Deep Dive 20/02/2024 – Very warm February and the SSW – Met Office Weather Forecast

This is an in-depth Met Office UK Weather forecast for the next week and beyond, dated 20/02/2024.

What is rain bunching and what is going on with the polar vortex? Is there more rain to come this week? Just how warm and wet has February been? All these questions are answered in this weeks Deep Dive

Bringing you this deep dive is Met Office meteorologist Alex Deakin.

00:00 introduction
02:01 wather set up
03:45 downpours Thursday
6:45 Rain warning Wednesday
8:40 Standing waves and rain 
11:05 Rain bunching
17:20 Weekend
19:03 February so far
22:13 Statosphere winds and SSW

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More rain to come this week just what is going on in the stratosphere and how warm has February been how wet has February been also standing waves and Rain bunching there is so much going on in this week’s Met Office Deep dive thank you very much for joining me thank

You for being there thank you for watching do hit the like button and do us a favor uh my name is Alex Deacon welcome along I am talking to you from Met Office headquarters in exitor in Devon if you’ve not seen a Met Office deep dive before we talk about the

Relevance weather situations and anything that takes our fancy in the world of meteorology over the next 20 to 30 minutes or so please share the love if you know another met head who might like a Met Office Deep dive then let them know about it and share this video

As I said please do hit like and please also comment now later on after I’ve recorded this I’m going to be in the comments probably from about half 4 till about 5:00 so if you’re watching there if you’ve watched it then got some questions got some comments uh join me

In the comments section but if you’re watching this later on don’t worry please keep your comments coming we will go through them we do read them all and try and reply to them uh comments about anything if you got any suggestions for future deep Dives anything like that

Please do put them in the comments section right as I said a lot to get through we’ll start with a bigger picture start with a satellite image over the last 5 days look at the swirls of cloud low pressure systems moving up between the UK and Greenland and all of

Our weather as a result has been coming up from the southwest uh when the air is coming up from the southwest it’s traveling over the Atlantic big mass of water and it’s also coming up from a warmer more tropical direction that is why it has been so mild and pretty wet over recent

Times in fact almost for the entirety of February you might say and well we’ll see a little bit of a change later this week but not a significant change why are we stuck in this weather pattern well as you might imagine if you’ve watched the Deep Dives before it’s kind

Of to do with the jet stream and the position of the jet stream it’s at the moment a little more south shifted than usual if we play through the sequence it kind of stays in that way the jet stream that fast moving River of air high up in

The atmosphere that dictates our weather patterns it’s just pushing low pressure systems just to the north of the UK and the weather fronts are bringing us the rain later this week it becomes even more south shifted it’s just dipping down to the South through Wednesday and particularly let’s fast forward it a

Little bit by the time we get into Thursday so now the Jets really dipping down here we’ve got this trough then sitting over the UK this dip in the jet stream that’s a breeding ground for more areas of low pressure to come in plenty of Isa bars on the chart so yes through

The rest of this week further spells of wet and windy weather are definitely on the cards let’s take a look at the rainfall then because actually for many of us Monday was a dry day and it’s still dry out there on Tuesday across a good chunk of Central and Eastern

England we do have this weather front bringing some rain that’s pushing further south and that will continue but it’s um kind of fizzling out as it does so that first bound of rain it then weakens but then look at this more rain coming in through Wednesday it’s going

To be a very wet night across much of the West uh Tuesday night into Wednesday night plenty more rain to come look at those brighter Colors Over The Hills in particular more on that in a moment and then once that’s out of the way we get this nasty looking line of showers

Coming in let’s play through this because this is quite a a nasty looking line quite a narrow line but just look how those brighter colors there you can see that line almost as it crosses the Midlands towards Eastern England some uncertainty about the timing and exactly how that progresses but this could be

It’s called a Squall line that’s quite a the brighter color let’s just rewind it a little bit see this really narrow line in there that signal the winds are changing direction rapidly across this this weather front and this strong wind V generates really strong gust of wind

And a really short burst of very heavy rain as that moves through so watch out for that as we go through particularly first part of Thursday especially over Central and Eastern parts of England there is a bit of uncertainty about how that progresses but it will bring a say

A spell of pretty Lively rain once it’s crossed through once that’s out of the way remember the jet stream’s dipping to the South now so we’re in the colder air we’re properly on the cold side of the jet so it has been a very warm February

So far again more on that in a moment but as that weather front moves through it will be introducing cooler air I’m going to put the freezing level on now uh the freezing level is how high up you have to go through the atmosphere how

