Last week, northwestern parts of the UK experienced their warmest weather of the year with highs of 23 to 27 Celsius. This week, temperatures plunge by more than ten degrees as unusually cold air arrives.
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00:00Hello and welcome to the Met Office Deep Dive. I'm Met Office Meteorologist and Presenter Aidan McGiven.
00:06We do these regular Tuesday Deep Dives, in-depth meteorological masterclasses every Tuesday on the Met Office YouTube channel.
00:15If you're not already a subscriber, please do hit subscribe. It will only encourage us to do much more of this sort of thing in the future.
00:23And there's plenty to talk about this week. I'm going to be covering the weather over the next few days.
00:29How cold will it get? Will there be any snow? How unusual is this weather pattern in September?
00:36And how will it change into the weekend and beyond into next week?
00:41Before I cover all of that, there's plenty to talk about in terms of last week's weather.
00:47Hard to believe we're only on day 10 of September. We've had more than a month's worth of weather to talk about so far.
00:53And there's much more to come, so stay tuned.
00:56This time last week, Alex Deakin did an excellent deep dive into cut-off lows.
01:02So if you want to find out more about what cut-off lows are and why a cut-off low was responsible for the extraordinary weather that many parts of the UK experienced over the last week, go back and check that out if you missed it.
01:12But this, I thought I'd start off with the infrared imagery for Saturday.
01:18The satellite image there showing UK in the middle, France. And France, in fact, had this cut-off low.
01:25So this area of low pressure that effectively became detached from the jet stream and left to meander around slowly on its own.
01:32It got stuck to the south of the UK. In other words, with the jet stream going way to the north of the country.
01:39And that low pressure over France pushed an awful lot of rain into southern counties of England, south Wales.
01:46And we saw several days of wet weather. Thursday, Friday, Saturday, Sunday.
01:51There were some breaks at times, but there was a lot of cloudy and wet weather across southern parts of the UK.
01:57Meanwhile, a very different story further north. The Midlands, parts of East Anglia, mid north Wales at times through Friday and Saturday.
02:07Northern England, Scotland, Northern Ireland having largely missed out on summer this year.
02:12Summer finally arrived at the end of last week and parts of Western Scotland, Northern Ireland, North West England experienced their warmest day of the year so far on Friday.
02:21That was, what, the 7th, 8th, 9th of September. Extraordinary warmth with temperatures reaching 24 to 27 Celsius widely across northwestern parts of the UK.
02:33It wasn't so pleasant in the northeast, of course. We had some har and fret at times coming in from the North Sea because we had this easterly wind arriving around the area of low pressure.
02:43Now that imported humid air from the continent, hence those temperatures. A few months later, an easterly wind might result in very different things.
02:52And I'm going to be talking about easterly winds towards the end of this deep dive, if you manage to watch that far.
02:57But yeah, extraordinary weather that stayed static for a few days before at all.
03:02If I fast forward, in fact, I'll press play. Fast forward to Sunday and into Monday, the bands of rain in the south pushed north.
03:10Very wet weather moving north across parts of England, Wales into eastern Scotland before peeling away.
03:15Finally, that cutoff low disappeared into the continent and then we were back to Atlantic driven weather.
03:20And this is the start of today, Tuesday, the 10th of September. And you can see this cloud streak from the Atlantic.
03:30And we go to the radar. Well, this is the last 24 hours of rain. Very different scenes for Scotland.
03:38Outbreaks of rain, heaviest in the west. More typical weather, unfortunately, for western Scotland having returned.
03:44But at the time of recording, this initial band of rain that has been affecting much of Scotland and Northern Ireland later on Monday.
03:51Well, it's been sinking south on Tuesday across northern England into parts of Wales and the Midlands with blustery showers following.
03:59But this, well, this is a cold front and it's sinking south and it's going to drastically change our weather patterns once again.
04:09So, end of last week, unusually warm weather. Later this week, next few days, in fact, unusually cold.
