Frostiest April for at least 60 years in UK

Frosty conditions have been common throughout April this year – Image: Shutterstock

April 2021 has provisionally been the frostiest in at least 60 years for the UK, topping the previous frostiest April in 1970, with records going back to 1960.  

April 2021 has already seen an average of 13 days of air frosts reported for the UK, topping the 11 days seen in April 1970. The frosty conditions have been replicated across the UK, with England (12 days), Wales (11 days) and Scotland (16 days) also reporting their frostiest April since it began being reported in 1960. Northern Ireland has currently seen 8 days of frost, not yet exceeding their current record of 11 days set in April 1983.  

Mark McCarthy, from the National Climate Information Centre, said, “We’ve been seeing a high frequency of frosts overnight throughout April thanks largely to persistent clear skies for most. This will be reflected in the end of the month statistics, which are already showing above average sunshine duration, as well as low minimum temperature readings overnight, with some parts of northern England and Scotland reporting minimum temperatures 3.5°C lower than the average for April.” 

The conditions have been particularly challenging for the country’s farmers and growers. Chief Horticulturist at the Royal Horticultural Society Guy Barter said, “Considerable damage has been experienced in many gardens with flowers such as camellia and magnolias being scorched, and cherry, plum and pear blossom injured so that the fruit crop will be reduced.” 

With the risk of frost persisting through this week, gardeners are being encouraged to keep tender plants inside for now, or, if they must go out, cover them with cloches or horticultural fleece. You can check out our previous blog post on the expected weather this week and over the bank holiday weekend.  

Current provisional frost figures across the UK in April (days):  

UK – 13 

England  – 12 

Southern England – 11 

Northern England – 14 

Wales – 11 

Scotland – 16 

Northern Ireland – 8 

The full weather statistics for April 2021 will be available at the end of the month.

Posted in Met Office News | Leave a comment

More changeable weather likely for bank holiday weekend

Contrary to some media reports, the early May bank holiday weather is likely to be more changeable than recently, with showers and cool temperatures for most of the UK instead of the clear sunny skies of recent days. 

As April draws to a close, cloud and showers will continue to move from northern Scotland south across the country. The showers will persist for many areas through the latter part of the week and into the bank holiday weekend. Central and southern England may see more persistent rainfall on Wednesday and Thursday and there is even a chance there could be some snow over the Scottish mountains.  

Met Office Chief Operational Meteorologist Steven Ramsdale said, “The weather heading into the bank holiday weekend is going to be a little more unsettled than we’ve seen in recent days. There will be rain or showers at times for many, which is likely to be welcomed by many farmers and gardeners after a dry April so far. However, as is the nature of showers, there will also be some good dry and sunny spells so it’s by no means a wash out. Temperatures are likely to remain near to or below the average for the time of year with temperatures by day not expected above 14 or 15°C.” 

These less settled and showery conditions are expected to continue into next week, but with a chance of a spell of more widespread rain and stronger winds, especially across southern parts, early on. Temperatures are likely to remain cool for the time of year.  

Despite the frost, it has been a sunny month with sunshine for the UK already (1 – 25th April) 33% above the monthly average with 5 days of the month still to go. A full roundup of April’s statistics will be available at the end of month. Visit the Met Office website for the latest forecast.  

Posted in Met Office News | Leave a comment

Climate Data Challenge – working in partnership for innovation

As Governments, organisations and individuals around the world seek to tackle the challenges of climate change, creativity and innovation are key to finding ways to improve our resilience to weather and climate.

Our scientists work at the forefront of global climate science, and with the approaching UN Climate Change Conference, COP26, we are stimulating innovation by bringing together a range of skills and information. Working with our partners we have been seeking new and exciting ways to better build solutions for understanding and visualising the risks and potential opportunities posed by a changing climate and the actions we take to manage it.

Our recent Climate Data Challenge virtual hackathon united around 200 participants from academia, other research establishments, Government departments, the Met Office and the private sector, and various disciplines including science, technology and policy. We worked with partners focussing on the challenges that climate change could bring to the following sectors: marine and coastal; nature-based solutions; and sustainable development. For each we invited delegates to tackle these challenges by imagining innovative ways to bring together data and other information.

