There have been one or two stories in the press today saying we’re in for another washout summer, which would rightly inspire collective misery across the country.
However, it’s a far too early to be writing off any chance of a decent summer season – after all, it doesn’t officially start (for us meteorologists) for more than two weeks (on 1 June).
It appears the news stories are borne out of the current position of the jet stream, a band of fast moving westerly winds high up in the atmosphere. But why is this important?
A quick Jet stream explainer
The jet stream tends to guide the generally wet and windy weather systems which come in off the Atlantic. So, if it’s over us or just to the south, we tend to get a lot of wet and windy weather – which is what we expect through winter.
If the jet is to the north of us, it guides that changeable weather to the north to give us more settled conditions – which is what we expect in the summer.
(You can read a bit more about the jet stream, how it impacted our weather last year, and any potential connections to climate change in a blog story we wrote last year).
What’s going on now?
Right now the jet stream is sitting to the south of the country and it is influencing the unsettled weather we are seeing at the moment.
Forecast chart showing position of the jet stream at midday on 13 May 2013
It’s fair to say that this is roughly the position it was in for extended periods during the exceptionally wet weather that we saw last year, particularly in June.
Crucially, however, the jet stream does move around quite a bit and it can change its track significantly in just a few days. So the current position of the jet stream does not mean that it’s stuck in that position.
Looking ahead
Much like our weather, it’s a huge challenge to predict the exact track of the jet stream more than five or six days ahead, so there’s still a great deal to play for in the outlook for our summer.
In short, it’s far too early to write-off summer 2013 based on the current position of the jet stream.
The UK is set to see some warmer temperatures this weekend as the colder than average weather seen so far this April eases its grip.
Temperatures have been steadily climbing since the exceptionally cold weather towards the start of the month, with today through to Friday set to see double-digit figures for many places.
On Saturday temperatures will be generally between 11 and 13 °C, feeling much milder than recent days. However, the weather will be wet, fairly windy and unsettled for many parts – with the best of any drier and brighter weather in the south and east.
Forecast chart for midday Saturday shows low pressure moving in from the Atlantic to bring mild but wet and windy weather for most of the country. High pressure still dominates in the south and east, bringing the best of any drier and brighter weather.
Warmest day of the year so far
Sunday looks set to be the warmest day of 2013 so far, with temperatures expected to be widely in the mid-teens Celsius. While the weather will be slightly more settled than Saturday, many places will see cloudy and breezy conditions with a risk of some light showers.
Once again the south and east will see the best of the weather, with drier and brighter conditions and temperatures of 15-18 °C – with a possibility that some isolated spots could reach around 20 °C.
Leading in to next week temperatures look set to cool slightly, but remain around average for the time of year.
Jet stream shift brings milder weather
The reason for the shift away from the colder weather is the re-alignment of the jet stream, a band of fast moving westerly winds high up in the atmosphere which tends to guide Atlantic weather systems. It’s these weather systems that bring us the mild and unsettled weather we normally expect at this time of year.
During the prolonged cold conditions the jet stream tracked far to the south of the UK, guiding those mild weather systems towards the Mediterranean. The UK, meanwhile, saw an easterly flow – bringing in cold conditions from the cold winter climes of north-east Europe.
Now the jet stream has started to shift its track, moving north to a position more in line with what we’d expect at this time of year. This means we expect to see milder, but also more unsettled weather coming in from the Atlantic over the coming week or so.
You may have heard talk of the UK possibly seeing some colder weather next week and that ‘things going on’ in the upper atmosphere may be playing a part.
The ‘thing’ happening in the atmosphere is known as Sudden Stratospheric Warming (SSW). When it does happen, it attracts a lot of interest in the UK because it is sometimes linked to the onset of cold weather in winter.
Here we shed a little bit more light on the phenomenon.
What is an SSW?
The term SSW refers to what we observe – rapid warming (up to about 50 °C in just a couple of days) in the stratosphere, between 10 km and 50 km up.
You may have heard of the jet stream which helps to steer Atlantic weather systems towards the UK. Well there are other jet streams high up in our atmosphere in both the northern and southern hemisphere which circumnavigate the Earth from west to east. One of these, the Polar Night Jet, circles the Arctic.
Sometimes the usual westerly flow can be disrupted by natural weather patterns or disturbances in the lower part of the atmosphere, such as a large area of high pressure in the northern hemisphere. This causes the Polar Jet to wobble and these wobbles, or waves, break just like waves on the beach. When they break they can be strong enough to weaken or even reverse the westerly winds and swing them to easterlies. As this happens, air in the stratosphere starts to collapse in to the polar cap and compress. As it compresses it warms, hence the stratospheric warming.