04:17Let me just show you the temperatures, first of all, before we get on to the weather of the next few days.
04:23Now, for the rest of Tuesday, temperatures still managing to reach 19 or 20 Celsius in the southeast.
04:30These are the maximum temperatures for Tuesday. But we're looking at 11 or 12 Celsius in the northeast.
04:36At this point, it must be said, there are weather warnings for the rest of Tuesday across northeastern parts of Scotland.
04:42Shetland seeing some very heavy rain and gusty winds as well for northeastern parts of the mainland of Scotland and Orkney, 50, 60 mph wind gusts.
04:51So, some very turbulent weather in the far northeast. There are weather warnings.
04:54But I'm not focusing on that in this deep dive because by the time many of you have watched it, well, Tuesday will be coming to an end.
05:01And there's more detail on all of that stuff in our short-range forecast on the Met Office YouTube channel, website and app and so on.
05:08But for the rest of Tuesday, as I say, we've got the colder air already starting to arrive into Scotland, Northern Ireland, Northern England, mid-teens or lower.
05:1719 or 20 still possible in the south. But look how the temperatures change by the time we get to Wednesday afternoon.
05:25We're looking at significantly colder air arriving. It doesn't quite show up on this graphic, admittedly.
05:31But there's another interesting thing I want to show you.
05:35First of all, here are the temperatures, the maximum temperatures on Tuesday.
05:39Mid-teens in the south, 11 to 13 further north.
05:43Although in some places, many places actually in the north, away from main urban areas, I suspect, and away from immediate coasts,
05:51temperatures will be staying below 10 degrees throughout Wednesday and perhaps throughout Thursday as well.
05:57And that can be illustrated. We just run this pen across the map.
06:03And this will show you the maximum temperature change across the UK over the next few days.
06:07So somewhere close to Southampton there.
06:11And we've got Tuesday, 18, 19 Celsius or so. And then those temperatures dipping.
06:16Mid-week, 13, 14 Celsius maximum temperatures before recovering into the weekend.
06:22Moving north, though, into the Midlands there, we've got maximum temperatures of 11, 12 Celsius.
06:29Over the Pennines, likewise, 11, 12, 10 Celsius.
06:32But into central parts of Scotland, these are the maximum temperatures, or near enough.
06:399s, 10s, these are the mountains, so 5, 6, 7 Celsius.
06:46So as you can see, Cardiff, we really are going to see this dip, this mid-week nadir in the temperatures.
06:55Depending on where you are, it's going to take place Wednesday, Thursday or Friday.
06:59Likewise, when it comes to minimum temperatures, we'll see a significant dip.
07:08Probably in the south, the lowest temperatures will be Friday morning or Saturday morning.
07:15Moving into mid-Wales there, you can see there's somewhere in mid-Wales 0.6 Celsius on Friday morning.
07:23So it is possible that even in the south, we'll see an air frost and a ground frost on Friday morning and Saturday morning.
07:31I think sheltered areas, prone spots like Benson or Sennybridge, somewhere like that, will likely see an air frost early Friday and early Saturday.
07:42Many other areas, urban areas, more likely low to mid-single figures.
07:47Further north, air frost and ground frost more likely, although those temperatures probably reaching a nadir Thursday morning, Friday morning.
07:55And as you can see, very close to freezing, or if not, just a touch below.
08:00So, unusually cold for the start of September, the first half of September.
08:08Why is it unusually cold?
08:10Well, the short answer is because we've got winds coming all the way from the Arctic.
08:17That's the short answer.
08:18We've got the jet stream pushing low pressure into the North Sea.
08:23And our winds over the next 24 to 48 hours are going to come directly from the Arctic, which is a cold place to get your winds, whatever the time of year.
08:35And as you can see, there's a long fetch of Arctic winds coming in for the middle of this week.
08:41These blue colours replacing the oranges that we experienced at the end of last week.
08:46So that's the short answer. It's unusually cold for the start of September because of northerly winds bringing air from the Arctic.
