Chasing the clouds

The last few weeks have been memorable for the cold and snow. Across the Midlands temperatures have not got much above freezing for a couple of weeks. And yet we’ve not really seen any really exceptionally low temperatures at night. Here’s why.

Radiation –┬ábut no nukes!

During the day we get warmth in the form of incoming solar radiation. Admittedly not that much when the sun is low in the sky in the winter but it’s still there. All objects then absorb some of that radiation and then slowly release it back into the air as heat (pics courtesy Hong Kong Observatory).

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While the sun’s up there, even if it’s behind the clouds, incoming solar radiation is usually more than that being lost so it’s generally warmer in the day.

At night, when there is no solar radiation, more heat is lost to the air than is gained so it gets colder. The main controlling factor over how cold it will get at night is cloud. Cloud cover acts like a duvet. Some of the heat radiated up from the surface bounces back of the clouds and warms the air below keeping temperatures up.

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If there are no clouds there is nothing stopping the heat escaping all the way up to space. This is when temperatures can fall to very low levels.

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The last couple of weeks have been mainly cloudy across the Midlands as the cold easterly airflow has brought cloud with it off the continent and North Sea. So although it has been bitterly cold during the day with snow and icy winds, the cloud cover has prevented temperatures tumbling far at night. A look at the minimum temperatures recorded at Pershore in Worcestershire (below) over that last week or so show values hovering around freezing – so not especially cold.

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Getting the breeze up!

Ironically the other element that has prevented temperatures hitting the floor has been the east wind which has had us all shivering over the last month. The wind mixes up the air, mingling cold pockets with warmer bits. The overall result is to prevent areas of still very cold air forming.

Below – east winds off the continent bringing plenty of cloud over the UK. A very common setup during March.

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Starry starry night

It looks like we might start seeing some more breaks in the cloud over Easter as winds fall a bit lighter. If we do the overnight temperatures could tumble – you have been warned.

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Floody Confusing!

We’re dealing with some very difficult and unusual weather conditions at the moment. A large winter storm in the Atlantic is bumping into cold air over the UK, turning heavy rain to snow in some places.

Our dilemma is that the ground is very wet and rivers very sensitive to rainfall. Our ability to predict flood risk is heavily dependent on getting accurate forecasts of the amount of rain entering the system. At the moment it’s really difficult to say how much of the precipitation will be rain and how much snow (which will not get into rivers immediately).

What’s happening?

The chart below shows the weather fronts (which bring the rain and snow) moving into the UK from the south west. Normally they travel straight across us bringing a spell of rain which then clears away. At the moment there is a big blob of cold air sitting over much of Europe, including the UK. It doesn’t want to shift out of the way and the weather fronts are being slowed and held back over the top of us.

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If we have a look at the forecast temperatures (below) for the same time (Friday night/Saturday morning) the contrast between the cold continental air and warm air pushing from the Altlantic is obvious. As the rain bearing fronts collide with this cold air the rain turns quickly to snow. The challenge is predicting exactly where!

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The difficulty is also illustrated by looking at the Met Office weather warnings that are in place on Friday morning.

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In fact the forecasts and warnings have been very accurate, with very heavy rain and significant flooding in Devon and Cornwall and disruption due to snow in the north Midlands.

However, the problem for us is elevation also makes a big difference to whether rain or snow falls. Overnight much of the high land of Shropshire and Herefordshire experienced significant amounts of snow (below), while lowland areas had over half an inch of rain. Trying to factor these differences into our flood prediction models is very difficult.

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A look at our rain gauges (mm in last 24 hours) on Friday morning (below) shows this well. Some locations recording over an inch of rain, while others, nearby showing very little. The radar for the previous 24 hours was showing fairly continuous precipitation across the entire area.

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The upshot of this uncertainty is that we’ve opened our incident room in Tewkesbury so that we can monitor the situation very closely and launch a quick response should the rivers begin responding quickly.

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The immediate outlook remains uncertain, but we do expect the rain to turn increasingly to snow through Friday night and Saturday. It should then turn very cold and dry. Our thoughts then will inevitably turn to the thaw of all the snow and what that will do to river levels. But that’s another story!

Water scene February 2013 – National update

Just a brief update on the national watery picture.

As in the Midlands, February was a pretty average month in terms of rainfall nationally. As you can see from the maps below this made a refreshing change for the areas that were deluged (blues) from April to December last year.