“Whilst there is a huge amount of climate data available, there is also a vast quantity of other data, and it is through the combination of these that we can gain new insights and understanding,” explains Dr Mark Harrison, Met Office Strategic Head of UK Applied Science. “Through the Climate Data Challenge we wanted to give people from a wide range of disciplines the chance to explore climate resilience in a safe and creative environment.”

Responding to the challenge

Over the two days of this event, 21 multi-disciplinary teams used datasets such as the UK Climate Projections to develop proofs of concept, visualisations and creative solutions to the challenges, generating their own ideas and drawing on expertise from the Met Office and our partners where required.

Responding to the marine & coastal ‘Bridging the gap between our experiences and the science’ challenge, one team explored how to analyse data and communicate climate change through community-sourced storytelling, supplemented by data visualisations and stylised scientific illustrations. You can see the STORY(coast)LINE prototype here.

Meanwhile, the 30 x 30 team took on the nature-based solutions ‘What is the best use for our land?’ challenge. They looked at how we could get to 30% protected land/ocean by 2030 and what the consequences of some of the different choices would be. The progress they made can be seen here.

Hot Stuff was the name of one of the teams tackling the sustainable development ‘supporting the most vulnerable communities’ challenge. They sought to categorise the effect of extreme heat events on power generation and use it to identify countries vulnerable to future climate change. You can see the interface they started to develop here.

An artist from Three Blind Mice developed a series of scribes of the event including to illustrate the outputs for each of the three themes. You can see these by following the hashtag #ClimateDataChallenge.

The legacy

“We were thrilled and inspired by the quality and variety of outputs at the end of the event,” recalls Professor Jason Lowe OBE, Met Office Principal Fellow and Head of Climate Services for Government. “The event illustrated how much insight can be generated by bringing together and visualising data from different disciplines and domain experts, even in a relatively short time.”

In the coming weeks the Met Office organising team plan to meet with some of the teams involved to see how we can support them to continue the work they started during the Climate Data Challenge. We are also exploring ways we can showcase the success of the event at COP26 in Glasgow in order to highlight the importance of using data innovatively to build solutions for climate resilience.

Involving a diverse range of organisations enabled hackathon delegates to benefit from a wealth of experience.

Jason said: “The effects of climate change and variability are already affecting people, and the challenges are likely to increase in the future. Events like the Climate Data Challenge are helping us better understand how society can manage the challenges on local, national and international scales. Avoiding the worst impacts of climate change will be easier with more people tackling the problem and trying to imagine inventive solutions.”

More to come…

Over the next few months, Met Office Academic Partners will also be hosting a series of hackathons. Follow #ClimateDataChallenge to find out more.

 

Posted in Met Office News | Tagged , , , , , , , , , , , , , , , , , , , , | Leave a comment

Records broken as temperate end to March offsets earlier cold spell

March 2021’s overall provisional climate statistics show a 0.9°C average temperature increase from 1981-2010 national averages for the month, as some high temperatures during the latter part of the month saw several stations break their existing records for their warmest March day.  

Particularly low temperatures started the month for large parts of the country, especially in southern England, but this was then balanced by late March temperature highs that brought the average temperatures for the month above what would normally be expected. For the UK as a whole, the March temperature was 0.9°C above the 1981-2010 average. Moreover, northern England, Scotland and Northern Ireland can look back on March as slightly warmer than average.  

The last week of March saw several notable station records for the warmest March day being broken. Bude (Cornwall) recorded its highest temperature ever for a March day at 23.4°C, with the previous record having been in place since 1968 when 21.2°C was recorded. It was a similar story for stations across the UK, as, among others, Kew Gardens (Greater London), Lyneham (Wiltshire), and Oxford broke long-standing March daily temperature records. Many March daily high temperature records were broken, with the vast majority being reported on 30 March, although temperatures remained well above average for many stations on 31 March.  

It wasn’t only daytime temperatures that were notably high. With temperatures staying above 12.7°C on 30 March at Kinloss (Moray) it was the highest March daily minimum temperature for Scotland on record.   

In terms of rainfall, the month was drier than average for southern, central and eastern areas, although northwest England saw more rain than usual, thanks to persistent heavy showers from 9 March to 14 March and rain from 24 March to 28 March. Cumbria saw 41% more rain than average, which was the most above average of any of the UK’s counties. The wettest day of the year so far was on 28 March when 177.2mm was recorded at Seathwaite in Cumbria. 