How does it move down through the atmosphere?
As it turns out, waves can only move around the Earth’s atmosphere in westerly winds. Fluctuations in our weather send waves up through the atmosphere to the easterly winds in the stratosphere, where they travel no further, and instead break and reinforce the easterly winds, bringing the easterlies lower. This pattern continues until the easterlies have moved down to the troposphere – the lowest part of the atmosphere where our weather is.
It can take anything from a few days to a few weeks for this process to take place.
What impact does this have on the UK?
We normally expect our weather to come in from the west – with a flow of relatively mild air coming in off the Atlantic.
When an SSW brings easterly winds this tends to alter our weather patterns slightly, weakening areas of low pressure and moving our jet stream further south. This leads to high pressure over the North Atlantic, ‘blocking’ that flow of mild Atlantic air and dragging in cold air from the continent to the east. Exactly how cold it might be depends on the details of where the air comes from.
SSWs don’t always result in this outcome – but a cold snap follows more often than not, so the SSW greatly increases the risk of wintry weather.
Can we predict these events in advance?
Currently we can reliably predict individual SSWs about a week in advance, and we can detect them early on with satellite and other observations. This means we have some time to see how they develop and may impact our future weather.
The Met Office will continue to monitor the current event and, as ever, will keep everyone up to date on any periods of cold weather through our forecasts and warnings.
The UK has seen some very wet and windy weather since the early hours of Sunday morning and that is set to continue in places for the next couple of days – but what has brought these disruptive conditions?
As is the norm, a low pressure which moved in from the Atlantic is to blame, bringing bands of heavy rain and strong winds (as you can see from the tightly packed isobars on the image below).
Forecast synoptic chart for 12:00 on Tuesday 25 September showing the low pressure over the UK.
Despite some reports to the contrary, this low is not what’s left of tropical storm Nadine, but is a completely separate entity – the remnants of Nadine are currently sitting close to the Azores far to the south of the UK.
Some warm tropical air dragged over by Nadine was sucked up into the low pressure, however, giving it some extra energy – essentially increasing its potential for strong winds and rain.
This isn’t unusual though, virtually every weather system we see will have had some input of sub-tropical air during its evolution.
There are two more notable features of this low pressure, however. Firstly, it has remained unusually active as it sits over the UK, leading to the strong winds and heavy rain.
This is due to the fact that, as the low pressure system moved north across the UK, it has also pulled in cooler polar air from the north. This cold air has come up against the warm sub-tropical air, re-invigorating the depression and allowing it to continue to deepen over the UK.
Secondly this low pressure is lingering for longer than we would often see. The reason for this is down to the position of the jet stream, a narrow band of fast moving winds high up in the atmosphere which ‘steers’ weather systems.
Normally the jet stream runs fairly directly from east to west and pushes weather systems through quite quickly. Similar to earlier this year, the steering flow of the jet stream is currently in a meandering mood – looking much like a river, curving north and south as it heads west across the Atlantic (we call this a meridional flow, with the more linear west to east flow being called a zonal flow).
When it meanders, weather systems can get stuck in the ‘peaks and troughs’ it creates – so they get stalled in one spot rather than moving on. The below picture of the jet stream as at 12:00 today shows with the steering flow of the jet over France and the UK in the resulting trough.
The weather system will move on during the day on Wednesday, but that still means the UK will have had three days of unsettled weather.
Like our weather, the jet stream can change rapidly and it’s difficult to forecast precisely what it will do for more than a few days ahead – so there’s no reason to expect it to continue to behave in this way and there’s plenty still to play for in terms of our autumn weather.
The low pressure system that is affecting the UK is unusually deep for September, with the lowest air pressure recorded so far being 973mbs. To find a similarly intense low pressure system in September you need to go back to 1981, when pressures below 970mb were reported over a period of 24 hours. Like this week’s, this low pressure system brought unsettled weather as it crossed the British Isles – starting in the Isle of Man and tracking east and then north to cover Cumbria, Northumberland, eastern Scotland, Orkney and Shetland. There have been other times when pressures as low as 970mb were recorded in some parts of the British Isles in September, such as in the Isles of Scilly in 1995 and others across the far north or west of Scotland or Northern Ireland, however none were as widespread as the low that pushed across the UK in 1981.
