This is an in-depth Met Office UK Weather forecast for the next week and beyond.
What happened to the high pressure that was forecast to remain close to the UK during the rest of April? In this Deep Dive, Met Office meteorologist Aidan McGivern looks back at a 10-Day Trend from the 17th April and assesses where it went wrong and where it went right.
What happened to the high pressure that was forecast to remain close to the UK during the rest of April? In this Deep Dive, Met Office meteorologist Aidan McGivern looks back at a 10-Day Trend from the 17th April and assesses where it went wrong and where it went right.
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00:00 Hello and welcome to the Met Office Deep Dive, our regular in-depth look at all things meteorological.
00:06 We do these every Tuesday.
00:08 If you enjoy them, don't forget to hit subscribe, send us a like and send us a comment.
00:12 You never know, one of your comments might appear on screen like this one.
00:18 What happened to the high pressure system you confidently forecast to be sitting over
00:22 us at the end of April a couple of weeks ago?
00:25 I got this comment a few days ago and in this week's Deep Dive I'm going to be looking back
00:32 at the forecast that we did at one particular forecast, the 10-day trend from the 17th of
00:38 April, in which we talked about a slow-moving area of high pressure.
00:43 And I'm going to be looking at where that forecast went right, where it went wrong,
00:47 because here at the Met Office, of course, we do forecasts all the time and people ask
00:53 us whether we look back at previous forecasts and assess how accurate they were.
00:58 So that's exactly what I'm going to be doing in this week's Deep Dive.
01:02 Now, in terms of talking about whether a forecast is right or wrong, it's important to remember
01:09 that the UK is a small group of islands at the edge of a continent and at the edge of
01:13 an ocean.
01:14 The weather day to day and from place to place is highly messy, particularly, as I'll explain
01:21 in a moment, at this time of year.
01:24 And a single statement like 'sitting over us at the end of April', 'high pressure sitting
01:30 over us at the end of April', doesn't really do it justice.
01:33 And actually, if you look back at our forecast, and I know I'm preaching to the perverted
01:37 here, it's these longer form forecasts such as the Deep Dive and the 10-day trend in which
01:43 we are discussing the nuances and the day to day weather of the UK, the uncertainties,
01:50 the regional variation, the microclimates, we try and do that in these forecasts because
01:55 simple statements like that don't really sum up the weather for many people as well as
02:01 they could.
02:02 And I'll give you this week as an example.
02:05 Now, this is a screenshot from the week ahead forecast that I did yesterday.
02:10 We've got double Aden now on screen.
02:12 I apologise for that.
02:13 You're going to see more of that in the next 10-15 minutes or so.
02:18 Bear with me, hope it's not too distracting.
02:21 So this is a screenshot from the week ahead forecast that I did on Monday.
02:27 And this shows the temperature across the UK on Thursday.
02:30 And the main message from the week ahead is that it's going to be warmer in many places
02:35 this week compared with the last couple of weeks.
02:38 Temperatures into the high teens, perhaps low 20s across some parts of central and north-western
02:44 UK, western Scotland, northern Ireland, into the Midlands and perhaps parts of East Anglia
02:49 for example.
02:51 But as I explained in the week ahead forecast, not everywhere will be warm.
02:55 As you can see, eastern Scotland, north-east England, a cool breeze from the North Sea,
03:00 some low cloud as well, just 11 or 12 Celsius here.
03:03 Likewise towards the south-west, cooler here because of cloud and outbreaks of rain that
03:08 we're expecting on Thursday.
03:10 So yes, warming up, but no heatwave.
03:15 And not everywhere will experience the warmth.
03:18 And it's not going to be dry or cloudy or sunny.
03:22 Everywhere there'll be mixed weather up and down the country each day this week, which
03:26 is always the case, which is explained in the week ahead forecast, five or six minutes
03:30 long.
03:31 And we go into those kinds of details in these longer forecasts.
03:33 That's the beauty of these longer forecasts as opposed to these kinds of things, these
03:39 kinds of headlines that give a single statement about the UK weather and, well, use a bit
03:46 of hyperbole.
03:47 And not everything is incorrect about this statement.
03:50 Maps show exact date, 22 Celsius mini heatwave arrives with country set to bathe in sunshine,
03:57 but it is overdoing it a bit.