Dry conditions were reported for much of the country, however, with Cornwall, Worcestershire, Tyne & Wear, and the Isle of Wight all having less than half of their average rainfall for March. Southern England overall was on average 36% down on the rainfall average for March, while only Scotland and northern England got to their March average, both at 100% in total each. The UK as a whole had 89% of the average rainfall for the month.  

Days of air frost for the month were in relatively short supply across eastern England, Scotland and Northern Ireland. However, large parts of the South West and Wales got to their March averages, while north-west Wales crept above the average.  

Mark McCarthy, from the National Climate Information Centre, said: “With the exception of some record-breaking highs at the end of the month, March didn’t deviate too much from what we’d expect to see for the time of year. It was warmer than average overall, but a cold first week balanced the milder end to the month. 

“It was dry for a lot of areas, but northwest England took the brunt when it came to the month’s rainy weather.  

“The most notable factor was the high temperatures at the end of the month, with a number of stations recording their warmest March day since we’ve been recording the data.”

Provisional March 2021Mean temp (°C) Sunshine (hours) Rainfall (mm) 
 Actual Diff from avg (°C)Actual% of avgActual% of avg
UK 6.4 0.9 101.9100  84.3 89
England 7.0 0.8 110.6 103 50.0 78
Wales 6.4 0.7 105.5 104 96.9 83
Scotland 5.3 1.1 91.1 98 140.4 100
N Ireland 6.8 1.0 74.1 75 74 78
Posted in Met Office News | Tagged , , , , , , , | Leave a comment

What kind of temperature swings are ‘normal’ in UK spring?

Ewe in snow with lamb

Spring snow fall can present challenges for livestock and livestock farmers. Picture: Shutterstock.

With forecasters predicting a marked shift in temperatures in the coming days, here we take a look at how commonplace temperature fluctuations are during a typical UK spring.

It’s certainly not uncommon for spring in the UK to see big differences in temperature. However, despite the lows expected in the coming days, the more climatologically notable weather for this time of year was actually the recent warm spell, which saw temperatures reach as high as 24.5°C in some places this week.

For reference, the highest daily temperature on record for March was 25.6°C in Mepal, Cambridgeshire, which was reported in March 1968. In contrast, the lowest daily temperature reported for March in the UK was –22.8°C at Logie, Aberdeenshire, in March 1958.

Dr Mark McCarthy, Scientific Manager of the National Climate Information Centre, said: “It’s not unusual to experience a wide temperature variation during a typical spring.”

On average the UK sees around 14 days each spring where temperatures fall to 0.0° C or below, and even residents in central and southeastern England can expect to see temperatures drop to this level around 10 days each spring. Coastal regions and islands around the southern UK witness the fewest spring frosts on average.

Dr McCarthy added: “Of course, you would expect the lowest and highest spring temperatures to occur at the beginning and end of the season respectively, but natural variation dictates that periods of lower than average temperatures are to be expected as we move through the season.”

The coldest spring in recent years was 2013, which was the ninth coldest spring in a series from 1884. The average temperature didn’t reach 6.0° C. During the season over 80 weather stations recorded daily maximum temperatures below 0.0° C and even the station at East Okement in Okehampton Devon recorded a daytime maximum temperature of –2.0° C on 11 March 2013.

Such variation in temperature during spring does present challenges to some areas of UK life, with sheep farmers particularly susceptible to temperature drops as lambing season comes in to full view. Farmers have to monitor the changing weather to ensure their ewes and new-born lambs are guarded against drops in temperature, but also ensure they can be outside if the weather allows for it.

Chief Executive of the National Sheep Association Phil Stocker said: “The sector has endured several cold weather events during recent springs and these have wrought considerable hardship to both stock and hill farmers. However, being aware of the approaching change in weather type will allow farmers to buffer the impacts by taking action to avoid the worst losses.”

The up-and-down nature of spring’s temperatures also provides challenges for the UK’s gardeners, with overnight frosts the most feared weather and provide particular issues for magnolia and camellia flowers, as well as many fruit blossom and young fruitlets. If these plants are flowering for gardeners, spring remains a time when they have to be ready to ward off potential damage with coverings to stave off frost.