High up in the sky you have to get before you get to 0o Celsius and at the moment it’s pretty mild across the UK can just make out Scotland there there’s uh uh Northern Ireland East Anglia there we’re in this dark green zone which is the freezing level is at least 1,400 M

Up you got to go 1.4 km up through the sky before you get to 0 Celsius but that will change as we go through the next few days pulses of Colder air Tri pushing across Scotland and then as that active front swings across the country during Thursday notice not only you’re

Going to see quite a strong wind there but you’re also going to see quite a drop in temperatures and the freezing level behind that will suddenly start to drop so that by the time we get to the end of Thursday most of the UK are in this

Bluer zone so the freezing level is 4 to 600 M and across Scotland and even Northern Island the freezing levels drop to only 200 M here you have to go 200 M up before you get to 0 Celsius which means means as the showers come in across Northern Scotland we will start

To see some snow returning at least to some of the higher routs here uh during the course of Thursday nothing unusual about that it’s February snow in Scotland but because it has been so mild well it’s a little bit of a change from the norm and even across parts of North

Wales and Northern England on the tops of the hills we could see a little bit of wintriness in the showers that come in on Friday so turning a little colder emphasis on the uh temperature later this week just going to back closer to average but because it has been so mild

You probably will notice the difference um let’s go back to the rainfall though because as I said plenty going on with this as the rain comes in tonight again especially in the west we do have uh met off his warning in place for this uh there’s the yellow

Warning for tonight and into tomorrow morning it’s just gone out the warning area out the warning time cuz it’s uh it’s from Midnight until 12 so in this Zone we could easily see um 15 25 mm of rain quite widely and more than double that on some of the hills now that is

Not a huge amount of rainfall for this part of the world uh to fall in 24 hours though the key thing is it has been so wet so that is likely to cause some further flooding issues over parts of Wales uh South Wales and extending up into the mountains of North Wales also

Across the southwest of England so we do have that met office yellow warning in place for tonight and tomorrow morning Tuesday night into Wednesday morning for that heavy rain because as I said could cause some certainly some local flooding and there’ll be a lot of spray and

Surface water on the roads so the roads like the M5 in particular the M4 in in South Wales not great for that morning commute on Wednesday morning now let’s go back to the rain because I want to show you something really quite cool here with the rainfall

And the way that it’s going to fall notice the winds coming in from the southwest this band of rain is kind of fizzling out as we go through this afternoon into this evening but this next pulse of rain as it comes in really quite interesting the heavier

Rain let’s Zoom right in on that actually heavier rain the brighter colors the oranges the yellows and the Reds and notice there’s a kind of stripy pattern down here so this is you can make Plymouth out there there’s Cardiff this is Devon corn so the Air’s coming

Up hitting d m you’ll notice here so get some heavier Rain Over The Hills you’d expect that but then there’s almost like a a a double stripe another zone of heavier rain here and another one of heavier rain and that is because we’re seeing the hills the Moors generating a

Bit of a wave going on so the air is rising it’s falling it’s rising and it’s falling and as it’s rising it’s cooling it’s condensing and it’s dropping more rain so you get this kind of striped pattern not just on here but also I can

Show you in other model out this is the accumulation of rainfall uh and again it just about make out the Southwest here but it’s also happening further north across Northern England these kind of lines of rain where the air is going up and coming down you get these The

Bouncing effect over the hills it’s going to kind of show that in a bit of a drawing so you’ve got the you’ve got the hill here and you’ve got the the air rising and going up over the hills let’s do that as an arrow so the air goes up over the mountain and

It comes back down the other side but it doesn’t stop there it then goes up again and it B it forms a WAV and everywhere every time you got the air going up it’s cooling it’s condensing and it’s falling as rain so you get more rainfall on this side then you get a

Drier slot because the air is descending and then you get more rain again as the air starts to go back up again so you’re get to get this standing wave pattern that’s reason Reas be common happens pretty frequently in uh in setups like this where you have this the winds kind

Of pushing in a very uniform Direction over the mountains you get heavy rain on this side of the mountains you get a drier slot on the Le side of the mountains and then you get another pulse of rain as the air starts to bounce up once again that’s a standing wave but

Now we’re going to go this is why it’s called the Deep dive we’re going to go really deep now into a brand new paper that um my colleague Nick silon put me on to this morning now Nick is Deputy Chief here at the Met Office very very clever