03:58 Yeah, could see 22 Celsius in one or two spots, but as I mentioned, other places will get
04:03 12 degrees.
04:04 And if you're living in one of those places where you've got 12 degrees, you'll be thinking
04:08 that forecast was entirely wrong because that's perhaps all you've seen.
04:12 You haven't watched the Met Office week ahead forecast where we explain those regional variations.
04:18 Likewise, mini heatwave, no such thing.
04:20 And country set to bathe in sunshine, lots of places this week are going to be cloudy
04:25 with some outbreaks of rain, particularly towards the south.
04:29 Another headline here, UK to sizzle hotter than Ibiza with half of England set for mini
04:34 heatwave.
04:35 That's that mini heatwave again.
04:37 Let's look at the hotter than Ibiza.
04:38 Well, I've chosen one of the warmer spots this week, Cambridge.
04:43 And what this forecast for Cambridge shows is that on Thursday, the 2nd of May, 21 degrees,
04:50 but with some cloud and some light patchy rain.
04:52 Ibiza, meanwhile, on the same day, 18 degrees, but with lots of sunshine.
04:56 In fact, it's sunny all week in Ibiza.
04:58 So yeah, if you're looking for some warm sunshine, Ibiza is probably still a better bet than
05:04 many parts of the UK.
05:06 And this kind of headline where it says UK set to sizzle hotter than Ibiza gives a slightly
05:13 wrong impression of what we are actually expecting this week, which, as I mentioned, is covered
05:18 in the week ahead forecast.
05:19 So let's go back to what we forecast in response to this comment, what happened to the high
05:26 pressure system you confidently forecast to be sitting over us at the end of April a couple
05:32 of weeks ago.
05:34 And I'm going to argue that we didn't actually forecast an area of high pressure to be sitting
05:38 over the UK at the end of April, the same as we didn't forecast that the UK would be
05:45 in for a sweltering April, which was suggested by Andrew Neil on Twitter, established journalist,
05:53 of course.
05:54 But he got this wrong.
05:55 The Met Office in general forecast that we'd be in for a sweltering April in the UK.
06:02 We didn't.
06:03 There was no forecast of that.
06:05 There might have been tabloid headlines that suggested that, but the UK Met Office didn't
06:10 forecast that in anything.
06:12 You can look back at our previous forecasts on YouTube.
06:15 They're all available, including this one.
06:19 This is the forecast in question, I think, that a lot of people are referencing in terms
06:23 of we forecast high pressure for the rest of April and the title there, slow moving
06:29 high pressure.
06:30 So let's take a look at what happened.
06:31 This is from the 17th of April.
06:33 You can go back and watch it yourself and make up your own mind.
06:36 But now, not saying that we get every forecast right.
06:39 There were things in this forecast that went well.
06:42 There were things that didn't go so well.
06:44 And so that's what I want to take a look at.
06:46 No forecast is perfect, but likewise, no forecast is entirely wrong either.
06:50 There are shades of grey in between, just as is always the case with the UK.
06:56 It's rarely sunny and sizzling everywhere, nor is it rarely raining everywhere either.
07:03 There are huge variations up and down the country, and it gets very messy when you look
07:08 at the regional and day-to-day variability.
07:12 But in this particular forecast, I titled it less wet but not entirely dry.
07:18 So that's the first thing.
07:19 I wasn't going with an entirely dry rest of April.
07:24 The thing I didn't forecast in this was a heatwave in April.
07:27 I wasn't forecasting widespread sunny skies either.
07:30 But what I did predict was that higher pressure would build through the weekend, the upcoming
07:35 weekend.
07:36 So that was the 20th, 21st of April.
07:40 And then this was what I showed for the following Tuesday.
07:43 So Tuesday, the 23rd of April, most likely weather pattern, higher pressure to the northwest
07:50 of the UK with this cool northeasterly airflow, and that higher pressure gradually moving
07:56 towards the northwest of the UK.
07:58 So let's take a look at what actually happened on Tuesday, the 23rd of April.
08:02 And this was the actual weather chart from that day.
08:05 Higher pressure to the northwest of the UK, cool northeasterly airflow.
08:09 So very similar.
08:10 This was a forecast for six days' time.