After weeks of heavy rain across parts of the UK, conditions are set to improve for many areas this weekend.
More heavy showers will affect some parts during the rest of the week, but by Saturday most areas will see drier weather with any showers few and far between. Temperatures will reach the low 20s Celsius.
Sunday will see the improved weather continue for a large part of England and Wales, with mostly dry weather and bright or sunny spells expected. However, the north and west of the UK, can expect some rain – which will be heavy in places – with strong winds.
Drier weather for many, with rain where it’s needed
Martin Young, Chief Forecaster at the Met Office, said: “As we move towards the weekend we will see a return to a more normal summer weather pattern for the UK. This will bring dry and bright conditions to southern parts over the weekend, and some much needed rainfall to the far north west of Scotland – where it has been exceptionally dry.”
Over the next few days, the jet stream is expected to move to its more usual position to the north of the UK, guiding rain-bearing low pressure systems from the Atlantic away from the country. This is why we expect to see a move to more normal summer conditions, with the south and east seeing the best of any drier and brighter conditions.
The above picture shows the position of the jet stream on 18 July 2012.
The forecast for 23 July 2012 shows the jet stream much further to the north.
Looking to the Olympics
There is understandably a huge amount of interest in what the weather will be doing at the end of next week in time for the Opening Ceremony of the Olympic Games. However, it’s still a little early to give a detailed forecast for the Olympic Stadium for the big opening event.
Sandie Dawe, Chief Executive at VisitBritain said: “The weather is a peculiarly British obsession, our international visitors come all year round for our temperate climate and enjoy a dash of unpredictability. Sunshine will help to get us all in the party mood, as we show the warmth of our welcome and the British know how to host not just a great Games but a great party too. Come rain or shine – Britain is the place to be in 2012.”
As ever, we’ll be working round the clock to make sure everyone – from the UK public, to athletes, coaches, and the organisers of the Games – has the very latest picture of what the weather has in store. For the latest information, keep up to date with our online forecasts and warnings.
Provisional mid-month statistics from the Met Office show that, after an exceptionally wet and gloomy June, the first half of July has continued the disappointing theme.
Normally at the halfway stage of a month we’d expect sunshine and rainfall to be somewhere around 50% of the average for the whole month, but the figures show a very different story.
Looking at the UK, we’ve already had more than the average rainfall we’d expect for the entire month – with 71.8mm (103% of the 1971-2000 average) falling up to the 15th of the month.
Within that there are some big variations, however. Northern Ireland and Scotland have seen close to normal levels of rain, with 35.9mm (49%) and 56.4mm (61%) respectively. England and Wales have seen much more relative to their monthly averages, with 81.2mm (150%) and 95.8mm (122%) respectively.
Looking in even more detail at Scotland, however, you can see bigger variations in rainfall patterns: Mid Lothianshire in south east Scotland has seen 105.8mm (167%) of rain, while the Western Isles in the far north west of Scotland have seen just 10mm (10%).
This illustrates how rainfall patterns have really bucked their usual trend, with the far north west – which is usually one of the wettest places in the UK – continuing to be one of the driest areas. This is partly due to the position of the jet stream, as discussed in previous articles.
Looking at sunshine, there is a more consistent pattern across the regions. The UK has seen 45.5 hours of sunshine, just 26% of the average for the whole month, so around half what we would normally expect to see by this time. So far, England, Wales, Scotland and Northern Ireland have all seen about a quarter to a third of their monthly quota – making for a very dull month thus far.
Temperatures have also been on the disappointing side – with the UK seeing an average temperature so far of 13.7 deg C, which is 1.1 deg C below the monthly average.
With figures only going up to the 15th of the month, it is of course impossible to say how the month could finish overall. It’s certainly too early to say whether this month will be a record-breaker, as that will depend on how the second half of the month plays out.
While more rain is expected in parts of the UK today there will also be some dry and sunny weather too. The good news is that the weekend is set to bring drier and brighter weather to many parts of England and Wales – while the far north west of Scotland is likely to see some much-needed rain.
Many news and weather articles, including our own blog, have reported recently that the UK’s wet summer is caused by the jet stream being situated further south than usual for this time of year.
However, some have questioned whether there is a link between this and the hurricane activity seen so far in the Atlantic.
So how is the Atlantic hurricane season shaping up and has it been influenced by the weather at higher latitudes?
Atlantic hurricane season so far
The Atlantic hurricane season officially started on 1 June, but in 2012 it began early with two tropical storms, Alberto and Beryl, forming in May.