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With a fairly average January too overall rainfall amounts in the last three months are returning to levels we’d expect. However, if you look at the cumulative amounts for the last six months to a year they are still way more than normal.

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Even though we’ve not had that much rain, the soil remains saturated. This is because temperatures are low so there isn’t much evaporation and there are fewer growing plants to suck water up. As a result when we do get rain it runs off very quickly, causing surface water flooding problems.

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River flows during February were very similar to January with many being average for the time of year. Some of the larger lowland rivers, such as the Trent, Thames and Avon, however, continued to see above average flows.

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Groundwater levels continue to show rapid rises in response to last years heavy rain as the water continues to percolate slowly down. Some sites, such as Skirwith (below) on the Eden Valley sandstone aquifer in Cumbria, are now recording record high levels for the end of February (solid black line).

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You can read our full national water report for February here: http://bit.ly/XoBB9x

The Midlands water scene – February 2013

After the extremes of 2012, February has been a pretty average month.

Rainfall in most parts of the Midlands was a little below normal, only the lowland parts of Shropshire recording significantly more than average. Indeed the far east and west of the area only received three quarters of February’s normal rain.

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This means that for two months running total rainfall in the Midlands has been less than the long term average. Is it the start of another trend?

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Despite the smaller amount of rain, river flows were generally above average for the month. This is partly because they were still recovering from flood levels at the end of January and due to continued run-off from saturated soils.

We’ve added a new monitoring site to our monthly water reports on the River Lugg at Butts Bridge in Herefordshire.

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Groundwater continues to respond to the heavy rain of the last year in most places. It can take many months for rainwater to find its way through the soil and into the deeper aquifers (water-storing rock).

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The delayed response can be clearly seen in the sandstone aquifers in Shropshire (below). They are, however, responding dramatically now!

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In contrast, the Nottinghamshire sandstone aquifers (below) are proving more stubborn and levels remain low despite all the rain.

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The first week of March has continued the drier trend and the immediate outlook is for much colder (but still fairly dry) conditions. We’ll see how it all pans out, but with recent experience I wouldn’t bet on anything!

You can read our full Midlands water report here http://bit.ly/Y0zfKk

2012 – A year of bonkers weather

It’s climate week. We’re encouraging people to start taking action to prepare for more extreme weather in the future. Whatever your views on climate change there is increasing evidence that extreme weather events are already getting more common in the UK and the trend is likely to continue in a warming world.

So let’s have a look at some of the conditions we experienced in the Midlands during 2012

A parched landscape

The year started with all parts seeing exceptionally dry conditions. Two successive dry winters and a dry (but dull and cool) summer in 2011 meant that by March the Midlands had recorded the driest 18 months on record. In the chart below red bars represent months that were drier than average. The bigger the bar the drier it was.

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The dry conditions were having a dramatic impact on the amount of water in our rivers. The flows in many such as the Severn, Avon and Wye were less than a quarter of what we would expect.

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The headwaters of some rivers such as the Teme in Shropshire (pictured below) and Lathkill in Derbyshire actually began to dry out completely and we had to begin rescuing fish that were getting stranded in isolated pools.

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Farmers and growers were getting increasingly worried about their ability to water crops, particularly if the weather stayed dry through the summer. We worked with them, relaxing restrictions on abstraction licences (which farmers need to take water out of rivers) and offering advice on creating new on farm storage reservoirs.

We were also getting very concerned about the amount of water stored in the rock beneath us (aquifers). Aquifers are vital both for use by water companies to supply mains water and also to individual homes and businesses that have their own private supply. By March levels right across the Midlands were at or lower than those recorded in the drought of 1976.

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Our models and predictions were telling us that even if we had above average rainfall over the summer of 2012, water resources would continue to be very low through the period. As a result the Midlands formally went into drought on 16th April.

Little did we know what lay in store!

The heavens open

On Wednesday 4th April it started to rain across the Midlands. Everyone was relieved and grateful for the respite in the drought conditions. The problem was that it didn’t stop! Every month except May until the end of the year was wetter than normal and in many cases much, much wetter.

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Indeed, by December the preceding nine months were the wettest in the Midlands since records began in 1910 (chart below). Even more remarkably, when taken as a whole across England and Wales, 2012 was the wettest year on record. This really is amazing when you consider that the first quarter of the year saw virtually no rain!