08:53But there's a slightly longer answer that I'm going to go into because it's the deep dive and that's what we do.
08:59And that involves looking at this particular weather pattern and how unusual this weather pattern is in September.
09:06So this weather pattern, we see low pressure just pushing into Scandinavia in between, say, Shetland there and Norway.
09:13And this area of low pressure and then higher pressure down towards the southwest near the Azores with northerly winds.
09:19That's a weather pattern that is similar to this, which is known as weather pattern number 26 in a climatology of weather patterns across the UK that was done by Rob Neill at the Met Office.
09:39And what Rob Neill and colleagues did was they looked at 154 years of weather patterns over the UK and came up by doing a cluster analysis,
09:50came up with the 30 weather patterns, what they look like and how all the 154 years of data could be basically sorted into 30 types of weather patterns.
10:03So this is weather pattern number 26. It's 26 because when Rob Neill and his colleagues looked at the 30 weather patterns, they ranked them.
10:15Number one was the most common weather pattern that had higher pressure to the southwest and a west-northwestly airflow across the UK.
10:23So that was the most common weather pattern looking at 154 years of data.
10:28Number two had lower pressure towards the west and southwestly airflow, etc, etc.
10:33So by the time you get to pattern number 26, actually, it's relatively infrequent, certainly infrequent during the summer and the autumn,
10:44which I'm going to show you now because actually another paper done by Rob Neill and Jonathan Wilkinson, both at the Met Office.
10:54This is 2021 from the Quarterly Journal of Royal Met Soc Journal, looked at the frequency of those 30 weather patterns for each season
11:06and the chance of thunder associated with each of those weather patterns in each season.
11:12So I appreciate this might be tricky to see, but basically top right here is the summer and the weather patterns are numbered from one to 30 on the bottom.
11:22And what the dotted line shows is the number of days from each of those weather patterns in the summer.
11:29So the most common weather pattern in the summer is pattern number seven. That's 10 days or so.
11:34Pattern number 26, which we've just looked at, that's the one that we're going to experience similar weather patterns to over the next few days.
11:41Very infrequent in the summer, hardly noticeable there on the dotted line.
11:47And winter, it's quite different. Again, the dotted line shows the frequency of these different weather patterns through the winter.
11:55Number 26 is one of the most common or at least maybe even the most common weather pattern during the winter.
12:02Now, the bars here are the number of days of thunder associated with each weather pattern.
12:08And likewise, during the summer, pattern number 26, not many days of thunder at all associated with this weather pattern.
12:15The pattern itself doesn't occur very often, but even when it does, not many days of thunder.
12:20But in the winter, this is the weather pattern that causes the most number of days of thunder.
12:27That's what those bars coming out of the graph show.
12:31So to summarise, basically, the weather pattern we're going to experience over the next few days has been analysed by Wilkinson and Neil in 2021.
12:40And it looks like it's very infrequent in the summer, doesn't happen very often, doesn't produce much thunder in the summer because of its infrequent nature.
12:48In the winter, it's much, much more frequent, much more regular effect in the UK, and you get more days of thunder as a result.
12:56In fact, the most number of days of thunder come from this weather pattern in the winter compared with all others.
13:01What about the autumn? It's kind of in between and we're right at the start of autumn.
13:07So I'd say it's more like the summer patterns at the moment.
13:12So, yeah, it's unusual to get a pattern like this in the summer, unusual to get thunder as a result of this pattern because the pattern doesn't occur very often at all.
13:22But in the winter it does occur, and these maps show where the thunder and lightning happen from various different weather patterns at different times a year.
13:30These patterns on the top are from summer months, and the darker colours on the maps show that you get a lot of lightning from certain weather patterns across central and eastern parts of England, for example, eastern Scotland, particularly the middle one there on top, so East Anglia and the Midlands.
13:46So that's a weather pattern in the summer that produces a lot of lightning across the Midlands, East Anglia and the southeast.