After a break of almost three weeks there were two more storms, Chris and Debby, in the latter part of June, with Chris becoming the first hurricane of the season.
Since then there has been another lull of almost three weeks in Atlantic tropical storm activity.
Has the Atlantic hurricane season been unusual?
In mid-July it is far too soon to make judgements about how unusual a hurricane season is likely to be based on activity so far. However, it was certainly unusual to see four tropical storms before the end of June as this has never been observed before in over 150 years of records.
The recent quiet spell in Atlantic hurricane activity is by no means unusual. Atlantic hurricane seasons are often characterised by bursts of activity followed by quiet spells.
The peak of activity usually runs from the second half of August through to October. Even in some years which turned out to be very active, early season activity was low.
For example, by this time in 2010 and 2011 there had been just one tropical storm. Each of these seasons ended up yielding a total of 19 storms.
Is the jet stream involved?
The jet stream which affects UK weather is much further north than where the majority of tropical storms develop and hence has no direct impact on their formation.
However, once a tropical storm develops and starts to move to higher latitudes the jet stream can influence where it ends up. For example, in September 2011 as Hurricane Katia moved northwards in the Atlantic it met a powerful part of the jet stream and was swept eastwards as a strong ‘post-tropical’ storm which brought stormy conditions to the northern UK.
Will it be an active hurricane season?
The Met Office seasonal forecast for Atlantic tropical storm activity issued in May predicted a near-average season with the most likely number of storms in the June to November period being 10. Since June two tropical storms have occurred so far (the two May storms fall outside of this prediction period).
One of the major influences on the season’s activity includes the existence of La Niña or El Niño conditions (natural cycles which affect sea temperatures in the equatorial east Pacific).
This would suppress hurricane activity in the Atlantic by disrupting the airflow over the regions where they usually develop.
However, it is worth remembering that it is 20 years since the quiet El Niño influenced Atlantic hurricane season of 1992. Despite being a quiet season overall, it still managed to spawn the deadly and powerful Hurricane Andrew which brought devastation to parts of Miami in Florida.
For more information on tropical cyclones worldwide visit our web pages or follow @metofficestorms on Twitter.
In fact, barring a warm and dry spell towards the end of May, the weather has been persistently dull and wet since April – which was also the wettest in records dating back to 1910.
Our weather here in the UK is complex and determined by many different factors, including the position of the jet stream.
This is the narrow band of fast moving winds which runs from west to east across the Atlantic high up in the atmosphere.
How does the jet stream affect UK weather?
Weather (or low pressure) systems bearing rain and unsettled conditions move across the Atlantic on a regular basis. The jet stream guides these systems, so its position is important for UK weather.
In summer, we would expect the jet stream to be north of the UK – dragging those weather systems away from our shores to give us relatively settled weather.
So far this year it has been to the south of the UK, guiding those systems straight to us. This is the position we’d normally expect the jet stream to be in during winter, when we are more accustomed to these wet conditions.
So why is the jet stream stuck so far south?
The jet stream, like our weather, is subject to natural variability – that is the random nature of our weather which means it is different from week, month or year to the next.
We expect it to move around and it has moved to the south of the UK in summertime many times before in the past. It has, however, been particularly persistent in holding that position this year – hence the prolonged unsettled weather.
This could be due to natural variability – a bad run of coincidence, if you will – but climate scientists are conducting ongoing research to see if there are other factors at play.
Changes in sea surface temperatures due to natural cycles may be playing a part, but there is more research to be done before anyone can establish how big a role they play.
Research has also suggested that reducing amounts of Arctic sea-ice could be affecting weather patterns, but more research needs to be done to confirm this link. Currently Arctic sea-ice is at a record low for this time of year.
Is climate change playing a role?
In the long term, most climate models project drier UK summers – but it is possible there could be other influences of a changing climate which could override that signal on shorter timescales.
If low levels of Arctic sea ice were found to be affecting the track of the jet stream, for example, this could be seen as linked to the warming of our climate – but this is currently an unknown.
However, it is very difficult to do this type of analysis with such highly variable rainfall events, so it may take many years before we could confirm how the odds of this summer’s wet weather happening have been altered by greenhouse gases.
We do know that the warmer air is, the more moisture it can hold. We have seen a global temperature increase of more than 0.7 deg C (since pre-industrial times) and this has led to an increase of about 4-5% in atmospheric moisture.
