This is an in-depth Met Office UK Weather forecast for the next week and beyond. Easterly winds arrive across the UK this weekend but how cold will it get, how long will the easterlies last and what caused them in the first place? Bringing you this deep dive is Met Office meteorologist Aidan McGivern.
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Forecast and any weather warnings are accurate at time of recording. To ensure you have the most up to date weather information, check the hourly forecast and live warnings on the Met Office website or app.
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NewsTranscript
00:00Easterly winds are on their way. How cold will those Easterlies be? How long will
00:05they last? And will they deliver any snowfall to the UK? I'll be answering all
00:10those questions plus much more in this week's Met Office Deep Dive. If you enjoy
00:14these kinds of in-depth discussions of the UK's weather and beyond, you'll love
00:19our YouTube channel. If you've not already subscribed to it, please do that
00:23now and hit like, send us a comment, we might answer you. And we do the Weather
00:28Studio Live, of course, on a Friday as well, so you won't want to miss out on
00:31those. Hitting subscribe will make sure that you do catch all our latest updates
00:36and it will only encourage us to do more of this sort of thing. The weather is
00:42going to change significantly over the next few days across the UK as high
00:45pressure arrives and then moves to the east, allowing colder easterly winds to
00:50turn up. So I'm going to be covering that at length in this week's Met Office
00:54Deep Dive. I'm going to be talking about why high pressure is arriving over the
01:00UK and then moving to the east, but I'm actually going to leave that towards the
01:04end because that's where it gets really quite complicated. I'm sure you lot will
01:08be fine, but it's going to hurt my little brain. So I'll do with a more
01:12straightforward warm-up first and that's why I'm going to start off by talking
01:16about the sunshine that we're all going to see over the next few days. Because
01:20we've not seen much sunshine, it's fair to say, recently and certainly at the
01:25time of recording there's an active weather front crossing the country.
01:28That's this band of rain here that is moving west to east, but that, apart from
01:34the showers that follow, is the last Atlantic-driven weather that we're going
01:39to see for some time because then it's all changed. What we've got at the moment,
01:43we've got that cold front moving through. It's attached to a rather deep area of
01:46low pressure that's affected Iceland during recent days and that's moving
01:50well to the north. Meanwhile, if I press play here, what you'll see, Tuesday
01:56evening, we're starting to see high pressure build in there. A bit of biscay
02:01initially, but it's just going to migrate northwards and as it does so, skies are
02:06going to clear. Yeah, we'll still see some showers overnight and some of those
02:10showers will be falling snow over the mountains of Northern England and
02:13Scotland. But, apart from those showers, actually plenty of clear skies so that
02:17by the start of Wednesday, it's chilly out there. Temperatures down below,
02:21freezing in some sheltered spots across the south, predominantly breezier further
02:26north and plenty of blue skies on offer for the start of Wednesday. Finally, a bit
02:31of sunshine, although not for everyone. Northern England, Northern Ireland,
02:34Scotland still prone to those showers. Still a slight Atlantic influence, still
02:39got a west-to-southwesterly breeze. But, as you can see, that high pressure is
02:43migrating north through the day on Wednesday. By the afternoon, this is
02:46four o'clock, that high is centred over the southwest. Got plenty of clear skies
02:52for England, Wales, southern Scotland, Northern Ireland, eastern Scotland. Still
02:55some showers in the far north and northwest, but they're fading away. And
02:59it's overnight, on Wednesday night, that clear skies become widespread across the
03:06country. That's going to allow these temperatures to fall widely as well. So,
03:10widely, a frost. And the lowest temperatures will actually be across the
03:15south and west of the UK, with temperatures dipping in tons of cities,
03:20minus one, minus two Celsius. But, I think in rural parts, minus two to minus five.
03:26We'll look at the temperatures in a bit more detail in a moment. But, as you can
03:30see, start of Thursday, yeah, it's a chilly start. There are some freezing
03:34fog patches around. But, for the vast majority, it's sunny skies and light
03:40winds. Although, there is a breeze blowing through the channel there, and
03:43across the west and north of Scotland, rather blustery still, around the
03:47periphery of this high. You can see the isobars tightly packed together there
03:51for Shetland, Orkney, the Outer Hebrides. And really, that high is sticking around.
03:56Thursday is a spectacular day. Waterable sunny skies up another country, all the
04:02way from Shetland down to Penzance. And it's just going to be very nice out there.
