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Climate Hustle

The Mid-Wales floods of June 2012: a taste of things to come?

Posted on 20 July 2012 by John Mason

Sitting on the western side of a landmass that already juts out prominently into the Atlantic, facing into the prevailing moist sou-westerlies and having lots of high ground just set back from the coast, Wales has a deserved reputation for having a wet climate. It is generally a very green country. Not all the time, it must be said: the summer of 2006 was marked by a drought so severe that farmers ended up having to cut sections of fence to allow stock to access what springs were still flowing, and it was the year in which a farmer friend was one of many who decided to have a borehole drilled as a new and more reliable water supply.

I've lived here in Mid-Wales for over 30 years now and having a strong interest in meteorology I've investigated all sorts of severe weather, from the Great Blizzard of 1982 to the EF-2  tornado that tore a path of destruction through a neighbouring village late in 2006. I've seen the aftermath of flash floods following severe thunderstorms and have even chased the occasional supercell. But I don't think any of these events have been so shocking as the Mid-Wales floods of June 8-9 2012.

Map showing the area affected by the 8-9th June 2012 floods

above: map of the area of Wales affected by the floods of June 8-9, 2012. Green triangles are summits, the highest of which is Plynlimon (752m). Large red circles are towns; small ones are villages. Roads are in red; rivers in blue. Graphic: author

Rain set in early on Friday June 8th as a deep area of low pressure off the SW of England moved NE straight across Wales. By the evening of the 8th, the low had moved off into the North Sea, but an associated trailing occluded front continued to sit over Mid-Wales overnight before finally drifting off northwards later on Saturday June 9th. The rain finally died out late morning on the 9th, almost 36 hours after it started.

This was a classic dynamic rainfall event: south-west winds with a long fetch down across the warm Atlantic brought moisture with them, only for it to fall out as rain when the air cooled as it was forced up over the Cambrian Mountains, generally 500-600m in height. It was a classic example of  “warm conveyor” driven, orographically-enhanced rainfall, made more efficient by the 'seeder-feeder' mechanism in which ice crystals falling from high, upper clouds seed the moist layers below.

However, what made this event stand out from the rest was the incredible 36-hour event totals. Now, in winter we commonly see dynamic, warm conveyor rainfall events giving 75-100mm of rain over, say, 48 hours due to the aforementioned mechanism. Three to four inches of rain in that timespan may sound a lot but the area's geography deals with it effectively: peat-bogs soak a lot up and the remainder runs off, putting streams in spate and the major rivers such as the Dyfi and the Rheidol spill out over their flood-plains as they have done for thousands of years. A few roads are closed for a couple of days then everything goes back to normal. But this fall was different. The highest rain-gauge total I have seen was 183mm in 36 hours, but cumulative radar plots indicate that in some areas of higher ground, 200-250mm or eight to ten inches of rain fell during the event.

Rainfall totals, day one (June 8th 2012)

above: cumulative radar rainfall totals for day one of the event, Friday June 8th 2012, from Netweather. The inset (top L) is a zoom-in on the Mid-Wales district.

The result was chaos on an unprecedented scale.

Talybont is a village some seven miles north of  the seaside university town of Aberystwyth. There has long been a settlement here, with Bronze Age, Iron Age and Roman remains to be found in the vicinity, but the village expanded with the growth of the lead and silver mining industry, which reached a peak in the Nineteenth Century, a time from which many of the original houses date. Two rivers pass through the village and meet just downstream from it: the Leri and the Ceulan. The Leri is a medium-sized river, but the Ceulan is little more than a stream. Talybont's village green, with its two old coaching inns, the White Lion and the Black Lion, sits in between them. It last flooded in 1963 when heavy late winter rains brought major snow-melt, and it is said a major factor in the flooding was that a tree had become wedged underneath the bridge that carries the main A487 road over the Leri. In 1990, the bridge was enlarged with a bigger and higher arch.

