This is an in-depth Met Office UK Weather forecast for the next week and beyond. After a dramatic few days of weather, we take an in-depth look at supercell thunderstorms and tornadoes. Bringing you this deep dive is Met Office meteorologist Aidan McGivern.
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00:00Hello and welcome to the Met Office Deep Dive, our weekly in-depth look at all
00:05matters meteorological and there are plenty of meteorological matters to look
00:08at this week. Supercell, thunderstorms, tornadoes, hail, 13,000 lightning strikes,
00:15two months worth of rain in just a few days and that's just looking at last
00:19week's weather. What's to come in the next week? Whatever happens it's
00:23certainly going to keep us interested I'm sure. Now before I get on with it
00:28just a regular reminder to hit subscribe if you haven't already done so. These
00:32deep dives are exclusive to our YouTube channel so if you love watching them
00:35make sure you hit subscribe, hit like, send us a comment and that way it will
00:40just encourage us to do more of this sort of thing in the future. And don't
00:45forget to tell your friends as well if you've got any friends who love talking
00:48or hearing about the weather then let them know that this exists. But yeah it
00:54was certainly a crazy few days of weather through the weekend and into the
00:59start of this week. Not for everyone of course. Scotland, Northern Ireland, parts
01:04of Northern England had a quieter time and I'll talk more about northern parts
01:10of the UK in a few minutes but I wanted to cover the really interesting weather
01:15that occurred across southern parts of the UK. First of all, lightning. Lots of
01:21lightning. More than we saw throughout much of the summer. This one was a
01:26lightning strike in Staffordshire. Staffordshire saw an awful lot of
01:30lightning on Saturday. This is from Saturday evening. But of course we saw an
01:36awful lot of rainfall which sadly led to submerged roads and cars, led to property
01:43flooding and transport disruption. This, the A421 in Bedfordshire, quite a
01:50scary scene there with that main road underwater. More on the rainfall totals
01:56in a moment. But a tornado as well in Aldershot on Friday. This was the damage
02:03from the tornado as reported by BBC South Weather. Well done BBC South
02:08Weather for not calling it a mini tornado. That's great. They're not mini
02:12tornadoes just because they happen in the UK. They're all tornadoes. No such
02:17thing as a mini tornado as some media outlets like to talk about. But yeah, some
02:23devastating wind gusts from that tornado there and it was confirmed as a tornado
02:28by Toro which is the official tornado research organisation in the UK. So head
02:34to their website if you want to learn more about tornadoes and look at the
02:37more recent tornadoes that we've seen. There was a tornado, not sure if it's
02:41been officially confirmed or not, but it looked like it was a tornado in Luton as
02:46well on Sunday. But I'm actually going to focus on the tornado in Aldershot
02:52which happened on Friday because it's a really interesting imagery from that.
02:57Let's go to the lightning map and let's zoom in a little bit. I'll just pause it
03:03there. Okay, where are we up to? This is 7 a.m. on Friday. So up to this point, no
03:10rain, no thunderstorms across the UK. We just had four days of virtually entirely
03:16dry weather across the UK, warm sunshine and so on. But Friday's the day things
03:22started to destabilise. What we saw over the near continent was something known
03:26as a cold pool, an upper area of low pressure and much colder air way up high
03:31near where the jet stream lies. And of course where you've got this build-up of
03:35warmth and humidity across southern parts of the UK at lower levels which we
03:40saw through last week. Build-up of warmth and humidity and then this upper area of
03:45low pressure causing the air to rise and much colder air way up. Well that leads
03:50to deep instability where the air is cooling off very quickly as you go
03:56higher in the sky and what that means is that anything that gets warmed up at the
04:01surface rises quickly because it's, you know, as it's rising always warmer than
04:06its surroundings and as long as it's warmer than its surroundings it continues
04:10to rise just like a hot air balloon. So what you've effectively got is all this
04:13warmth and humidity at the surface, upper cold pool comes along from the south
04:19destabilising the air and boom, you get thunderstorms. Not just that but you get
04:24something known as wind shear and that's what we saw on Friday and that's a key
04:30ingredient for some really lively storms. Now let me explain a little bit about
04:36wind shear and why it's important for thunderstorms. Imagine you've got that
04:40rising air, the updrafts, and you've got the difference between the warm and
04:44humid air at the surface and the colder air above but you've got no wind shear.