04:07Yes, some patches of cloud. But, otherwise, it's predominantly just a pleasant, crisp
04:13winter's day. It's going to feel quite chilly after that cold start, but
04:17temperatures not far from average. We're talking about five to eight Celsius up
04:23and down the UK. You'll notice, by this stage, the high is starting to edge away
04:29towards the northeast, and that's the sign of the next big change. It doesn't
04:33hang around on top of the UK for long. It is going to set up a new home over
04:39Scandinavia. More on that in just a moment, but let's talk about the
04:42temperatures over the next few days, first of all. You'll notice I'm skipping
04:46the weather for the rest of Tuesday and Wednesday. More on that, of course, on
04:50our shorter-range forecasts in this deep dive. It's more useful, I think, to go a
04:54bit further ahead. So this is the temperature trend for maximum
04:58temperatures over the next few days. You can see Tuesday, Wednesday, Thursday,
05:01Friday there. They're just slowly ebbing away, really. And so we're starting off
05:06Tuesday, 10 degrees. By Thursday, 6 to 8. Friday, 5 to 7 Celsius. But it's during
05:16Friday and into the weekend, as we start to see the Easterlies arrive, as high
05:23pressure moves into Scandinavia, that's when wind chill will become a little
05:28more important. So Thursday, these are the wind chill temperatures, by the way.
05:33Thursday, 2 o'clock, it's not that windy. These are the actual temperatures. These
05:38are the feels-like temperatures. Knock a degree or so off, because it's not that
05:43windy, apart from the south and the far northwest. But by the time we get to
05:47Saturday, take a look at this. These are the feels-like temperatures. Saturday
05:51afternoon, and we're talking about 0 to 2 Celsius across the UK, because now
05:56we've got high pressure over Scandinavia, and we've got this cold, this raw feeling
06:01Easterly wind. Now the actual temperatures, the maximum temperatures on
06:05the thermometers, are not especially low. We're talking about 3 to 5 Celsius. Not
06:12crazy cold air. 5 or 6 Celsius in places on Sunday. So yeah, it's going to feel
06:20cold, but the most important thing is it's going to feel cold, rather than it
06:24being notably cold. Temperatures are going to be a little below average, but
06:29these are the feels-like temperatures. It's the wind that's going to feel cold.
06:33There will still be some sunshine around through the weekend, but more towards the
06:39northwest of the UK. There'll be more cloud arriving into the south and
06:42southeast, and with that wind, of course, towards the south and southeast, it's
06:48that feel of the weather that is important. The air itself isn't
06:52especially cold. Before I explain why, and look at the temperature trend into
06:58next week, a final thing I want to look at is the minimum temperatures, because
07:02they are going to be particularly cold over the next few nights, and then
07:04perhaps less cold into the weekend and the start of next week, because it will
07:09be breezier. So although the character of the air, the feel of the air, will be
07:13colder by day because of the stronger breeze, by night, conversely, we're going
07:18to see the lowest temperatures over the next few nights because of the light
07:21winds and the clear skies. So these are the temperatures by towns and cities
07:25over the next few nights. Thursday morning's temperature is particularly low.
07:29In fact, we could get an idea, Thursday morning, of what it will likely be away
07:36from towns and cities just by running the pen across the map here. So minus one,
07:40minus two, we've got minus threes there. Tending to be coldest, I suspect, where
07:46we've got the axes of the high around. You can see the winds circulating around
07:51the periphery of the UK there, the south. The far northwest there, not particularly
07:56frosty because we've got a stronger breeze coming up. Northern Ireland,
07:59temperatures hovering around zero or just below. But yeah, come further into
08:04the central part of the UK, and that's where you could see temperatures of minus
08:09three, minus four Celsius or so in some sheltered spots. So widespread frost
08:15Thursday, that's going to be the frostiest start. Still a frost around on
08:19Friday, still temperatures a few degrees below freezing, more towards Northern
08:24Ireland, central and southern Scotland, northern England because the centre of
08:28the high is drifting away. You'll notice those temperatures still around one to
08:33three Celsius in the south but less widely below freezing because we've got
08:37this breeze picking up with the high moving to the east. And likewise on
08:41Saturday, ones and twos there across the south. Yeah, frost in places but we're not
08:48talking about well below freezing at this stage because of that breeze
08:51helping to stir the air up in its lowest layers. But by this point it's Northern
08:57Ireland, it's parts of central and western Scotland where we're likely to
08:59see the clearest skies into the weekend and the lighter winds and as a result
09:04temperatures below freezing in those locations. So high pressure will be
09:10moving away. We've got the crisp sunny winter's days, Wednesday, Thursday, Friday.
09:17But as high pressure drifts east, here it is on Friday, well to the east of the UK.