Guy always looking for Trends always looking for interesting meteorological tidbits and he spotted that this pattern set up this morning it is highly likely to be being enhanced the stripy uh nature of the rain not just by that standing wave but also by something else and there something else else has been

Highlighted by a former colleague of ours Eddie Carol in this uh brand new paper you can see in U sphere Eddie Carol used to be a chief meteorologist here at the Mets office not anymore but a very very clever guy and he’s still heavily involved in meteorology and he

Has been looking at the way standing waves uh impacted the rainfall that fell over the Lake District in December 2015 um a lot of heavy rain fell there was serious serious flooding and I’m going to show you this diagram now this is from that paper because this is really really

Interesting blew my mind when I saw this this morning now okay bear with me because it’s a pretty complicated diagram say you go back to if you really want to uh study it that’s the title of the paper so you can go and find it for yourself online but this diagram

Particularly struck with me today now this is a crosssection across the lake District this is the these are the hills and the mountains uh this is in uh kilometers along the bottom there a cross-section of the atmosphere going up through the atmosphere here okay now the darker

Shading the the the dark Grays and the blacks that is where the air is rising so that’s speed of the air in uh meters per second so the darker shade is where the air is rising as you expect that I should have said the wind is going is

Blowing this way the general wind direction is from left to right uh in this case it was a Westerly so west to east so the air is hitting the mountains and it’s rising that’s what the dark uh the dark Grays and the black is suggesting here then as it’s coming down

The other side the air is is um descending it’s descending where it’s white and the pale gray and then it’s rising again because you’ve got more hills and mountains here now the key thing here is this purple line which is the freezing level or in this case more

Relevant to think of it as the melting level so as snow is falling it’s hitting this line which is around 0 Celsius and it’s turning back to rain and the line here notice the freezing level in this cross-section actually drops here because the air is rising the rising air

Is also cooling as well as it’s condensing which is meaning that the freezing level drops so as you’re going up the hill actually the freezing level will start to drop and then as you go down the hill with the air descending the freezing level starts to rise once

More so this is the line at which snow turns to rain and that’s dictated by these dotted lines which you can just about make out here so anything above this purple line is snow anything below it is rain now here’s the crucial thing snow falls slower than rainfall you’ll

Have noticed that if you’ve ever seen any snow been a lot of it around this winter but you will notice that snow because it’s bigger it’s a larger Mass it falls slower than rain so so the trajectory of snow in these dotted lines here along the path and as you get to

The air here it’s actually it’s actually starting to rise so what you see is the snow coming down hits the fre the melting level and it rains and it rains the rain comes down at a steeper angle because it’s it’s um heavier it falls faster but what happens because you’ve

Got this dip in the freezing level you get this kind of Zone here where the snow isn’t in it’s just continuing to travel on its way until it hits until it intersects here with that Rising freezing level again so you get a more concentrated area where the snow turns

Into rain and as a result down here you get an enhancement of the rain so you get that enhancement of the rain anyway because the air is going up but you get a double enhancement you get that secondary boost because of the way it interacts you get a shadow where there

Isn’t as much precipitation and also you get this enhancement this secondary en enhancement rainfall bunching that’s called and that’s in a brand new paper I say by Eddie Carol go go and check it out if you want to it’s really really fascinating blew my mind this morning

Say special thanks to to Nick from the guidance unit upstairs for pointing that out to me we’ll try and maybe put a link to it in the chat as well if we can and that’s kind of shown here but this is the real time data for what’s going on

With the rainfall tonight and tomorrow morning across the Southwest so we’ve got um Devon and Cornwall here the black lines are the freezing level and the colored zones are where the air is De it’s again it’s that Vertical Velocity but now we’re looking down instead of a cross-section we’re looking down at

Devon and Cornwell and you can see where they’re corresponding with the descending air and the um and the gradient of the freezing level is changing you get those two or three different zones there across South de in particular which should fingers crossed for me and my cycle to work in the

Morning mean that in exitor where I am there will will be a bit of a rain shadow and hopefully the rain won’t be as heavy here as it will be in other parts uh as shown here so we’re in this little Zone here in exitus hopefully bit

Of a rain shadow here whereas the rain will be pretty heavy across South Devon down towards Plymouth and again in this next Zone uh somewhere across Dorset and Somerset so we interested to see if that happens and again you can see it similar patterns a bit further north across the

UK as well so yeah fascinating meteorology that’ll be happening on Wednesday morning across the UK wow that was good wasn’t it right where are we talking about next let’s go and look at the longer term and the longer range forecast let’s get rid of the jet stream because well now let’s