04:49Wind shear is basically the change of the wind with height. Either it's changing
04:54direction as you go higher in the atmosphere or it's changing speed. One or
04:59the other or both. If you've got no wind shear then you've got an updraft forming,
05:03the air rises, it cools, it condenses, you get a cumulonimbus cloud forming, then
05:09it starts raining. As the rain falls it pulls colder air down with it, known as a
05:15downdraft, and that, if there's no wind shear, will basically fall on top of the
05:21updraft and kill it off and subsequently kill off the thunderstorm. So single cell
05:26thunderstorms, that's what these are called, when there's little or no wind
05:29shear, they tend to not last that long. Short, sharp, shower, rumble or two of
05:36thunder possibly, then it's all over in around an hour or less sometimes. This is
05:42often known as popcorn convection as well. These little popcorn showers can
05:46pop up here and there, they don't last long and they don't cause too much
05:51concern apart from a bit of a drenching if you're caught under one. But they will
05:57occur when there's little or no change in the wind with height. Either the winds
06:01are light through the atmosphere or they're all travelling in the same direction
06:04as you go higher up with the same sort of speed. So the downdraft falls on top of
06:10the updraft basically. Now if there is wind shear, let's say the wind is
06:15strengthening as you go higher up in the atmosphere, then the updraft will tilt a
06:20little bit and so the rising air will move to the west, for example, faster at
06:2810,000 feet, 20,000 feet compared with the surface. So the whole thing's tilted.
06:33So the updraft moves up in a diagonal line, then the downdraft falls but it
06:38doesn't fall back in that same diagonal line so it doesn't cut off the updraft.
06:43What it does do is it hits the ground and the colder air that's coming down
06:48from above spreads out as it hits the surface and this, known as a gust front,
06:53can then travel along and meet a wind coming in a different direction,
06:58slightly detached from the original cell of the thunderstorm. And as those winds
07:04collide they can form another updraft and this new updraft causes a new cell
07:11and a new downpour and rumble of thunder or something and a new diagonal updraft
07:18a new downdraft, a new gust front and then subsequently meets another wind
07:24coming in a different direction, another updraft and so on. Meanwhile the
07:28original updraft and downdraft have sort of dissipated and you've got what's
07:34known as a multi-cell thunderstorms where each storm leads to another storm
07:41cell in a slightly different place and then another storm cell in a different
07:45place and so on. These storm cells keep popping up maybe in a line or something.
07:51That's known as a multi-cell storm so they last longer, they can last for a few
07:55hours and they can be a bit more vigorous because they don't die off so quickly.