09:26We've got these isobars ganging together. We've got also, here's the jet stream by
09:33the way, the main branch of the jet stream running well to the north of the
09:37UK. Now if this was summer it would be rather hot across much of the country
09:42because we'd have all this hot air moving north. But of course when you have
09:44a continental influence, this time of year the continents cool down over
09:48winter, it's a colder feel. But whilst the main branch of the jet stream is running
09:53well to the north of the UK, we've got this other branch that's coming in like
09:59so. You might just be able to make out those arrows and this is just going to
10:03cut off from the main branch. It becomes so elongated around like this, so
10:07elongated that this area cuts off. We see it in action, disappears entirely by
10:13Saturday. There it is, cuts off and forms its own little low
10:18pressure system that's just to the south of the UK over France. And it's that plus
10:23the high moving into Scandinavia that caused the isobars to bunch together and
10:28this southeasterly wind. And that low will be introducing some precipitation
10:33into the south and east especially as we go into the weekend. So we've got the
10:40winds coming increasingly from the east or southeast as the high pressure moves
10:43away and as that low pressure to the south of the UK forms by Friday and into
10:48the weekend. But that low and the wind direction will cause, unfortunately you
10:55might think, a reduction in the sunny skies. Let's go back to Friday's weather
11:01on here. You can see 2 p.m. Friday, there's the UK, there's that closely
11:08packed isobars there across southern parts of the UK and you can see more
11:12cloud coming in across much of England and Wales. And these showers, mostly rain
11:17showers actually, but there's a few tiny dots of white mixed in as well so some
11:22wintry content because the air is cooling down. But they're generally light
11:27and they're mostly affecting on Friday the far southeast and perhaps along the
11:31south coast. Brisk wind making it feel cold as I said. Feels like temperature by
11:36Saturday 0 to 1 Celsius. Maximum temperatures on the thermometer 4 or 5
11:42Celsius. And here's Saturday's weather. You can see the cloud cover at this stage
11:46becoming more extensive across England, Wales into southern Scotland, Northern
11:50Ireland. And these blues across the Midlands, East Anglia, the southeast
11:55just indicating the showery precipitation moving in from the
11:59continent because of that that area of low pressure basically. Into Sunday and
12:04it's very similar. A lot of cloud, quite low cloud, quite gloomy once again across
12:09England, Wales. I think brighter skies because of the wind direction coming
12:13from the east across Cornwall, West Wales, more especially Northern Ireland,
12:17Northern and Western Scotland. But for much of central and eastern UK it's
12:23gloomy. We're going to see those outbreaks of light precipitation. I call
12:28it precipitation because it's going to be a messy mixture of rain, sleet and
12:33snow. And here's why. We've shown this kind of graphic before in the
12:39deep dive but I'll quickly explain what's going on. It shows the height of
12:45the zero degree isotherm, the height of the freezing level above sea level. So as
12:50you might know as you go further up in the atmosphere the air tends to cool
12:55down. It doesn't always. Sometimes you get what's called an inversion where the
12:57temperature goes up with height but on average the temperature goes down as you
13:01go further up. And the height of the zero degree isotherm, the point in the
13:06atmosphere at which you go below zero is important when forecasting whether it's
13:11going to be rain, sleet or snow falling out of the clouds. So when the air is
13:15below zero you're going to get snow and sometimes a little bit below that as
13:22well because as the snow falls down from the clouds any falling precipitation
13:27will also cool the atmosphere. Something called, well what happens is the
13:33air comes, the precipitation comes down and it evaporates as it falls through
13:40the atmosphere and you get something called evaporative cooling. Same thing
13:43when you come out of the shower, you've got little water droplets on you, you feel a
13:47bit colder because that water is evaporating and it needs energy to
13:51evaporate so it's extracting the energy from your body causing you to cool down.
13:56Rain clouds or snow clouds, the precipitation comes out of the cloud,
14:00extracts heat energy from the atmosphere in order to evaporate as it falls
14:05through the atmosphere. So that's why it's that cooling that means that you can
14:10normally expect snow at about 200 metres below the cloud or the zero degree
14:20isotherm I should say. So where you've got zero degrees go down 200 metres
14:25you've still got snow even if it's slightly above freezing and that's what
14:28this is showing. Sometimes it's 300 metres, sometimes it's 400 metres, it
14:32depends on a few things, most notably the intensity of the precipitation so the
14:36heavier it's falling the further down you can go, 300 or 400 metres. Now the grey
14:42colours on this chart indicate where the air will definitely be cold enough for
14:47snow so that's 200 metres or below. When you get into these darker blues it's
14:52marginal. This is 9 a.m. Thursday so let's skip forward to let's say Friday
14:58evening and what you can see is, this is the UK by the way, there's London, there's
15:03Belfast, Stornoway and much of the UK is covered by the grey colours so that
15:10would indicate if you've got falling precipitation it would be coming down as
15:14snow but there's not much falling precipitation at this stage. It's coming