08:13 Now going further ahead, of course, forecasts get more uncertain.
08:17 And as a result, for the Thursday, 25th of April, I showed a few scenarios here.
08:23 But the most likely one showed that higher pressure edging a little further towards Greenland,
08:28 further to the northwest of the UK.
08:30 Again, this cool northeasterly airflow.
08:34 What actually happened, something a little different, something quite different maybe
08:40 if we're going to argue.
08:42 But it shows that higher pressure, this is the actual weather chart for the day, higher
08:48 pressure further west than forecast.
08:53 So further over here.
08:54 And as a result, more of a low-pressure influence.
08:58 For example, this low over here getting closer to the UK.
09:01 So really it's on the Thursday, the day eight in the forecast, where things started to differ
09:09 in terms of the forecast and the reality.
09:12 So what happened?
09:13 Well, if you watch that 10-day trend, I talked about wave number five pattern, which is a
09:20 very slow-moving weather pattern that we often see in the spring and summer in the Northern
09:25 Hemisphere.
09:26 And let me explain just briefly what that means.
09:30 Now we're looking at the Northern Hemisphere here from the North Pole view.
09:35 And what it's showing here is North America, just to get your bearings, North America here.
09:40 We've got Europe, we've got the UK here, and we've got Asia here.
09:46 And what I talked about in this 10-day trend was this was the most likely setup across
09:52 the Northern Hemisphere for the following week.
09:56 And the orange areas show where, well, they roughly correspond to areas of higher pressure,
10:03 and the blue areas roughly correspond to areas of low pressure.
10:07 Geologically speaking, this is a chart that shows the 500 hectopascal geopotential anomaly.
10:12 So it's basically the pressure anomaly way up in the atmosphere, about 18,000 feet above
10:18 sea level.
10:19 But just for the sake of keeping things simple as they could possibly be, we're going to
10:24 talk about areas of high pressure and low pressure corresponding with the oranges and
10:28 blues.
10:29 And so in this forecast, I talked about this area of high pressure, or this ridge, becoming
10:34 established just to the east of the Rockies.
10:38 And then downstream of that, we've got this ridge established itself just to the east
10:44 of the Urals, and another one just to the east of the Himalayas.
10:49 And when those ridges become established, they can be difficult to shift because you
10:54 get this self-positive feedback system whereby the land heats up because of high pressure.
11:02 That helps to sustain the high pressure and so on.
11:04 So those three significant orange areas become self-sustaining.
11:08 In between, you get an orange area there across the Atlantic, so another ridge, and another
11:13 one there over the Pacific.
11:15 So five ridges.
11:17 And the important thing to note is that the more of these ridges, the more wriggly the
11:23 jet stream is, and the more slow-moving the weather patterns become.
11:28 And so I talked about this wave number five pattern, and I even showed the forecast for
11:33 two weeks' time, which was how that could change or stay the same for the start of May.
11:41 So this was a forecast for the two weeks' time, and it showed the same sort of thing.
11:46 A ridge over North America there, a ridge just east of the Urals, and a ridge just across
11:51 northeastern parts of Asia.
11:53 So a very slow-moving system, with a ridge to the northwest of the UK, and low pressure
12:01 there across parts of the Atlantic and Central Asia and so on.
12:07 So the idea was that this self-sustaining pattern would mostly remain the same for the
12:14 rest of April.
12:15 And I think that's where the impression came from that we'd have high pressure for the
12:18 rest of April.
12:20 However, what happened in reality?
12:24 So this is the forecast I just showed for two weeks ahead, which was for the start of
12:29 May.
12:30 This is how things are now looking for the start of May.
12:33 This is this week's weather pattern.
12:35 And if I just step out of the way, it's, well, potential there for a spot the difference
12:40 game.
12:41 Now, there are some things that are very similar.
12:44 We've still got that ridge to the east of the Rockies, across northern parts of USA
12:51 into Canada.
12:52 We've also got a corresponding ridge across parts of Scandinavia on both images, and another
13:00 ridge there across eastern parts of Asia.
13:03 So we've got those big ridges still more or less in the same place.
13:08 But one of the biggest differences when you look at these two charts is what's happening
13:13 to the UK's weather.
13:15 Instead of having this ridge to the northwest of the UK, we've got quite a strong low pressure
13:23 to the southwest of the UK.