However, Chief Horticulturalist at the RHS Guy Barter said, “Cold weather in April is not altogether bad, spring flowers, ornamental cherries and daffodils, for example, last longer when it is chilly.”

Posted in Met Office News | Tagged , , , , , , , , , | Leave a comment

Atmospheric carbon dioxide passing ‘gloomy’ milestone

This year the climate is passing a gloomy threshold. The concentration of carbon-dioxide in the atmosphere is reaching 417 parts per million: an increase of 50 per cent since humans embarked on the industrial revolution and began to emit greenhouse gasses at large scales.

Measurements at the Mauna Loa observatory in Hawaii are revealing that CO₂ concentrations have already been above this level on some days, and are expected to remain above this symbolic threshold for around three months.

Atmospheric rise in carbon dioxide from the industrial revolution to the present day

Next year, CO₂ concentrations are expected be more than 50% above pre-industrial across most of the year, and will continue to rise until global emissions reach net zero.

Prof Richard Betts MBE is Head of Climate Impacts Research at the Met Office, and one of his areas of research is forecasting the rise in atmospheric carbon dioxide. Prof Betts said: “Humans began burning fossil fuels at large scales at the end of the Eighteenth Century, and it took about 200 years for the atmosphere to see a 25 per cent increase in the atmospheric concentration of carbon dioxide, but only another 35 years to reach this year’s sorry milestone of a 50 per cent increase.”

You can read more about CO₂ passing this symbolic threshold in Prof Betts’s article in Carbon Brief.

Posted in Met Office News | Tagged , , , , , , , | Leave a comment

Did the weather fuel climate-changing forest fires in the Amazon in 2019?

In 2019 there was a sudden spike in Amazon fires that captured the world’s media attention. Between them the blazes destroyed 906,000 hectares of rainforest – an area larger than the island of Corsica.

Amazon fire

In their natural state the forests of the Amazon draw down atmospheric carbon, but during fires this carbon is released to the atmosphere. Pic: Shutterstock.

The thousands of fires produced around 400 million tonnes of carbon emissions adding to the atmospheric concentration of greenhouse gases and exposing millions of people to increased air pollution, including the residents of São Paulo, 1,700 miles away.

With an increase of carbon emissions up by 80 per cent on the previous year, a team of scientists led by the UK Centre for Ecology & Hydrology and the Met Office began a research study to discover if meteorological conditions could account for the fire surge during the year.

The team examined data on land use, population density, rainfall patterns, humidity, temperature and lightning to build a picture of the likelihood that factors related to weather conditions may have fuelled the increase. The team’s results indicated that 2019 was not an unusual year meteorologically and this led to the startling finding that there was only a seven per cent chance that weather conditions triggered the increase in fires in the main deforested areas of the Amazon between June and August.

Amazon fires during 2019 compared with 2002-2019

Figure 1. Maps of observed percentage of burnt area.
Left: Observed burnt area, for June–August 2002–2019 annual average.
Right: Difference between June–August 2019 and 2002–2019 average.

A key aspect of the research was comparing meteorological conditions for any given year with the extent of burned areas, following forest fires. A significant finding was that observations show that the burnt area was higher in 2019 than in previous years in regions where there had already been substantial deforestation.

Although fires can start from natural sources of ignition within the Amazon, previous studies have shown that fires in the region are rarely ignited without human intervention. The humid rainforest vegetation is not adapted to frequent burning and consequently there is higher tree mortality when fires do occur.

“Our conclusion is that human activity – landscape changes or fires started to clear forest – caused the increased level of burning and not the weather,” says Dr Doug Kelley of UKCEH, who led the research.

Dr Kelley added living trees hold water, and this can suppress the spread of wild fires, even in drought periods. However, in deforested areas, felled wood and vegetation may be drier, acting as fuel for a fire.

Pied tamarin

The pied tamarin is one of the world’s most threatened primates. Confined to a small part of the Brazilian Amazon rainforest, the Critically Endangered monkey is affected by several factors, largely linked to deforestation. Picture: Shutterstock

The Amazon is crucial to regulating the world’s climate as it absorbs large amounts of carbon dioxide (CO2) from the atmosphere. However, being a significant store of carbon, this may be re-released as CO2 during deforestation and burning.