15:20out of that low pressure that's over the continent so these blue colours here,
15:24there's Paris into the London the southeast and in that general area
15:31actually what we've got is milder air coming up. Now initially there is some
15:36colder air over the southeast so could see some snow coming down from the
15:41southeast on Friday night but as the night progresses these milder colours
15:48start to move in so the freezing level rises and that's because the low
15:53pressure system that contains the precipitation is also carrying some
15:58milder air and by early Saturday what we've got here is some of the blue
16:06colours across say the Midlands, East Wales, East Anglia and the south of
16:10England, patchy, mostly light precipitation and it's falling out of
16:16clouds that are in these kinds of colours as far as the freezing level is
16:22concerned. So 200 metres or below across parts of Wales in the southwest, the
16:26Midlands, more like 400 metres, 500 metres across the East Midlands, East Anglia and
16:31the southeast and remember that this is fairly light precipitation so it's not
16:37going to lower that freezing level much as it falls and so what I'm trying to
16:44get at here is that there will be snow in places but it's mostly light and it's
16:48going to be very marginal falling through the night and on Saturday
16:52morning in many places particularly towards the southeast very marginal
16:56about whether it is snow or whether it's rain or sleet so effectively we've got
17:01early Friday night cold air across much of the UK and then this area of rain
17:08sleet and snow moving out the continent wrapped up within it, milder air so more
17:13likely to be rain but it then moves westwards across the UK and it's across
17:20parts of the West Midlands as you can see towards the southwest and Wales
17:23where it is possible that we'd get a small amount of lying snow by the start
17:29of Saturday but we really are talking about small amounts this is the rolling
17:35snow accumulation between now and up to Saturday morning so if I fast forward to
17:40Saturday morning this is the total amount of falling snow between now and
17:43Saturday most of it across England and Wales happens basically on Friday night
17:48as you can see that's Friday and then we skip forward to Saturday morning and
17:54yeah what we see one centimeter or so across parts of Wales, Central, Southern
18:00England, Southwest so minimal amounts but yeah you might wake up in those
18:05locations to a small amount of snow on the ground it might make things look
18:10quite exciting but it's not going to be a huge amount as you can see and this is
18:17the lying snow depth early Saturday and as you can see it's patchy it's a
18:22centimeter or so at most patches of snow and after that well we're talking about
18:28that air that I mentioned coming up from the continent wrapped around the low
18:32that's actually going to cause a rise in the freezing levels into Sunday and what
18:38we're more likely to see if it goes that far is yeah late Saturday into
18:45Sunday these freezing levels are around four to six hundred meters what does
18:51that mean it means that this patchy light precipitation is mostly going to
18:57be rain perhaps some sleet perhaps some snow over the hills but nothing
19:03particularly dramatic it's generally going to be a gray and damp day on
19:08Sunday so it's Saturday morning that at the moment is the main period that we're
19:13looking at for the chance of a small amount of lying snow in one or two spots
19:17then through the weekend we've got the milder air coming up from the southeast
19:21turning it mostly back to rain. Nowhere near as snow and nowhere near as much snow as
19:28Japan has seen. Take a look at this this is absolutely crazy this is a video
19:36let's play it back it's just there we go here's a video showing I doesn't want
19:44to play okay this maybe if I press play that might work there we go this shows
19:50the snow in a place called Obihari I think should have checked that
19:56it's it's on my graphic I'm going to show you the correct place name in a
19:59moment they had 120 centimeters of snow in 12 hours which is a national
20:05record 120 centimeters of snow in 12 hours here's another video of some poor
20:13fella shoveling all that snow I'm gonna play there we go imagine having to deal
20:19with that much snow so yeah puts the UK's weather into perspective that's
20:25for sure actually these locations aren't far from the snowiest city in the world
20:30good pub quiz question for you what's the snowiest city in the world
20:35it's actually in Japan a place called Aomori City in the far north of Japan
20:42now let's zoom right in here Obihiro there is that's the location with 120
20:49centimeters of snow in 12 hours and the cause of that is this area of low
20:52pressure which has been sitting there for some time and it's going to continue
20:56to sit just to the northwest of Japan and we've got what you know you might
21:03describe in the US as lake effect snow basically very cold air coming from
21:07Siberia really cold air coming from Siberia low pressure over the
21:14Sea of Japan causes instability and moisture and then you just got these
21:18cold very cold moisture laden winds coming in from the west bringing heaps
21:24and heaps and heaps of snow and they're expecting more snow there over the next
21:29few days so absolutely incredible now let's move on to what happens next week
21:35as this is well a little uncertain but also there's a lot of interesting stuff
21:39to talk about with this now first of all there's actually quite a lot of
21:46confidence about the general weather pattern into the start of next week as
21:50you can see by this graphic this shows the probability plot for various weather