08:01Now a supercell thunderstorm is similar in that the updraft is detached from the
08:07downdraft but the difference is that you've perhaps got more wind shear,
08:13you've maybe got some directional wind shear as well as the wind changing speed
08:18with height and you've got a very vigorous updraft, a very fast updraft because
08:23you've got this deep instability and as a result not only is the updraft going
08:30in a diagonal line as you go up through the atmosphere but it can start to rotate
08:34and that rotating updraft can become a tornado and that's what we saw in Aldershot
08:42on Friday. So I just want to play the radar and the, well actually it's just the
08:48lightning for now. Now this is 9am, you see London there, this is Kent and this
08:54is 9am, lightning starting to occur. Very little actually, let's put the rainfall
09:00radar on, very little rain elsewhere, this small cluster of thunderstorms that just
09:05started, there's Aldershot. I'm just going to play it forward and it will pause
09:11at exactly the time that the tornado was reported, 12.25 on Friday, so there we go
09:18it's paused. I'll zoom right in and what you can see, there's Aldershot and the
09:25heaviest rain, let's zoom in further actually, turn off the lightning, the heaviest
09:32rain has already gone through Aldershot when the tornado hit, so these brighter
09:38colours here, that's where the heaviest rain is, just to the north and the west
09:43of Aldershot and the storm is moving in that direction. Now this is a supercell
09:49thunderstorm, we know this because it produces a tornado but not just that, it's
09:54the shape of the system as well and if we look at the satellite image,
10:00fascinating, this is what we call an anvil of high cloud that's extended out to the
10:06west and so what we're seeing here is the upper winds are going faster than the
10:10surface winds, so you've got that wind shear and the upper winds are blowing this
10:14high cloud further west than the rainfall radar is producing rain at lower levels
10:24and also further west than the actual surface winds are blowing. So we've got
10:31this wind shear and also if I draw some arrows on here you can see the structure
10:36from above, the typical structure of a supercell thunderstorm, so here you've
10:41got the most intense rain, that's where the downdraft is happening, so you've got
10:45a downdraft that's spreading out from this really heavy area of rain and
10:52potentially hail as well, bear in mind that rain has just gone through Aldershot,
11:00you've also got, that's called the forward flank downdraft, I didn't have to
11:07read that off the thing, and then you've got this rear flank downdraft here,
11:16so another downdraft area here but that's the main downdraft coming out of there,
11:21then you've got this warm moist inflow coming into here,
11:28that's the updraft as it comes in feeding into the rear side of the supercell thunderstorm
11:37and the whole thing rotates and just in there you've got this rotating updraft and
11:46so that's where the tornado happened, just there, Aldershot 1225.
11:52Now some of you might have watched the movie Twister or indeed Twisters and
11:58you'll know that you don't want to, if you're storm chasing, approach a supercell
12:03thunderstorm from the wrong direction, if you were to approach it from here you've
12:07got to go through the really heavy rain before you get to the tornado, you're not
12:11going to see the tornado, that's a really dangerous thing to do, you go through
12:15that, you get the really heavy rain, low visibility, then you're hit by the
12:18tornado, so storm chasers tend to come in this side, you've got the better visibility,
12:23you can see the tornado on the rear side of the storm as it's moving that way
12:30and yeah, that's a much safer way to chase storms.
12:34Additionally, down here you've got the gust front and that can also be
12:42an area of formation for further thunderstorms and that's actually what we saw,
12:47if I run the sequence further forward a little bit, oh, gone a bit too quickly,
12:54and put the lightning back on, just in this area here it developed into further
13:00thunderstorms that went through the southwest of England, mostly Somerset,
13:06parts of Devon as well, later in the day, this is 4pm on Friday, but this area of
13:11thunderstorms was really lively, going through Gloucestershire and Gloucestershire
13:16itself, the county of Gloucestershire saw 2,700 lightning strikes on Friday
13:22alone and you can see how many of those lightning strikes there were, so this
13:25supercell thunderstorm, it sort of split into two, we had all these lightning
13:31strikes go up to the northwest and then this other cluster, these new
13:34thunderstorms that developed just on the southern flank of it as it moved west,
13:39so a very, very lively day there, and then just playing it through for the
