the percentage of the average so a quarter more than the average if
23:19you like across northern Scotland and Western Scotland in terms of rainfall
23:23down at 69 71 percent for the Midlands and East Anglia and the sunshine hours
23:28again here East Anglia 111 percent to 10 percent more than average whereas
23:33Western Northern Scotland 85 percent compared to their usual average so yeah
23:39all of that information is available on the Met Office press release that was
23:43out this week one more I'm going to show you on this because I like the map it's
23:46nice it's purple I like this colors on here but it was a windier than average
23:51August as well another reason I wanted to show you this one of the reasons why
23:54it was windier than average we had a storm the only storm of the summer
23:58months storm Lillian on the 22nd I think it was so yeah almost right across the
24:04board it was slightly windier than average and again with the wettest and
24:08windiest weather generally across Scotland through this month that's where
24:11we've seen the strongest winds but also notice unusually windy across northern
24:15parts of Wales and even southern Wales there down across the across the Gower
24:19especially windy through this August so lots to pick out in the stats there just
24:25thought I'd show you some of the highlights do like talking about those
24:29as we go through so yeah it has been an interesting month and an interesting
24:35season with that storm through the 22nd some windy spells and the wettest
24:41weather across the north and the west a couple more things I just wanted to
24:45highlights before we finish up climate conversations new video available on our
24:53YouTube channel so check that out it's about sport sport was the climate theme
24:57through last month we've had the euros have had the Olympics still going on now
25:01the Paralympics so how is climate change impacting sport really interesting
25:06episode of climate conversations you can check that out we'll put the link in the
25:10chat as well and just want to highlight something else as well it's cloud
25:14appreciation day a week on Friday Friday this 13th of September so search
25:21that up find out more information about that because yeah a day where you can
25:26look up and just appreciate the clouds good for your mental health as well as
25:30learning a little bit about meteorology so just wanted to flag that as well as
25:35always would love your comments your questions I will get in the chat about
25:40four o'clock live answer any questions you've got there but we do read all the
25:44comments as well so if you've got any suggestions about how we can do this
25:46better what you'd like us to cover in future deep dives then do let us know in
25:51there please do hit the like button it does as a big big favor and share the
25:55love let everyone know that we're on here because not everyone does a couple
26:00of shout outs next door you don't get to see them but Angel has been running the
26:05show in terms of the cameras and everything and she's quite new so a big
26:08shout out to Angel for doing that today and Simon's helping her out in there as
26:11well so yeah it's a team effort this Met Office deep dive it's not just the
26:16presenters doing it so yeah big shout out to them for helping out next door as
26:21well and to you for watching do share the love do hit the like button and it's
26:25ah I don't know if it's Aiden or Annie doing the 10-day trend tomorrow I should
26:30have looked that option either Aiden or Annie will be doing the 10-day trends
26:34tomorrow so we'll find out a bit more about the wet weather through the course
26:37of the weekend and the outlook into next week as well so make sure you tune
26:41in for that okay I'll stop rambling now thanks very much for watching see you
26:45again soon