21:55patterns that will affect the UK for the next two weeks the time runs along
22:02the bottom there up to the 17th of February and you can see one maybe two
22:07colors are dominating this graphic the first color that will be absolutely the
22:13most likely weather pattern for Monday the 10th of February is the Scandinavian
22:19high there are different flavors of Scandinavian high and different flavors
22:24of easterly wind I know whenever we get easterly winds or a hint of easterly
22:30winds in the UK a lot of people on social media get excited and the
22:34newspapers get excited as well because they conflate it with the beast from the
22:37east in 2018 but we've had plenty of easterly winds in the winter since 2018
22:43and I'd say the vast majority of them haven't been anywhere near as cold or as
22:48snowy as the beast from the east it all depends on how much of a fetch the
22:53easterly has so you can have an easterly that just picks up air from the near
22:58continent or you can have an easterly that brings air all the way from Siberia
23:01into northern Europe and then the UK and that's what we saw in 2018 and that's
23:06not what we're expecting early next week so this type of Scandinavian high that
23:12is just on the map here is the most likely situated over Denmark let's put
23:18the temperature anomaly on and hide the graphic this is the most likely weather
23:25pattern for the start of next week basically a continuation of what we've
23:28got through the weekend feeling chilly in the east to southeast winds a lot of
23:35cloud perhaps some patchy rain and sleet in places but temperatures not massively
23:40below average degree or two as you can see by the key over here but because the
23:46easterly is bringing air from the near continent and not all the way from
23:50Siberia into Eastern Europe and then across the UK the temperatures aren't as
23:56cold as some of the more classic easterlies that have brought a lot of
24:00snowfall in the past and that is the most likely situation for the start of
24:05next week continuation of the chilly gloomy for many a little bit damp
24:10weather that we'll see over the weekend but not your beast from the east
24:15classic however whilst that is the most likely weather pattern for much of next
24:22week as you can see that continues through Wednesday into Thursday to play for
24:27Friday and Saturday and so on one of them here this is the second most likely
24:34weather pattern for Thursday the 13th of February and this does show as you can
24:38see by the temperature anomalies there a colder easterly an easterly with a
24:45longer fetch coming in from say well Eastern Europe or Scandinavia or perhaps
24:51Russia temperatures several degrees below average now I must stress that
24:55this is only the second most likely weather pattern for the mid to end of
25:00last week more likely this is I'd say a small chance more likely is a
25:06continuation of that less cold southeasterly either way we've got high
25:11pressure nearby it's blocking Atlantic systems from coming in and whether
25:15you've got this or whether you've got the other type of easterly there's a lot
25:20of reds high-pressure dominating that's what that means and Atlantic weather
25:24being kept at bay the Atlantic weather low-pressure systems are indicated by
25:31these blue colors that only start to creep in towards the end of this two
25:35week period the middle of February and this is where the forecast encounters
25:40a crossroads basically will we get more benign southeasterly winds rather chilly
25:48but nothing particularly dramatic will we get a very cold easterly with
25:53disruptive snow currently assessed as only a small chance for later next week
25:59but not the most likely or will the Atlantic try to return and to try and
26:05answer that I want to look back at how the easterlies started in the first
26:11place why we're going to get a week of easterly winds and what could encourage
26:19those easterlies to stick around for longer and perhaps become more potent
26:23and likewise what could cause those easterlies to break down and and move
26:28away and allow more unsettled weather to move in let's take a look at this
26:33first of all this shows the likelihood of easterly or westerly winds out to the
26:38next couple of weeks the data on the top that goes out to the 18th of February in
26:42the middle of the month and the colors represent the likelihood of easterlies
26:47or westerlies easterlies the more likely they are the darker the red more likely
26:51westerlies are the darker the blues so the top run of models or the most
26:57recent run of computer models is on the top there and as you can see strong
27:01blues over the next few days strong likelihood of westerlies and then it
27:04switches from Friday into next week the strong reds that indicates this strong
27:09likelihood of easterly winds but the strong likelihood of easterly winds for
27:14next week hasn't always been the case when you look back at historic computer
27:19model runs and you go back to say Thursday of last week Thursday the 30th
27:24of January and these bottom three rows are indicating westerlies are more
27:29likely than easterlies throughout much of the first half of February the blues
27:33they're dominating and it was really around the end of Thursday that we
27:39started to get this signal for easterly winds becoming more likely these reds
27:45starting to appear but it wasn't until the weekend that confidence grew
27:50significantly because these reds became stronger and more long-lived through
27:55next week so something in the atmosphere changed at the end of last week to
28:01switch the models from favoring westerlies for the first half of February
28:05to favoring easterlies over the UK and that's why at the end of last week of