13:44rest of the weekend, those arrows are going to stay on there for a moment,
13:51we saw more heavy downpours and thunderstorms developing through Saturday
13:56and again some very, very big storms that went through the Midlands in
14:01particular, Staffordshire had I believe 1,700 thunderstorms on Saturday itself,
14:10just double check the number on that so I get it right,
14:171,752 and for the UK as a whole, mostly it was the south and central parts of
14:23England on Friday, the UK as a whole, 6,432 strikes of lightning and on
14:30Saturday, 6,589 strikes of lightning, they were more centred on Saturday,
14:36across the Midlands they were further south on Friday, further thunderstorms
14:41happened of course on Sunday, not quite as many lightning strikes on Sunday but
14:47some very heavy rain and we've got Roatwoburn on the graphic here and they
14:52saw some extremely heavy rain, in just a few hours they had their September
14:58average rainfall and then throughout Sunday and into the start of Monday
15:03they'd had a huge amount of rainfall for just a day and a half, I'm just going
15:09to show you some rainfall totals here, so Roatwoburn had the most amount of
15:15rain, these are 48-hour totals, 147 millimetres in total, their average for
15:23September is 55, so that's 267% of their September average in three days,
15:31apologies that's three days rainfall, Saturday, Sunday and Monday because of
15:36course we saw more rain on Monday with an amber warning, again some very heavy
15:41rain and it wasn't just Roatwoburn, South Newington there, Chipping Norton,
15:46parts of Northamptonshire, so really it was Oxfordshire, Bedfordshire, Northamptonshire,
15:51Hertfordshire, Warwickshire, yeah central parts of England really really got a
15:56deluge and you can see that on this map here, this shows the percentage up to
16:03this point of September, up to the 23rd of September, percentage of September's
16:08average rainfall, Scotland, Northern Ireland, Eastern England, drier than
16:12their September average but across this part of the Midlands into central
16:17southern England, a large swathe, more than double their September average
16:23rainfall and that roughly coincides with where we saw all these flashes of
16:30lightning, these are the number of lightning strikes for the first 23 days
16:34of September and you can see this big cluster, these were Friday's lightning
16:38strikes so moving up through Kent, home counties into parts of central
16:42southern England, Gloucestershire there, really a focal point for all those
16:46thunderstorms and then Saturday's thunderstorms a bit further north but an
16:51awful lot of storms, like I say six and a half thousand on Friday, roughly six and a half
16:55thousand on Saturday as well across much of England and Wales, very little further
17:00north like I say. And just to finish on that, it is something that the Met Office
17:08have been talking about in terms of future projections of climate change, that
17:13is that these kinds of summer style big thunderstorms and very intense rainfall
17:19rates are likely to extend further into autumn so into September, late September
17:27and so on and so one of the projections from UK climate projections is that
17:33future climate change is projected to bring about a change in the seasonality
17:37of extremes and that is an extension of the convective season from summer into
17:42autumn and yeah the very very very high rainfall totals that we saw through the
17:47weekend. Woburn by the way, 24-hour record for Woburn and their records go back
17:52more than a hundred years. This is some very extreme weather that we saw in the UK.
17:57Speaking of extreme weather, there's also on the other side of the Atlantic expected
18:03to be some extreme weather, we've got Hurricane John which is currently
18:07affecting Mexico, that's on the other side of North America but, or Central
18:13America but now we've got this tropical storm, potential tropical storm 9 and
18:21this is likely to become a hurricane in the next 24 hours and it's likely to
18:28move through the Gulf of Mexico and hit Florida as a major hurricane sometime
18:34around Thursday night UK time so this could be a very destructive beast, this
18:40could bring a large amount of sea surge, damaging winds and flash flooding
18:46because there's likely to be along its path two to four hundred millimetres of
18:51rainfall so yeah that's something that meteorologists all across the world will
18:55be monitoring over the next couple of days as that likely becomes a major
18:59hurricane. Now I mentioned wind shear as something that helps to sustain
19:05supercell thunderstorms but wind shear does the opposite for hurricanes
19:10because wind shear where you've got the change in wind with height tends to
19:14tear apart hurricanes, it makes them less powerful. At the moment there's a bit of
19:21wind shear coming off the Yucatan Peninsula and that is hindering the