13:25 Still higher pressure out towards Iceland and just south of Greenland there, but this
13:30 strong low over the UK.
13:34 What seems to have happened is that when you look at this big, broader view, the weather
13:42 patterns over the UK have shifted westwards a little, and we've ended up stuck under low
13:49 pressure to the southwest of the UK for this week in particular.
13:55 And that's really important because, for starters, we were forecasting high pressure to be sitting
14:03 to the west or northwest of the UK rather than sitting over the UK for the end of April.
14:10 But what's gone wrong is that that higher pressure has ended up further away from the
14:17 UK, out further west, and we've had lower pressure more influential over our weather
14:23 than we were expecting.
14:25 So the bigger picture across the globe hasn't varied too much from what was initially predicted
14:33 in terms of that very wriggly, that very stuck pattern, basically a series of ridges and
14:42 troughs that are stuck.
14:45 We've been stuck in a rut, but it's been a slightly different rut to what we were expecting
14:51 back in the middle of April in terms of lower pressure being more influential across the
14:56 UK and that higher pressure influence further west in the Atlantic.
15:01 And this comes down to why it's particularly difficult at this time of year to get longer
15:06 range forecasts accurate for a small country like the UK.
15:11 And it's because you can get very close a week or two in advance to this broader scale
15:19 pattern across the northern hemisphere, but if there's a few hundred mile shift in where
15:27 the lows and highs are sitting, then you end up stuck with a very different weather pattern.
15:33 Now if all of this at a different time of year was moving through much more quickly,
15:38 then that difference of 100 or 300 miles wouldn't make too much of a difference because it would
15:44 just be a timing issue.
15:46 It would all be moving through.
15:48 It's the fact that these weather patterns become stuck.
15:52 So if you're expecting to be stuck under an area of high pressure and it moves slightly
15:57 so that you're stuck under an area of low pressure, you get very different weather as
16:00 a consequence, even if the broader pattern is roughly similar to what you were predicting.
16:07 And why is it a difficult situation at this time of year?
16:12 Because going into May, the jet stream is particularly weak and a weak jet stream ends
16:19 up more wavy and these weather patterns get stuck more often.
16:23 This shows the average speed of the jet stream in May.
16:27 There's the UK, there's Europe, here it is coming out in North America and it is weaker
16:35 in May than at any month of the year.
16:38 It then gets stronger every month, June to July to August, September, October and then
16:42 it peaks in January.
16:45 This is the January jet stream.
16:48 So the January jet stream is particularly strong and that's to do with temperature contrasts
16:53 across the Northern Hemisphere.
16:54 So through the autumn and into the winter you get colder and colder air to the north.
16:59 You get a strengthened polar vortex.
17:02 We've covered those in previous deep dives and 10-day trends.
17:06 The stratospheric polar vortex is very strong circulation that sits above the North Pole.
17:13 And with these strong west to east circulations, you're looking at much bigger weather patterns
17:20 on a much bigger scale through the autumn and winter.
17:24 And it's much more easy to see these large areas of low pressure coming and going.
17:29 You can predict large areas of low pressure seven, eight days ahead.
17:33 Whereas when the jet stream is weak and tenuous as it is in May, it is much more difficult
17:40 to predict, well, areas of low pressure that kind of just wobble around and they're not
17:48 steered particularly strongly in any one direction.
17:51 So this week is a great example of that.
17:54 This is the current state of the jet stream and the areas of low pressure and high pressure
17:59 across the Atlantic in Europe.
18:02 And as you can see, the jet stream is quite weak, but it's very wavy.
18:07 So we've got this area of low pressure off the coast of Canada.
18:11 We've got this other one.
18:12 And the jet stream is looping quite wildly around these areas of low pressure.
18:18 And what happens if we go forward over the next few days is that these areas of low pressure
18:25 cut off from the main flow of the jet stream.
18:28 So they become these closed circulations.
18:31 And this one in particular to the south of the UK, as you can see, the jet stream winds
18:37 are circulating around it.
18:39 That allows later Wednesday some warmer air to be pushed into many parts of the UK, as
18:46 I mentioned earlier.
18:47 Not everyone will be affected by the warmth, but there will be some warmer air arriving
18:52 from the central and southern parts of the Europe around that area of low pressure.