28:12course if you watched our forecast we were talking about a lower likelihood of
28:14easterly winds but now at the start of this week we're talking about a much
28:18greater likelihood of easterly winds something in the atmosphere changed and
28:23that led the computer models to flip and talk about easterlies rather than
28:29westerlies so what exactly happened this is where it gets fun you might say
28:35complicated you're more likely to say and I'm just going to take a look at a
28:41different point of view across the northern hemisphere here you can see the
28:46Pacific North America and the Atlantic and the UK and this is on Thursday this
28:51is when high-pressure builds over the UK and what I want to draw your attention
28:55to is the sequence of events that have led to high-pressure forming over the UK
29:02so you go back to the time of recording start of the forecast the jet stream is
29:06relatively flat across North America and the Atlantic it's flowing west to east
29:12it's bringing us low pressure it's bringing us the band of rain crossing
29:16the country on Tuesday skip forward to Thursday and I just want to draw your
29:21attention to this part of the jet stream it dives south that part dives south
29:26which you could imagine it's like cracking a whip it causes this part of
29:33the jet stream to push to the north of the UK and take an area of low pressure
29:38to the north as well and by Friday a strong jet stream powerful jet stream is
29:45well to the north of the UK well to the north of Scandinavia allowing high
29:48pressure to form over Scandinavia so one way to imagine it is that this dip in
29:54the jet stream on the other side of the Atlantic causes equal and opposite
29:59effects to happen on our side of the Atlantic jet stream pushes to the north
30:03high pressure of Scandinavia forms but another way of looking at it which ends
30:08up being a bit more complicated but quite useful for tracing the origins of
30:11this disturbance in the jet stream is talk about something called upper
30:16tropospheric divergence the troposphere of course is the lower part of the
30:21atmosphere just below the stratosphere the jet stream tends to flow around the
30:26planets in the upper troposphere you get highs and lows in the upper troposphere
30:31the same as you get highs and lows at the surface this is showing the jet
30:35stream it's showing the highs and lows at the surface but on Thursday if you
30:40imagine with the jet stream dipping on the other side of the Atlantic we've not
30:44only got surface lows but we've got a low pressure here as well so we've got a
30:49low pressure here and to the north of the UK we've got high pressure of the UK
30:54but we've got this deep low moving out of Iceland we've also got a low pressure
30:59in the upper troposphere what happens when he gets a surface low pressure and
31:05a low pressure in the upper troposphere is you've got all this rising air that
31:09then wants to escape it diverges so the air is rising through the atmosphere and
31:14then in the upper troposphere it's escaping in all directions that's what's
31:18causing it to be low in the upper troposphere so let's draw some arrows to
31:24illustrate that air in the upper troposphere diverging these arrows are
31:30coming out in all directions from that area of low pressure some of the arrows
31:34are going to be bigger than others because in some directions those arrows
31:39are moving more air than in other directions and that's partly to do with
31:45the way these weather systems are moving about in the way that the planet is
31:47spinning but in this situation the biggest arrows are actually coming into
31:52Scandinavia so we've got the air escaping from that low escaping from the
31:57dip in the jet stream and then moving into Scandinavia where the air then
32:00sinks which is causing that high pressure over Scandinavia to build upper
32:05level divergence in the upper troposphere and then sinking air
32:09Scandinavian high once that Scandinavian high forms it becomes a blocking high it
32:14disrupts the jet stream so much that it sticks around through next week so the
32:18question I want to answer now is where that upper divergence has come from that
32:24disturbance in the jet stream can we trace the origins of it because that
32:27might allow us to predict whether it's going to happen again and encourage the
32:34high pressure of Scandinavia to stick around or if it doesn't happen again
32:38will we see a return of the westerly winds around the middle of the month
32:41that's what this is going to help us to answer so it's going to be worth
32:44sticking around and if I can just about explain it we'll see in a way that we
32:51all understand maybe it will help us to predict how the weather could change
32:57mid-month so that area in which the arrows are moving out of the area of low
33:03pressure can also be shown on this diagram it's not my graphic it's coming
33:07from a website www.aliciambentley.com really useful website with lots of
33:13technical graphics and so this one isn't particularly easy at a glance to
33:19understand what's going on but let me paint a picture for you we're looking
33:23straight down at the North Pole there's the North Pole in the middle there we've
33:27got North America there's the Atlantic there's the UK and there's Scandinavia
33:33so in this instance we're interested in Scandinavia because that's where the
33:38high pressure is building and this is a snapshot of midnight 5th going into the
33:456th of February so the start of Thursday or the start of yeah the start of
33:51Thursday the point at which high pressure is beginning to build over
33:55Scandinavia and the arrows I drew on the other graphic can also be seen here
34:00these black arrows there's lots of them but there's in particular a lot of black