19:26development of this storm but as it moves away from that area there's
19:31actually very little wind shear as it moves north and as a result it is likely
19:36to develop very quickly we've got high sea surface temperatures there as well
19:40so yeah that's something we'll be keeping a close eye on over the next few
19:44days. Back to the UK though and back to this week's weather and it's all about
19:51to change once again so we saw summer-like humidity, warmth and
19:57thunderstorms through the weekend as the autumn equinox occurred. Now however you
20:03like to define autumn it has truly begun and this week it's going to feel
20:07certainly more like autumn because the jet stream is going to become more
20:11involved with our weather once again. This streak of the jet stream currently
20:16sitting above the Atlantic moving towards the UK over the next few days
20:21and it's south shifted compared with climatology so more typical position for
20:27the jet stream is somewhere closer to northwest Scotland or between Iceland
20:30and Scotland so it's shifted south that's been a common theme with our
20:34weather over the last week year or so and yeah here it comes again into
20:40Wednesday and it's picking up areas of low pressure it's helping to spin them
20:44up and carry them towards the UK. Now these areas of low pressure they're not
20:49going to be particularly deep they're not going to bring particularly strong
20:54winds the strongest winds will actually be we put on the wind gust graphic there
20:59and actually be across parts of northern France you can see this swathe coming in
21:03there on Thursday night but the UK is likely to see further wet weather which
21:10of course many places don't need particularly central and southern parts
21:14of England and Wales so more wet weather to come because of that. Let's take a
21:19look at the wet weather over the next few days we've got showers out there for
21:23Tuesday afternoon mostly dry in the south thankfully showers though coming
21:28into Scotland, Northern Ireland, Northern England some longer spells of wet
21:32weather for northeast Scotland. Fast forward 24 hours and here comes that low
21:38pressure really it's from the word go that showers in the south become more
21:43and more prolonged after a largely bright start for Scotland, Northern
21:47Ireland as well as parts of Northern England some fog patches around but
21:51otherwise largely bright the rain arrives into the south and southwest
21:57and that rain becomes heavier it does move through now 25 to 40 millimeters of
22:04rain is expected across some western and southern parts of the UK but it does
22:09move through so nothing as extreme as we saw over the weekend. Then into
22:15Thursday the start of Thursday it tends to stall now all the while we've got
22:19colder air coming into the north of Scotland so touch of frost overnight
22:23here in some sheltered spots over the next few nights it stays bright but
22:27showery in the north and while we've got that cold air in place these systems are
22:31coming up from the southwest and they're tending to stall and this is a cause for
22:36concern because you can see those bright colors on the rainfall forecast across
22:40northeast England particularly through Thursday morning so it could be a
22:43tricky start to the day for Newcastle and Leeds perhaps but the rainfall will
22:50mount up most over the hills North York Moors the Pennines and so on and there
22:55is a Met Office warning in force and this is the area it covers for Thursday
23:02there's the risk of 80 to 100 millimeters of rainfall over some of the
23:07wetter spots like I say North York Moors parts of the Pennines more widely 25 to
23:1350 millimeters within that warning area so risk of localized flooding risk of
23:17transport disruption now once that clears through another big change
23:23happens in the UK's weather we've got much colder air arriving
23:35there's Thursday's weather the front stalling across northern parts and then
23:39low-pressure eventually moves away Friday morning the rain continues to
23:46move south but eventually brighter skies arrive as the winds change to
23:51northerlies across the whole of the country and then the kind of weather
23:54that northern Scotland's experiencing over the next few days sinks south to
23:59cover the whole of the UK by Friday afternoon so we've got these northerly
24:02winds directly from the north for a time certainly across northern parts of the
24:06UK so much colder than average it doesn't last long though we've got a
24:12cold start to the weekend a widespread ground frost a touch of air frost in
24:17places to start off Saturday but a ridge of higher pressure building in and that
24:23will lead to a lot of fine weather for Saturday and into Sunday certainly
24:27across southern parts of the UK just a few showers further north I'm going to
24:31talk about temperatures in just a minute but while we're on the subject of mostly