The scientists say deforestation creates a series of vicious cycles. It reduces the Amazon’s ability to store carbon and absorb CO2 while also increasing the risk of wildfires, which emit greenhouse gases and damage plant life that would otherwise retain carbon and remove CO2 from the atmosphere. While the Amazon ecosystem ‘takes care of itself’ by recycling its own water, deforestation will reduce its ability to do this, and make the rainforest less stable.

Met Office climate scientist Dr Chantelle Burton, a co-author of the study, says: “It is important to be able to understand what is driving changes in fire trends, especially in globally important carbon sinks such as the Amazon. This could help inform decision making around land management practices to minimise unintended impacts – both on short timescales and for future responses to climate change.”

The study – Low meteorological influence found in 2019 Amazonia fires -was funded by the Natural Environment Research Council. It has been published in the journal Biogeosciences.

Posted in Met Office News | Tagged , , , , , , , , , | Leave a comment

Scottish Flood Forecasting Service marks 10th Anniversary

The Scottish Flood Forecasting Service (SFFS) is today (Tuesday 9 March) marking a decade of supporting Scotland’s preparedness and response to flooding.

Staffed around the clock 365 days a year, the service has been combining hydrological and meteorological data to provide strengthened flood forecasting for Scotland over the last 10 years, and is a close partnership between The Scottish Environment Protection Agency (SEPA) and the Met Office.

Its team of experts across Scotland has delivered more than 3800 national flood guidance statements. These five day forecasts are issued daily to a range of 230 organisations including emergency services and local authorities, helping emergency responders prepare in advance for the impacts of flooding.

Climate change is likely to bring increased risk of coastal, river and surface water flooding. With sea level rise and more extreme weather, including wetter winters, more intense rainfall and thundery weather in summer, the increasing importance of the service to help Scotland prepare and adapt is clear.

The service has played a critical role in forecasting impacts from some of the most significant weather events of the decade, including storms Desmond and Frank in 2015. These saw SEPA issue its first severe Flood Warning since the formation of the SFFS, and its highest number of Flood Warnings in one day (101 warnings) respectively.

Terry A’Hearn, SEPA CEO, said: “Over the last ten years, the SFFS has proved a vital service to help Scotland prepare in advance for the worst impacts of flooding. Preparation is crucial, as we saw in the last fortnight when towns from Aberdeenshire to the Scottish Borders flooded.

“The threat from flooding is real and growing. As well as posing a risk to properties, infrastructure and our environment, it can pose a serious threat to life. We and our partners at the Met Office, together with flood management authorities, community responders and communities themselves all have key roles to play as Scotland learns to live safely with flooding.”

Over the last 10 years, the Met Office has made continual improvements to the radar network and other weather technology. This enables the SFFS to issue more detailed daily flood guidance, with ever increasing lead-time and confidence.

Met Office Chief Executive Professor Penny Endersby said: “Climate change is not only resulting in a warming climate but also more intense rainfall which has a crucial bearing on the SFFS’s operations. The risk and impacts of flooding are likely to increase over the coming years, meaning we will need to continue to adapt our services.

“Forecasting skills are constantly evolving and developing thanks to world leading, cutting-edge science and technology and as we look to the future it is clear that we will need to continue to work together if we are to ensure that Scotland will continue to stay safe and thrive.”

The service was also recognised by the Royal Meteorological Society for its pioneering work to provide the UK’s first operational 24-hour surface water flood risk forecast during the 2014 Commonwealth Games in Glasgow. Showing street-level surface water predictions for the first time, the project was the result of joint research with Scotland’s Centre of Expertise for Waters, CREW, and has helped improve understand of flooding that doesn’t arise from rivers or the sea.

The launch of the SFFS in March 2011 was supported by the Scottish Government.

Kenny Wratten, Chair of the Local Authorities Resilience Group Scotland, said: “Getting that five day forecast of flooding every morning from SFFS helps councils plan and deliver flood mitigation and response. The earlier we get an indication of what’s expected and where, the better we and other responders can gear up to make best use of available resource in areas we know are most likely to be affected.

“For councils this might include extra maintenance in trouble spots, considering road closures, the deployment of flood protection measures, and making arrangements to assist people. More than anything it’s about working together using best knowledge to support our communities.”