34:05arrows coming out of that low pressure area between Iceland and Greenland and
34:09moving into Scandinavia so in this diagram it shows a lot of different
34:14things it shows the jet stream it shows various different technical things but
34:18the important idea is that we've got the arrows they're indicating this
34:22upper tropospheric divergence where the air is moving out of an area of low
34:26pressure and then the arrows points to where that air is moving into and
34:30therefore building the surface pressure if we rewind the clock 24 hours and go
34:38back to midnight tonight then this shows that those arrows actually can be traced
34:47to a disturbance on the other side of the Atlantic and there are more arrows
34:52here there's the eastern side of Canada there's Greenland so just south of
34:57Greenland over the Atlantic we've got a lot of arrows moving in a number of
35:01different directions but the biggest arrows are pointing towards the west or
35:06southwest of the UK they're helping to build the high pressure to the south of
35:09the UK and just taking those off and going back to the time in question we've
35:16got these lows here and basically if you can imagine the air is coming out of
35:21those low pressure systems and then sinking in the region of this area of
35:26high pressure that's what we're seeing with this graphic these arrows but those
35:30arrows can be traced to a disturbance 24 hours earlier just over the
35:37northeastern parts of the US and Canada and we've got arrows again moving out
35:42we've got this area of divergence this area where air is escaping from the top
35:46of the troposphere and rewinding it step further and it all gets a bit messy and
35:52bit hard to make out a clear signal because we've got North America we've
35:56got the Rockies and they help to break up the signal a little it's much clearer
36:00when this moves over the ocean but the arrows in question are at this point
36:06over eastern parts of the US and Canada so or western parts of the US and
36:12Canada I should say and going back 24 hours before again so this is the end of the
36:17weekend this is Sunday night I believe and we've got the arrows quite a lot of
36:24arrows pointing into eastern parts of north western parts of North America
36:29would help if I got my directions right and coming out of the Pacific and just
36:34skipping forward again and we're seeing more and more of these arrows by this
36:39time Saturday night more and more of these arrows or Friday night I should say coming out of the Pacific most
36:48especially coming out of an area just to the northeast of Hawaii and the final
36:55one final one to point out because this is the crucial bit this is Thursday
37:02night going into Friday the 31st of January and it's this area here that I've
37:08traced back from the Scandinavian high all the way across the Atlantic all the
37:13way over North America to Thursday night and it's an area over Hawaii where we've
37:18got loads of arrows kicking off exploding out in all different
37:22directions but the biggest arrows heading east or northeast into the
37:28Pacific towards North America and basically this is a tremendous area of
37:33energy coming out from the surface area of low pressure into an upper area of
37:38low pressure and spreading out in all directions in the upper troposphere but
37:42most especially heading east across the Pacific. This energy that helps to perturb the jet
37:51stream at this point and that disturbance in the jet stream then
37:54begins a chain of events that ends up with Scandinavian high-pressure building
38:00and lasting through next week. So what was that low pressure that caused that
38:06tremendous disturbance and chain of events that have led to our weather being
38:11affected? It was this. This was last Thursday. Hawaii slammed by triple threat
38:17of severe thunderstorms, blistering wind, blizzard-like snow. A deep area of low
38:22pressure really quite deep hit Hawaii and brought terribly impactful weather
38:28and you know very unseasonable weather for Hawaii they don't get lows like this
38:33often and yeah it brought a winter storm basically for Hawaii and it was
38:39deep enough that it then just produced this huge amount of energy that went
38:44into the upper troposphere and propagated into the jet stream causing a
38:48chain of events causing a disturbance that has led directly to Scandinavian
38:53high-pressure and easterly winds for the UK. Bearing in mind this happened on
38:57Thursday, Friday. That low happened on Thursday and Friday and that was when
39:03the computer models realised basically that the jet stream was going to be
39:08impacted in that way and we were going to see that chain of events that led to
39:12the easterly winds being forecast for this next week or so rather than
39:17westerly winds. So perhaps before Thursday, Friday the models were
39:22underestimating how deep the low would be, how it would interact precisely with
39:27the jet stream, all of that stuff but it just goes to show that a small little
39:31disturbance on the other side of the planet can then cause a chain of events
39:36that ends up completely upending our ideas of what the weather will do in a
39:42week or two weeks time. So the reason I'm covering all of this is to ask the
39:50question whether it could happen again and if it happened again what would be
39:57the consequence for the Scandinavian high that's going to stick around for
40:01next week or are we going to go back to business as usual, westerly winds and
40:06this is why there's a somewhat of a crossroads in the forecast around the
40:11middle of February. There's one scenario in which high pressure over Scandinavia
40:17moves further north and we get much more potent. Let's draw some arrows on the map.