18:56 But along this weather front that is sitting across southern and southwestern parts of
19:00 the country, there'll also be increasingly...
19:03 Let's put the rain on.
19:09 ...some thundery showers developing later Wednesday and into Thursday morning.
19:14 And that low pressure, because of the closed circulation of it, just wobbles about to the
19:23 south of the UK for a few days before the next couple of lows turn up into Friday.
19:32 These two lows, but again, these are associated with this closed circulation.
19:38 So it's a bit like a spinning top that starts wobbling and meandering all over the place.
19:43 It's difficult to put an exact steer on where it will end up.
19:48 These areas of low pressure that are detached from a weak jet stream are much more difficult
19:54 to predict than if you've got this strong, powerful west to east jet stream in a straight
19:59 line across the Atlantic, similar to how things often happen in the autumn and winter.
20:06 But that then leads to a lot of uncertainty heading into the Bank Holiday weekend.
20:11 So we've got the thunderstorms in the southwest and south on Thursday morning.
20:16 The front associated with those moves north to bring some outbreaks of rain across central
20:20 parts on Friday.
20:22 And then skipping ahead, there's a brief lull as we go into the start of the weekend.
20:29 Before these areas of low pressure start to influence things through the weekend.
20:34 Now this is the start of Saturday and there's a little ridge across many parts of the UK,
20:39 but it looks likely that these areas of low pressure will eventually come to the UK, or
20:44 at least sweep in close to the UK.
20:48 So through the weekend, some drier weather around at first, but increasingly these lows
20:54 will bring areas of cloud, outbreaks of rain or showers in with them, and eventually some
21:00 warmer air.
21:02 But because these lows are just kind of circulating on their own with this closed circulation
21:08 around them, they're kind of just wobbling about at the moment, which is why, once again,
21:13 these weather patterns are stuck, they're slow moving, and it is difficult this time
21:18 of year to get a precise handle on them.
21:21 But after that, just to leave you with some positive news, after that, let's take a look
21:28 at one of these graphics again.
21:30 Again we've got the polar view here, we've got Greenland there, we've got the UK there,
21:34 and this is the most likely pattern across the Northern Hemisphere into next week.
21:41 And things have shifted.
21:43 We're not seeing the same sort of pattern across the Northern Hemisphere as we've seen
21:46 over the last couple of weeks.
21:49 We're starting to see the ridges to the east of the Urals and to the east of the Rockies
21:56 decline.
21:57 Things get moving, things become unstuck, but as a result the UK ends up with higher
22:02 pressure.
22:04 And although there's a lot of uncertainty at the moment in terms of exactly where that
22:09 higher pressure could end up, as is often the case, and that of course would be really
22:14 important in terms of the day-to-day and regional variability and so on, this shows the most
22:19 likely weather pattern for next Tuesday, and it's got that higher pressure moving in from
22:25 the southwest.
22:26 And it looks most likely that that higher pressure will be close to the UK, most likely
22:31 to the west or southwest through next week.
22:35 And just to rule out some things that I'm not predicting, not predicting a heatwave,
22:39 not predicting that it will be widespread sunny skies and dry weather everywhere, but
22:45 it does look more likely than not that that higher pressure will move in from the southwest
22:50 and sit somewhere to the west or southwest of the UK through next week.
22:54 And so there's the potential for the weekend after the bank holiday weekend to be drier
22:59 and more settled, following perhaps a lot of cloud and showery rain at times for this
23:05 particular bank holiday weekend.
23:08 I'm hoping that all made sense, but I just wanted to, you know, address a comment that
23:15 we've had a few times recently about, you know, our predictions of higher pressure over
23:19 the UK for the rest of the month.
23:21 And without trying to be too defensive, talk about the challenges and limitations of our
23:28 longer range forecasts, why sometimes they're right, why sometimes they're wrong, and why
23:32 they're never completely wrong and they're never completely right.
23:36 There's always something, a shade of grey in between.
23:39 So hoping you enjoyed this one.
23:41 Let us know in the comments if you've got any other feedback, if you've got any other
23:45 requests for future deep dives.
23:46 Love to hear from you.
23:48 But that's all from me for this week.
23:49 Bye bye.
23:49 Bye-bye.