Put the jet stream back on actually because what we’re going to see we see this dip in the jet stream towards Friday we’re in the colder air lots of showers around what happens at the weekend will be determined by what happens out here complex interactions between a couple of areas of of low

Pressure and the Jets it’s a bit all over the place it’s kind of splitting bifa here and that adds to complications with this area of low pressure as we see over the next few days how that then topples around and then drives back down and comes towards the UK as we head

Towards Saturday and Sunday that complex interaction with the jet splitting and the models aren’t in complete agreement with where this low pressure will go as we head into Sunday and it could be depending on how it interacts with the jet stream it could get picked up and

Spun up into quite a nasty looking area of low pressure some models have been doing that for Sunday so it is something that we need to watch but at the moment there is quite a lot of uncertainty and this is the spread these are the postage stamps the model output

When we run them many many times from what we call an ensemble forecast so instead of just looking at one computer model you run it many many times and you get lots of different solutions from which you can work work out a kind of probability of of or a chance of what

Certain things are going to happen now if you look at these top ones you probably can’t see them very well I can go full screen maybe on this you can see those top ones there all pretty much in agreement there’ll be some heavy sorry I should have said these are all showing

Where it’s going to rain on Sunday and you can see that heavy rain in agreement there across those top four but then if you look at number 13 for example that’s in agreement with a lot of heavy rain but you look at number 23 or number 32 they both have pretty much dry

Conditions across the South so there is quite a lot of a range of spread because of that complex interaction with the jet stream how that low pressure will manifest itself nothing too unusual about that again we’re talking five days ahead you wouldn’t expect a detail forecast at this stage for for Sunday

But do keep up to date with the forecast for Sunday particularly if you live in the south because there could be some more wet and windy weather on the way um Annie is doing the 10day trend she will have more on that tomorrow no doubt right where should we go next let’s have

A quick look at February so far because as we’ve been discussing it has been wet and uh very very wet but also very very mild and there is more rain to come and there is a dip in the temperatures to come but only closer to average but this

Graph again we’ve shown these before on the Deep dive is showing the cumulative temperature throughout February so each day we mark on the the average mean temperature and you can plot that across the month and compare it to the average and compare it to previous records and

I’m showing this one because notice the last couple of days we just jumped right up to that red line which is the highest we have recorded so getting close to well it is close to record breaking values for the mean temperature across February so far still plenty of time to

Go extra day this year as well so still um 10 days to go so that can change but at the moment we are running close to record-breaking values for February this is with the data up to the 19th so up to yesterday temperature distribution across uh I don’t want to show that one

That was a bit of a tease wasn’t it for uh for some sudden stratospheric warming action uh what we wanted to show here was this temperature map so quite a range across the country but notice Northern Scotland in the whites there at the top that’s the only area where

Temperatures are close to average so far the rest of the UK in the pinks and the Reds and that dark red there I know that dark red offends some people but it is Extreme this and we have dark blue for when it’s cold extreme but it isn’t cold

It’s been very warm so that’s why we have the dark red colors there covering uh Eastern and Central England indicating that temperatures are running 3 and2 de above average in these at least and as you saw earlier that mean temperature is even higher than that similar maps for where we’ve seen the

Heaviest rainfall as well and and in particular that that blue that darker Blue Zone that is where we’ve had 200% of the average February rainfall so we’ve had twice the monthly average already and again that is with data up to yesterday so still 10 days to go up

To the uh 29th uh that zone there Lincolnshire down towards Cambridge twice the monthly average rainfall on anywhere that’s blue has already had 125% of the February rainfall uh you’d expect it to be around 6% so you’d expect it to be in this brown this pale brown color at the stage because there

Is still 10 days to go so that would be bang on average but all the rest of the UK are already above the average and many places have already had their month’s worth of rainf as I said some parts have already had twice their monthly rainfall average so it has been

Very mild it’s also been pretty wet okay let’s change Tech now and take a look at what’s going on in the Stratos field rather has been going on in the stratosphere because it has been a little bit Bonkers don’t take my word for that though because um Dr Simon Lee

On Twitter also thinks thinks so uh Simon Lee is a world leading expert in uh the polar of Vortex and sudden stratospheric warming you should follow him on on X as it now is regularly posting about what’s going on high up in the stratosphere really interesting guy really really fascinating post on there