Paul Laidlaw, Resilience Manager for the Scottish Flood Forum said: “Services from the SFFS are invaluable to the Scottish Flood Forum (SFF), as they help us to work with communities to build actions that help them prepare for flooding. The Flood Guidance Statement in particular helps us and other recognised community responders get enough of a heads-up to prepare where flood recovery resources may need to be deployed, to help those impacted.

“It also provides an excellent learning tool for us to help communities develop flood plans and response exercises, which alongside SEPA Flood Warning messages helps them better avoid, reduce or prevent damages that all too often can leave physical and psychological scars.”

In addition to the five-day flood forecast shared with emergency responders and those with flood-risk management duties, plans are also in progress for a public-facing three-day flood forecasting product, being developed in consultation with the public.

SEPA is Scotland’s national flood forecasting, flood warning and strategic flood risk management authority. As well as working in partnership with the Met Office to forecast for flooding, it operates Floodline in Scotland to warn the public and emergency responders when flooding is likely.

It issues regional Flood Alerts – early advice that flooding is possible – across wide geographical areas covering the length and breadth of Scotland. It also issues local Flood Warnings in areas where it has developed river system monitoring and flood risk impact knowledge. These warnings are issued at shorter notice when it is more certain that a specific area will be affected.

Vincent Fitzsimons, SEPA’s head of flooding, added: “There are some simple steps you can take to help prepare for flooding, including signing up to Floodline, preparing a flood plan, familiarising yourself with how to shut off energy and water supplies, having key contact numbers to hand and considering the use of flood protection products. And when flooding does strike, remember these five important tips;

1. Do not walk, drive or swim through flood water.
2. Do avoid any form of direct contact with flood water as it could be contaminated.
3. Do not use any electrical appliances.
4. Do move to higher ground and wait for the emergency services to find you.
5. Do co-operate with emergency services and your local authority who are responsible for co-ordinating relief measures.”

Further advice can be found at floodlinescotland.org.uk.

Posted in Met Office News | Tagged , , , , , , , , , , , , , , , | Leave a comment

Record low temperature for the UK this millennium

Overnight the recorded minimum temperature at the weather station at Braemar dropped to -23.0°C, one of three stations in the UK to dip below minus 20.0°C. With the media reporting on the ‘extreme’ conditions, we have analysed our records since 1961 for temperatures dropping below that threshold to observe the trend and to put last night’s records into a longer-term context.

Frequency of very cold nights in the UK

Last night’s temperature at Braemar (11 February 2021) was the lowest recorded in the UK since 1995.

Minus 20.0°C is extreme cold. In fact it is even 2.0°C colder than the recommended temperature for the inside of a domestic freezer. So experiencing temperatures below -20.0°C in the UK are not common and not usually widespread.

Dr Mark McCarthy is the head of the National Climate Information Centre. He said: “The records from last night help to put the UK’s climate into context. For example, for all its roar, temperatures of below -20.0°C weren’t recorded during the Beast from the East during the end of February and the beginning of March 2018. But that event was certainly extreme by other measures, including widespread low daytime temperatures.

“Before this week we’d have to go back to December 2010 to see days where the UK temperature falls below minus 20.0°C. In that year eight days saw temperatures below that value.”

Since 1990, four years have recorded days below minus 20.0°C, but in the 30-year period up to 1990, 14 years reached that level.

Mark McCarthy added: “Temperatures of this level have always been extreme events in the UK, but what our records show is that these events have become less frequent. However, the UK’s climate record displays a huge range of natural variation and even though our climate has warmed by approximately 1.0°C, there is still the possibility of a severe cold-weather event: they’re just not as regular as they used to be.”

The lowest temperature recorded in the UK is -27.2°C on 30 December 1995, at Altnaharra; and on 10 January 1982, at Braemar.

From today temperatures are not expected to dip below minus 20.0°C as warmer conditions start to push in from the Atlantic reaching all parts of the UK by Monday.

Posted in Met Office News | Tagged , , , , , , , , , , , | Leave a comment

Multiple drivers create challenges for forecasting weather direction

How times change. This time last year, we were in the early days of a month which went on to become the wettest February for the UK in a series stretching back to 1862.