40:24So there's one scenario in which high pressure later next week moves further
40:31north and we get a more potent easterly. That's not the most likely outcome. It's
40:36one possibility but it's looking like a small chance at the moment. There's
40:40another scenario where we keep more of the same high pressure over say Denmark
40:48and more of a southeasterly with low pressure sitting out here and something
40:55of a blocked situation where we've got benign southerly winds or perhaps some
41:00rain coming into the west at times but otherwise not much movement. We've got
41:05the high to the east, low to the west and then there's a third scenario where
41:11these lows in the Atlantic move in. The jet stream becomes more powerful and
41:18when the computer models are suggesting these three different scenarios and
41:23we're not entirely sure more than a week ahead which one's the most likely, we
41:27look at teleconnections. We look at things like what's happening over the
41:30Pacific and there's one important thing that could take place in the Pacific
41:34over the next few days which could, like the storm over Hawaii, cause a chain of
41:40events that would lead to a strengthening of the high pressure over
41:43Scandinavia and the easterlies becoming more long-lived into the middle of
41:48February and perhaps colder and that chain of events could be sparked off by
41:53the Madden-Julian Oscillation phase and I'm not going to explain what the Madden-
41:58Julian Oscillation is in detail here. We've covered it in a previous Deep Dive
42:02and there's an explainer on the Met Office Learn About Weather channel on
42:05the Madden-Julian Oscillation but just very quickly it's a pulse of enhanced
42:13tropical rainfall that tends to slowly circle around the world in different
42:18phases. At the moment this enhanced precipitation, this enhanced area of
42:23thunderstorms is over the maritime continent and Indonesia, Borneo and so
42:28on. It's going to move into the Pacific and there are two possibilities, there's
42:34some uncertainty in the modelling over the next few days about whether it
42:37becomes particularly active over the West Pacific by early next week or
42:45whether it fizzles away. The European model would suggest it's going to fizzle
42:50away but the Met Office model would suggest it's going to become more active
42:54and the Met Office model recently has been modelling it a little bit better so
42:58there's a potential for that enhanced area of thunderstorms to move into the
43:01Pacific and that could do similar things to that Hawaii storm. It could cause the
43:05jet stream to again absorb all this energy, become more wobbly across the
43:11world and allow that high pressure to become reinforced across Scandinavia.
43:16Meanwhile there's something else we're looking at which is the stratospheric
43:20polar vortex and the stratospheric polar vortex is this very powerful ring
43:25of winds that circle around this very cold air called the stratospheric polar
43:30vortex in the stratosphere above the North Pole and throughout the winter the
43:37stratospheric polar vortex and the wind circulating around it have been very
43:40strong. That tends to enhance westerlies across the northern hemisphere, it's not
43:46the only thing that affects the westerlies but it can act as a tailwind
43:50and enhance those westerlies basically. This graphic from ECMWF shows the
43:57strength of the winds circulating around the stratospheric polar vortex and at
44:02the moment they're very strong and each of these lines represent a different
44:06computer model simulation so there's 50 odd computer model simulations that show
44:11the strength of those winds and most of them decelerate those winds through to
44:17early next week. The reason behind that is because of the Scandinavian high
44:21pressure. You can have high pressure in the lower atmosphere that then because
44:26of the jet stream becoming very undulating and weaker that can have an
44:30impact and slow the stratospheric polar vortex and as a result we've got this
44:36weakening but it rebounds through next week. That's what the suggestion from
44:40these models are, it rebounds. So the main question mark is whether the
44:46weakening of the stratospheric polar vortex, the mean wind at the height of
44:50the stratosphere, many kilometers above the surface, whether that
44:57weakening is going to encourage the Scandinavian high pressure to stick
45:03around for longer and whether the Scandinavian high will cause it to
45:07weaken further and there'll be this positive feedback type of thing and
45:12maybe the Madden-Junian oscillation phase will also feed into that or once
45:17it rebounds later next week could it then force the Westerlies to return at
45:21the surface, force that high pressure over Scandinavia to weaken and we end up
45:25with low pressure over the UK, milder, wetter and windier and so on. So two main
45:30question marks. The strength of the enhanced thunderstorms that are moving
45:35into the Pacific as a result of the Madden-Junian oscillation phase and how
45:40much they will have a consequence and set off a chain of events that cause the
45:43jet stream to wobble about and strengthen the Scandinavian high by
45:48middle of February and how much the stratospheric polar vortex will weaken
45:53or wobble or move and how much of a consequence that will have on the high
45:58pressure over Scandinavia or will it restrengthen and send us a powerful jet
46:03stream and low pressure from the middle of February. We simply don't know which
46:09route it's going to go at the moment. We can say what those options are, we can
46:14say what the consequences would be if one of those options was more powerful
46:17than the other. My feeling is that we're going to end up with a battleground
46:22situation with low pressure just sitting to the west of the UK, high pressure to
46:26the east and a subtle difference in the position of those systems will lead to
46:32either cold and dry or mild and wet. It will only take a subtle difference and
46:36we're talking about 10-14 days ahead so it would be foolish to say for sure
46:42which exact scenario we're going to see at this stage but of course we'll keep
46:47the data right here at the Met Office on all of that and yeah please do hit
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