So follow him on on Twitter on X that was his post from a couple of days ago this Winter’s Arctic stratospheric polar vortex is to be honest a bit bananas so there you go from the experts it’s a Bonkers year for the polar vortex uh

What has been going on if you watch last week’s with Annie you’ll know that we were expecting the winds to reverse in the polar vortex now um this is all tied in with sudden stratospheric warming or ssw now sudden stratospheric warming ing is where the air high up in the stratosphere above

The North Pole suddenly jumps up in temperature now that’s useful in some ways because it’s indicative of what’s going on with the polar vortex but we’re really really really interested in what’s going on more to do with the polar vortex than actually the temperature the fact that the

Temperature uh suddenly jumps up is just an indication that the polar vortex is is weakening or or is being displaced what’s the pole of Vortex well that is the winds kind of like form a Donuts strong winds around the North Pole happens every winter due to what’s going

On high up in the atmosphere you set up these strong winds that that spin around the North Pole throughout the winter what can happen is that that can get displaced dislodged just messed around with when that happens you see a sudden spike in temperatures but more importantly when the polar vortex gets

Messed around with or or changes shape or gets a little bit out out of shape that can in some cases lead to effects lower down towards the atmosphere and have impacts across the UK there is a link between when sudden stratospheric warming happens and an increased chance of seeing blocking weather patters which

Is an increasing chance of seeing easterly winds which is why people get excited about it because it can lead to snowier conditions but it doesn’t always as has been the case this year and the polar vortex in particular as with Simon Lee said earlier has been particularly

Crazy this year and it’s showing no signs of becoming even less Bonkers because we’ve seen this graph this is what um Annie showed a version of this last week that was suggesting that the winds around the polar of Vortex would reverse temporarily they did but really

Really briefly and now this blue line is showing that they are recovering so the PO Vortex is getting back up to speed and in the next week or so it’s expected to get back up to here and close to the average now this red line is the average

Speed wind speed of the polar vortex as we go through March and towards April notice it’s dropping off because that’s what happens as you go through to Spring the polar vortex dies if you like the tilt of the earth means it all warms up and you don’t get that polar vortex

Anymore it sets up every Autumn and it goes away every spring and that’s the mean line that that the average line of those winds showing you that it’s dipping off but what’s Happening Here is quite interesting with this year’s polar vortex it is now picking up speed again

But it is expected to dip back down again and really start to weaken or at least wobble significantly and it never looks like recovering after that so there is a decent chance this could be the end of the poll of Vortex this season even with it dropping back down

Though cold Fanatics snow mantics don’t get too excited because even if it does drop down like this it’ll be a couple of weeks before we see any effects because it’s all got a ripple down from the stratosphere remember this is going on high up in the stratosphere it all has

To come down through the atmosphere and that takes a couple of weeks and by which stage we will be into the middle or latter part of March it’s going to be very very hard for us to get anything colder significantly cold with um um Arctic air or anything like that or cold

Conditions cold Arctic air coming back across the UK so don’t get too excited but what this does show is that we’ve seen a few of these episodes with the polar vortex weakening a few times through the winter and this could be an early death of the uh of the polar

Vortex happening potentially in the early part of March when normally the polar of Vortex doesn’t die until the middle of April you want more on that to say go and follow Simon Lee um on Twitter SLX and there’s also a video about what polar vortex is and sudden

Stratospheric warming on on our other YouTube channel learn about weather we will put a link to that in the chat as well no doubt did I want to say anything else I’m not sure I did I think that’s pretty much it for this week thank you

So much for being there thank you for watching got any questions got any comments I will be in the chat as I say from about halfast 4 to 5:00 but um even if you’re watching later than that please do keep your comments coming and I’ll dip in and out over the next few

Days and try and answer some of those comments we would love your feedback I would really love you to hit that that the like button that does us a big big favor it spreads the word and it means we can keep doing more and more of these

Big shout out again to my former colleague former Chief Eddie Carrol so you can go and check out that paper once more and also to Nick silon deputy chief upstairs we’ll have to bring him in one day uh for a chat we’ve talked about him and Dan Harris and the other Deputy

Chiefs who really do help us out with these um with these deep Dives big shout out to Simon who’s next door and doing all the all the camera work as well well I know that um you just see me and Aiden and Annie and Alex but there’s a bit of

A team of us here in the content team that have to put these things together and it doesn’t just come together miraculously takes a bit of work from the whole team the whole content team so big shout out to Simon Hammond who is running things next door right now but

That’s it from me for this week uh join Annie Tomorrow on the 10day trend goodbye