13:51A lot of people have been asking lately whether the number of days of thunderstorms has been reducing in recent decades because of global warming, and that's definitely something that we're going to look into in a future deep dive, but not this one.
14:05I want to focus on this one, is this middle map in the bottom, that's how the weather pattern that we're currently experiencing over the next couple of days in the winter produces thunder and lightning, and where that thunder and lightning happens.
14:20Basically it happens in the north and west of Scotland, parts of Northern Ireland and northwest England.
14:25And finally, before I explain why you're more likely to see thunder and lightning from this weather pattern in the winter in the north and northwest, this shows the distribution of thunder and lightning for each season from all weather patterns.
14:42You've got spring on the far left, so the most frequent lightning is across London, the home counties, the Midlands, southern parts, much less across central and northern Scotland.
14:53A bit more of an extreme version of that in the summer, a lot more lightning in the east and southeast of England compared with the northwest of Scotland.
15:02Autumn, so that's the season we're heading into, you get a fair bit of lightning through the English Channel, warm seas of course, and still some lightning in the south but increasingly western Scotland.
15:13Then by the time we get to winter, it's the north and west of Scotland, northern and western Northern Ireland, parts of west Wales that see more lightning than other parts of the UK, so it completely flips.
15:23Summer, more lightning in the southeast, winter, more lightning in the northwest, and it's more lightning in the northwest that comes as a result of weather patterns like the one we're experiencing in the next few days.
15:36So why are we more likely to get lightning in the northwest from this kind of weather pattern?
15:41Before I mention that, I just want to explain why we're seeing this weather pattern in the first place, given that I just mentioned that it's so unusual.
15:51Let's turn that off and put the jet stream on.
15:54The reason is this enormous area of high pressure over north-west Asia, north-west Russia, and that's basically blocking weather systems from progressing from the Atlantic, so it's sitting there stubbornly and it's acting as a block.
16:15What we're seeing over the next few days is the jet stream coming out of the Atlantic.
16:21It's wanting to push these areas of low pressure, these weather fronts through, and it would succeed.
16:28It would move these things through.
16:29We'd be in a changeable westerly sort of regime if it weren't for this area of high pressure.
16:35That's acting as a block, and so watch what happens.
16:38The jet stream comes along, tries to move its way eastwards.
16:42Everything gets stuck.
16:44Low pressure gets stuck over Scandinavia, and the jet stream bunches up.
16:48It becomes more amplified, so you get it pushing north over this area of high pressure in the Atlantic and then diving south over the UK by Thursday.
16:58So everything gets bunched up.
17:00You get a tighter gradient between high pressure and low pressure because everything's bunching up.
17:05You get a much stronger northerly flow as a result, and with the jet stream diving to the south of the UK, we're under what's known as a trough,
17:14and that means low pressure in the upper atmosphere as well as low pressure in the lower atmosphere, so all this rising air as a result.
17:23Not only have you got low pressure and low pressure, rising air, you've got a significant temperature contrast.
17:30So the air coming from the Arctic is moving over much warmer seas, of course, and because you've got colder air above, warm seas below.
17:39All of those factors combine, and you get an awful lot of convection, so you get rising air, showers, and thunderstorms.
17:46That's why this kind of weather pattern, you end up seeing a lot of convection, cumulus and cumulonimbus clouds,
17:55and as a result, those areas that are exposed to the north and north-westerly wind over the next few days will see the most frequent and liveliest downpours,
18:06and that's why it's northern Western Scotland, north-west England, northern Ireland, parts of Wales,
18:11where we'll see those lively showers moving through, cumulus and cumulonimbus clouds.
18:17The risk of lightning is still there over the next few days, but it's not as high as you might get in the winter
18:24with the same kind of weather pattern, those temperature differences aren't quite as stark,
18:28and that upper trough does move along, so it's not going to produce that instability for too long.
18:35It does move along through Thursday and into Friday, so things do stabilise.