Storms Ciara, Dennis and Jorge, which swept in west from the Atlantic, brought a large proportion of the month’s total rainfall on moisture-laden and relatively warm winds

So will February 2021 follow a similar pattern? Met Office weather presenter Aidan McGivern said: “What we have started with this month is a different pattern, especially in the north, with more of an easterly influence than one from the west, and there are several reasons for this, but the major factor is the strength of the westerly flow, which has and is being influenced by opposing climate drivers.”

A key driver of winter weather in the UK and Western Europe is the North Atlantic Oscillation (NAO) – a measure of average atmospheric pressure difference between an area of high pressure near the Azores and the semi-resident low pressure between Iceland and Greenland. Aidan McGivern said: “This sounds complicated, but what you have to remember is that when the pressure difference is greater than average you have a so-called positive NAO which steers low-pressure systems toward the east across the Atlantic; a negative NAO slows down the flow, and can even reverse it completely by bringing the flow from the east. That is the prospect, a much colder prospect, that we see in the forecast for at least a few days from the weekend.”

So as the NAO is a principal determinant of the UK’s weather in winter, what drivers can affect the NAO? Aidan McGivern continued: “When meteorologists are trying to gain an insight into longer-term forecasts it is like trying to work out who is going to win a marathon. You can study the form of individual athletes but on the day any number of factors can determine the outcome: and, of course, the favourite doesn’t always win because of the ‘chaos’ inherent within a sporting event, or the atmosphere.”

In meteorological terms there are a number of influences that can affect the positive or current negative phase of the NAO and they can all play out at different timescales.

Atmospheric events even as far away as the tropical Pacific can exert an influence on the strength, shape and position of the jet stream and subsequently the NAO.

As this winter approached, La Niña – the cool phase of the El Niño Southern Oscillation – was taking place. La Niña has historically been linked to colder conditions during late autumn and early winter followed by increased rainfall and milder conditions during the second half of winter.

However, as winter progressed, it soon became apparent that other drivers would provide an opposing influence to La Niña and at shorter timescales.

Often a big driver for affecting the phase of the NAO is a Sudden Stratospheric Warming, where a breakdown of the Stratospheric Polar Vortex (SPV) – a ring of westerly winds high up in the stratosphere – causes a disturbance and a potential reversal of direction which can influence the jet stream below, causing it kink and buckle. An SSW event in early January helped to disrupt the jet stream and contribute towards a south-shifted Atlantic storm track during recent weeks. The SPV has not yet recovered and so can still exert an influence.

The Madden-Julian Oscillation (MJO) is an eastward-moving phase of enhanced tropical rainfall that primarily affects the Indian and Pacific Oceans and occurs on weekly to monthly timescales. When rainfall is enhanced in the West Pacific, it has been shown to lead to weather patterns close to the UK that induce a greater chance of cold weather during winter. At the start of this week, the MJO moved into an active phase over the West Pacific. A strengthening high pressure block to the north of the UK during the next few days, helping to push the Atlantic storm track even further south and resulting in easterly winds, is consistent with what might be expected from this phase of the MJO.

Aidan concluded: “In recent years, meteorologists have become more aware of the many different influences on the UK’s winter weather – from the stratospheric polar vortex to rainfall patterns in the tropics. Some influences, such as La Niña, can be identified well ahead of the start of winter. Others, such as the MJO, operate on much shorter timescales. Add into the mix the inherent chaos of the weather and you’ve got all the ingredients for a particularly challenging forecast.”

Paul Davies is the Met Office’s Chief Meteorologist and he has the ultimate responsibility for production of the forecast. He said: “Our team of meteorologists have the complex task of producing a forecast taking into account not only the global drivers, but also looking at effects closer to home.

“Some of the differences in the forecast can be quite marginal depending on exactly the area in focus. For example, in Scotland you may have coastal communities lashed by heavy rain, whereas their inland and uphill neighbours may notice a marked change from heavy rain to snow. But all of these weather conditions present hazards, but communities rely on the Met Office to cope with the ever-changing and multi-hazard nature of winter weather and produce a forecast which helps to keep people safe.”

Posted in Met Office News | Tagged , , , , , , , , , , | Comments Off on Multiple drivers create challenges for forecasting weather direction