24:35fine weather here's how Saturday starts a much quieter day certainly for
24:41southern and central parts compared with last weekend and you can see plenty of
24:45sunshine for northern and central England's southern areas just some
24:48showers coming to the north and west of Scotland Northern Ireland parts of Wales
24:52and the southwest and then into Sunday and more of the same effectively a
24:56bright start to the day a few fog patches a few more showers coming into
25:00the northwest but otherwise a lot of fine weather through the weekend but
25:04temperatures are going to be suppressed we're going to see below average
25:08temperatures these are the actual values so staying relatively normal across
25:14southern parts through the next few days this is Thursday 18 Celsius in the
25:18south but we've got the cold air in place further north certainly feeling
25:21cold where we've got the rain in the northeast of England for example eastern
25:24Scotland 11 to 14 Celsius at best then into Friday those temperatures are
25:28coming down so 13 to 15 in the south 9 to 11 further north and by Saturday
25:34what have we got similar sort of things again just 10 to 12 Celsius in the north
25:39these are the daytime temperatures of course overnight temperatures are going
25:43to be lower still main urban areas high single figures but low single figures in
25:49some shelter spots and perhaps in some of the most shelter spots a touch of air
25:53frost how does that compare with average let's just go back to that so this is
26:01how the maximum temperatures compare with average Wednesday's below average
26:06across the UK about average on Thursday in the south but you can see
26:10significantly below average so three or more degrees below average across
26:14southern Scotland northern England parts of Wales into Friday that below average
26:19air is across the whole of the UK and it stays cold into Saturday so below
26:25average temperatures as we end the week as those northerly winds arrive how
26:30about into next week because we well we end the period we go into the weekend
26:36with a lot of fine weather but it doesn't last because next area of low
26:43pressure soon turns up that's likely to bring some very windy weather across
26:48western parts of the UK on Monday morning and a band of rain then that low
26:55tends to stick around through next week or at least become replaced by another
26:59low and it generally turns cool and showery with most frequent showers
27:03across western parts this is the mean sea level pressure so the pressure
27:09pattern anomaly for the week of next Monday to the following Monday and it
27:14shows low pressure anomaly over the top of the UK indicating that we're just
27:19going to see low pressure basically through next week no signs for any
27:22really significant rain and wind but it's likely to be unsettled blustery
27:27showers mostly but some longer spells of rain at times particularly in the
27:32West and especially at the start of next week and perhaps though after those
27:37stronger winds at the start of next week it is going to stay blustery and as a
27:41result it's going to stay cool as well so cooler than average conditions for
27:44that same period that same seven day period and finally this shows the same
27:48period it shows the basically rough position of where you'd expect the jet
27:56stream big low over the UK south shifted jet stream around that so quite
28:03a strong signal there from the computer models that we're going to see low
28:06pressure arrive at the start of next week after a relatively fine if rather
28:10cool weekend and that low pressure is likely to last for at least a week or so
28:15bringing generally cool and showery weather beyond that this is from the
28:20Met Office output again this shows that seven day period low pressure centered
28:26over the UK and parts of Scandinavia now beyond that what the Met Office output
28:31the longer range output is showing is a build of pressure this is for the
28:35following week going into the middle of October a build of pressure towards the
28:39West and more definitively over the UK by 14th the 20th of October now the
28:47European model output isn't showing that at all very mixed signals from there so
28:51yes certainly we wouldn't say we're confident about that signal but it is
28:56just a hint there so what I would suggest we take away from that is an
29:03unsettled week next week low pressure of the UK cool and showery some hints of
29:08something a little more settled developing around the middle of October
29:11but it's a long way away and we know how these things can change as you get
29:17closer to them and we'll keep you updated right here of the 10-day trend
29:21tomorrow with Alex Deakin and we'll have another deep dive next week of course so
29:26don't forget to subscribe so you won't miss that and thanks again for watching
29:30bye bye