This means that when we do get unusual weather patterns such as we’re seeing now, it’s likely there will be more rainfall than the same patterns might have produced in the past. In short, it seems when it does rain, it is heavier.
Taking into account this effect, perhaps it’s not surprising new records like those for this April and June are being set. In fact, the wettest July and November in the records dating back to 1910 happened in 2009, making a total of four record wettest months in the past four years. If wet months occurred randomly, we would expect only one record to have been broken since 2006.
For temperature, April (2011), May (2008), July (2006), September (2006) are all recent warmest records. Again, this is much more frequent than would be expected if temperatures were not rising.
What about elsewhere in the world?
Looking at the bigger picture, the jet stream may be having an impact elsewhere in the northern hemisphere.
It is stuck in a persistent pattern of waves, with one of these ‘waves’ taking it to the south of the UK.
Figure shows upper level wind patterns in early July 2012, with the northern hemisphere jet stream marked with arrows.
The figure gives a picture of the upper level winds for the first week in July, but the wavy nature of the jet stream has been persistent throughout June.
Meanders of the jet north and south can be seen across the US, the Atlantic and into Europe.
While the wet weather in the UK has been under a southward meander of the jet stream, the recent Russian floods near the Black Sea appear to have been beneath the next trough to the east.
The US heat wave is also beneath a northward meander and a ridge of high pressure.
Provisional Met Office figures for June show double the average amount of rain has fallen, making it the wettest June since records began in 1910.
This is the second record breaking month of rainfall this year, with April also topping the rankings. The period from April to June is also the wettest recorded for the UK.
It is also the second dullest June on record with just 119.2 hours of sunshine, narrowly missing out on the record of 115.4 hours set in 1987. To complete the disappointing picture, it has also been the coolest June since 1991 with a mean temperature of 12.3 deg C.
Unsettled weather has never been from the UK during the past three months, with only the latter half of May seeing a spell of prolonged fine weather.
Movements in the track of the jet stream, a narrow band of fast flowing westerly winds high in the atmosphere, have contributed to the weather we have seen.
This June has seen periods of heavy and prolonged rain, as well as short but exceptionally heavy showers.
The total UK rainfall was 145.3mm – exactly twice as much as you would normally expect compared to the 1971-2000 average. This beats the previous record of 136.2 mm set in 2007.
Looking at individual countries, it has been the wettest June on record for Wales and Northern Ireland, the second wettest in England, and the eighth wettest in Scotland.
Many areas have seen extremely high rainfall – with 83 (out of 237) observation sites marking their wettest June on record. Some of these aren’t significant as they have very short recording histories, just a year in some cases, but others have been operating much longer - Otterbourne in Hampshire been operating for 119 years.
One of the key features of the month has been that the far north west of the UK, traditionally the wettest part of the country, has been remarkably dry.
Up until quite late in the month, a few stations in this area were below their record lowest rainfall amounts – but they just missed the record books with rain falling in the last few days of the month.
Provisional figures from the Met Office to the 27th of June show it has been a disappointing month on all fronts – with many areas being exceptionally wet, very dull and cooler than average.
Currently it is the second wettest June on record for the UK with 130.1 mm of rainfall – just 6 mm off the 2007 total, which was the wettest June in the records which go back to 1910.
It is already the wettest June on record for Wales, with 186.3 mm of rain beating the previous record of 183.1 mm set in 1998. England and Northern Ireland are currently second wettest in the records.
Looking at the figures for the whole of Scotland, it has been a relatively unremarkable month, but there have been big contrasts. North Western areas have seen an exceptionally dry month, while southern and eastern parts have joined the rest of the UK in the wet conditions.
It should be noted that totals for this month so far do not include the heavy rainfall yesterday, and with more rain in the forecast it is likely more records will fall by the end of the month.
It may not be surprising that, with so much rain, sunshine hours have been well down across the UK. There have been 104.2 hours of sunshine so far, just 62% of the long term average (from 1971-2000).
With some sunshine in the forecast, the totals will increase, but it is likely to be one of the duller Junes on record – but it’s not possible to say where it will finish. The current dullest June on record was in 1987, with 115.8 hours of sunshine.
Temperatures have also been below average, with a mean temperature of 11.9 C (0.7 C below average) making for the coolest June since 1991 in the UK.
One of the main reasons for the wet and dull weather so far this month has been the position of the jet stream, which has been much further south than usual. This has meant that rain-bearing Atlantic low pressure systems have been over or close to the UK for the whole month.
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