18:39But as you can see, this is Wednesday, a lot of showers across the map.
18:44It's a day of sunny spells and showers basically, bright skies, but one minute it's sunny, next minute a downpour,
18:51and the showers will be most frequent across the north and west of Scotland, northern Ireland.
18:55They'll be funnelling through what we call the Cheshire Gap, so this part of north-west England into the Midlands, affecting Wales as well.
19:02North Devon, North Cornwall, perhaps fewer showers there along the south coast because of a bit more shelter,
19:08and really it's those areas that have the wind coming in from the sea that will get the most frequent showers.
19:14Similar again on Thursday. Thursday's another very showery day, as you can see, but it's starting to stabilise,
19:24so perhaps the showers are becoming fewer.
19:27And then by Friday, a cold start because we've got that cold air in place, we've got clear skies overnight,
19:32that's why it's one of the coldest mornings.
19:35But by Friday, that upper trough is moving away, and we've got a ridge of higher pressure starting to move in from the west.
19:45So, yeah, it's looking really quite unusually cold over the next few days.
19:51And one final point on how unusual it is, take a look at the temperature trend for Edinburgh.
19:57I've chosen Edinburgh because, well, most places in the UK exhibit a similar temperature trend over the next few days,
20:03but my brother's in Edinburgh at the moment over the next few days.
20:06He's come all the way from Thailand, so he's not used to cold weather,
20:09and of course he's going to experience unusually cold weather in Edinburgh for the time of year.
20:15It wouldn't be unusual in the winter, this kind of weather pattern and these kinds of temperatures,
20:19but unusual for the first half of September.
20:22So you could say he's unlucky, but of course he's lucky for visiting such a beautiful city,
20:26one of my favourite cities, and he's lucky to have such a great family as well.
20:30So this is the temperature trend for Edinburgh.
20:32The blue boxes show the forecast temperature for the overnight period out to the next couple of weeks.
20:38The red boxes show the temperature forecast for the daytime.
20:43And it's around the middle of this week that the blue boxes dip down to this blue line
20:48that you might just be able to make out on the chart there.
20:51That's what we call the extreme in climatology.
20:55So this is from the ECMWF computer, and ECMWF have looked back at decades of weather patterns,
21:06and this blue line represents the extremities, what you'd look back and say,
21:13well, that was a fairly extreme number for the overnight value for September for this time of year.
21:20Another way of looking at that is to look at this kind of map,
21:24and this shows how extreme the temperature is on Thursday across Europe, by the way, compared with climatology.
21:34So where you've got, here's the UK, where you've got these darker blues,
21:38we've really not seen these kinds of temperatures very often,
21:41maybe the top 5% or the coldest 5% of occasions in the last 30 years or so.
21:47Likewise, these dark reds across parts of Finland into northwest Russia show that it's unusually warm here.
21:55So what you often get in this kind of situation, where you get this buckling of the jet stream and the amplified airflow,
22:02is you get northerly winds in one place, but then they have to come up and act as southerly winds in another place.
22:08This seesaw effect happens, and on balance, of course, they average each other out
22:13and don't have a big impact on the global climate.
22:15But it just goes to show where you've got very cold weather, unusually cold weather in one place,
22:19you normally have unusually warm weather in another place.
22:22So that's something interesting to consider.
22:24Anyway, let's go back to this, because you might be interested in what happens beyond Thursday and Friday.
22:34And it's all change again.
22:36Here's Thursday. As I mentioned, that upper trough moves away.
22:41And then finally, it stops bunching up.
22:45The pattern becomes unblocked and weather patterns start to move through.
22:50More specifically, the Atlantic jet stream starts to move back into more of a climatological pattern.
22:56So a pattern that we'd more typically expect at this time of year for the UK.
23:01And that is that it will lie just across northwestern Scotland heading into the weekend
23:07and bring this area of low pressure between Iceland and Scotland and this weather front.
23:12So that means it's a weekend of contrasts once again, but very different contrasts compared with last weekend.