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Some of the rain we experienced was very intense, particularly in June and July, resulting in many incidents of flash flooding. One example of this happened on 14th July when very heavy thunderstorms developed over Shropshire. Up to 2 inches of rain fell in a few hours leading to some serious flooding problems around Ludlow and Tenbury (pics below).

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However, it was the persistence of the rain rather than the intensity that resulted in the long and drawn out flood events that plagued the Midlands over the last nine months. The soil became completely saturated meaning that any rain that did fall found its way very quickly into streams and rivers. There have been seven significant periods of flooding in the Midlands since last April and most of them were caused by relatively small amounts of rain. To make matters worse, several of the wet spells have been close together meaning that river levels have not had chance to recover from the previous flood event.

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River levels and flows were generally well above average for the whole period from April to December. The chart below highlights the exceptionally high flows on many Midlands rivers through November and December. Peak levels observed on the big rivers such as the Severn, Avon and Wye were generally between a one in ten and one in twenty year return period and many towns (Worcester, Tenbury, Gloucester and Evesham) saw the highest levels since the catastrophic flooding of 2007.

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The hydrograph from the River Teme at Stanford Bridge in Worcestershire is typical of many Midlands Rivers. Frequent periods of high levels and showing a rapid response to any heavy rainfall. The flood warning theshold at Stanford Bridge is about 3.8 metres. This was breached nine times during 2012.

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The regular flooding kept us (and our emergency response partners) very busy. Our incident room in Tewkesbury was open for fifty days between April and December and our flood barriers at Shrewsbury, Ironbridge, Bewdley, Worcester and Hereford were up and down on a very regular basis. Many of our new defences, built since the 2007 flood received their first real tests. Sadly, despite all our efforts, many homes and businesses did flood during 2012, although many thousands more were protected.

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Ironically farmers and growers who in April were worried about whether they would have enough water for their crops found their ground waterlogged and, in many cases, completely inaccessible. Crops have been ruined and feed costs for livestock have escalated as normal grazing has been restricted or prevented.

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The intense rainfall over the summer also led to many instances of soil erosion. Thousands of tonnes of soil was washed off fields, damaging agricultural land, polluting local streams and in some case blocking roads!

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On balance

Eighteen months of dry weather have now been more than balanced by an exceptional period of wet conditions. When we declared drought in April we said that it would not be resolved by a few weeks of rain (I remember doing the interviews!). We were right! It’s taken the wettest year on record.

Even water levels in the aquifers, which take much longer to respond to rainfall, have now recovered (see chart below). Indeed, in some parts of the country (such as Dorset) groundwater levels are so high that they are causing flooding problems of their own. Who would have guessed that in April?

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The future

So have we had a vision of the future during 2012 or just been subjected to the natural variability of the British weather? I don’t know, and I’d suggest neither does anyone else. It’s not possible to attribute any single event to climate change. However, if we do accept that the world is warming, then the typeof weather we’ve seen during 2012 (and indeed in the years before) are more likely to occur. Warmer air holds more water and changes to ocean temperatures may change the global scale air circulations (such as the jet stream) which have a huge impact on our weather. Extremes (of all sorts) are more likely.

One of the most dramatic (and scary) model predictions on future climate change in the UK that I’ve seen is for river flows (below). Based on medium/high emissions scenario (which is closest to where we are) the model suggests that by 2050 summer flows on rivers like the Severn and Avon could be 50-80% lower than now. That, clearly, would be game changing! Conversely average winter flows could be slightly higher than now

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Whether you believe in climate change or not, we’re encouraging everyone to think about extreme weather and how we can be better prepared. Over the last decade tens of thousands of people across the Midlands have been affected by extreme weather events. Surely a bit of thought about your levels of risk and how it may affect you, your family or your business in future makes sense!

Try these links:

Climate week – all the details http://www.climateweek.com/

Check your flood risk http://www.environment-agency.gov.uk/homeandleisure/floods/31650.aspx

Practical actions for businesses and organisations http://bit.ly/Umoa2r

Check out how vulnerable you are to extreme weather events http://www.environment-agency.gov.uk/research/137639.aspx

Climate proof your business http://bit.ly/Iawxab

Advice to farmers on coping with adverse weather http://bit.ly/Xkre4q