23:21And the biggest rainfall totals, let's see, 12-hour rainfall totals here.
23:30The biggest rainfall totals through the first, well, through the daytime on Saturday, likely to be western Scotland.
23:38And you can see that big contrast between western Scotland, Northern Ireland, northwest England, north Wales and the rest of the UK
23:45as those Atlantic systems once again return to northwestern parts of the country.
23:50We're talking about, in a 12-hour period, 25 millimetres or more.
23:54The rain focused across higher parts of Scotland.
23:59If we look at 24-hour totals, this is between 6pm Friday up to 6pm Saturday.
24:06And you can see widely 10-25 millimetres.
24:10Some parts of western Scotland could see 40-60 millimetres of rainfall, most likely western hills.
24:16But in the same period, no rain at all across much of Wales, central and eastern England and the south.
24:24So a stark contrast this weekend compared with the weekend that's just been.
24:31That's Friday night and then you can see that rain coming in.
24:35Really it's raining from the word go across the northwest of the UK on Saturday.
24:41And it continues to rain, unfortunately, if you live in those areas and you're fed up with the rain that we saw so much of through the summer.
24:49We're back to that kind of weather pattern.
24:51Outbreaks of rain for Scotland, Northern Ireland, northwest England, north Wales.
24:54We put the cloud on and you can see a very different day for the south and southeast.
24:58A hazy sunshine. There'll be some high cloud out there but otherwise mostly fine.
25:02Also risk of gales in the northwest, by the way, in northwestern parts of Scotland.
25:06And then later in the weekend that front sinks south.
25:12But a bit of uncertainty about how far it sinks south and how active it will be.
25:17It may just fizzle out and tend to disappear because what happens next is that it's all changed once again.
25:28This is the most likely weather pattern for, and this is regime 3 out of 30.
25:33So, as I said at the beginning, these are ordered in terms of how common they are across the UK.
25:37So this is one of the more common weather patterns, as I mentioned.
25:40Higher pressure to the south and then outbreaks of rain coming into the west.
25:44And these numbers here show the temperature anomalies expected.
25:48So a degree or two above average and it will certainly be feeling warmer in the south.
25:52So that's the weekend's most likely weather pattern.
25:55But there are increasing signals that that high in the south will develop more widely as we go into next week.
26:01It's not going to be completely plain sailing.
26:03There'll still be some rain around, particularly during the transition period.
26:06But this is the most likely weather pattern by the time we get to the end of next week.
26:11And it's got higher pressure over Scandinavia.
26:14And this has been a signal in the modelling for the last few days or so.
26:18So it's looking reasonably confident. You can never say for sure at this range.
26:22Higher pressure over Scandinavia and an easterly wind with temperatures on this below average.
26:29This is what this would suggest, although there's some disagreement here.
26:32Because the European projection, the European model, ECMWF, also shows higher pressure to the north of the UK as the most likely weather pattern.
26:44This is for the average of Monday to Monday of the following week.
26:48But it also shows slightly warmer than average weather across the UK.
26:54And I suspect that's more likely going to be the case because we'll see a bit of sunshine.
26:59It will also be becoming increasingly dry.
27:03So drier than average for the UK and for Western Europe.
27:07And this time of year, there's no particularly cold air to the east to bring to the UK to lead to those negative temperature anomalies.
27:16So with less rain, better sunshine, it possibly will be around average or slightly above.
27:23Certainly warmer compared with this week.
27:25But those people who love winter weather might be complaining that we've got the northerly winds this week and the easterly winds next week.
27:36And why can't they happen in January?
27:38Because if they did, we would be seeing very different consequences as a result.
27:44Who knows? Maybe that will happen this winter.
27:47Whatever happens, we'll keep you informed here at the Deep Dive.
27:49Hope you enjoyed this one. If you did, tell your friends about it.
27:52And we'll be back again next week with another in-depth look at the UK's weather.
27:56Bye bye.