By the night of Friday June 8th, the Leri was already in raging spate, but early the following morning both the Leri and the Ceulan reached unprecedented levels. Water burst in through the backs of houses that had stood close to the Leri for at least two hundred years, flooding their ground floors feet deep. The Ceulan's waters tore across the car-park of the White Lion, ripping up metres of tarmac. The rivers merged together on the green and then as one surged on down the valley, where they overwhelmed flood defences with ease and swept across caravan sites, leading to emergency aerial evacuations taking place involving local lifeboat crews and RAF helicopters. At Dolybont, the Environment Agency's river-level gauge on the Leri went, quite literally, off the scale. From there, the floodwaters continued on down to affect the fishing village of Borth before reaching the open sea. Days later, boat crews out fishing reported finding slicks of floating debris from houses and caravans, several miles out to sea. This incident involved neither snowmelt or wedged trees: it was, plain and simple, the sheer amount of rain that fell.

Chaos: looking upstream along the Leri after the flood's peak, June 9th 2012

above: chaos - after the flood's peak, looking upstream along the Leri in Talybont, on the afternoon of June 9th 2012. Photo: author

On July 11th 2012, the BBC reported that, according to the UK Government's Committee for Climate Change:

“Four times as many homes and firms risk flooding in the next 20 years if the UK does not prepare for climate change.”

Criticising the government for cutting funds for flood defences, the committee said that these will be needed more than ever if climate projections prove accurate. This news follows the publication of the US National Oceanic and Air Administration's (NOAA) annual state of the climate publication, in which a supplementary report looks at attribution and asks whether extreme weather events can be linked to climate change.

Weather is of course influenced by climate, so that if the climate changes in any way one would expect the weather to respond accordingly. The NOAA research looked at a number of extreme events and ran estimates of the likelihood of them happening in a world with pre-industrial CO2 levels. They found that with some events, climate change almost certainly had a role. The 2011 Texas heatwave was 20 times more likely to happen as a result of the climate change to date; warm winter extremes such as the second warmest November on record in the UK, in 2011, were now a staggering 60 times more likely to happen. Some events were more influenced, however, by other factors such as man-made infrastructure: the 2011 Thailand floods were cited as an example.

In the UK, climate contrarians tend to blame all severe floods on man-made infrastructure: concreting-over of soil, building on flood-plains and so on. Let the events of June 8-9 2012 be a lesson for them: the Leri and the Ceulan are upland rivers that drain boggy, acid moorland where there is barely a speck of concrete to be seen and only a few single-track roads giving access to farms. It is too early to take these disastrous floods and nail them firmly to the notice-board of climate change, given that such research is a time-consuming process, but the unprecedented ferocity of these floods begs the question, “is this a taste of what is to come?”

Screengrab of a video of the Talybont flood, 9th June 2012

above: a taste of things to come? Screengrab of a video taken by Ifan Lewis of Talybont.

The question is partially answered by the Clausius-Clapeyron relation: put simply, this holds that in a warming trend, for every added degree Celsius the air can potentially hold 7% more water vapour. It therefore follows that moist air-masses, such as those advected towards the UK from the sub-tropical Atlantic, can become substantially moister in a warming world, leading to more intense rainfall-rates and increased event-totals. It is important to point out that prolonged, multi-day dynamic rainfalls have always been a feature of the Welsh climate as we are especially prone to 'warm-conveyor' synoptic setups - as are the mountains of Ireland, the Lake District and Western Scotland. What we need to watch out for in the coming years are any further indications of increases in intensity of precipitation during such events in any of these areas. The November 2009 flood disaster in Cockermouth, Cumbria may be another example: it was another classic “warm conveyor” which dropped huge amounts of rain over the Lakeland fells.

Moving from individual events to broader trends, whilst over the pond in the USA the heat records have been going down like ninepins, here in the UK the summer has been dismal, with the only records getting broken involving rainfall. Many areas have seen in excess of 200% of the average rainfall in June, based on 1971-2000 climatology. Why have we been stuck in a weather-rut for so long?

The answer lies in the large-scale waves that mark the course of the Polar Jet around the Northern Hemisphere. This band of strong winds aloft marks the boundary between the warmer air of the tropics and the colder air of the polar regions: upper ridges are where warm air has pushed northwards whereas upper troughs are where incursions of polar air are sinking southwards. The ridges bring warm, settled conditions in summer, whereas the troughs are where stormy weather is generated. And the problem - both for the USA and for the UK - has been that through June and July, the system got stuck.

North Altantic 300hPa chart showing the jetstream and upper ridges/troughs

above: GFS 300 hPa windfields over the North Atlantic for July 14, 2012. The Polar Jetstream is clearly marked as the band of stronger winds (green to red-orange) and the series of ridges and troughs is annotated. Chart from Wetterzentrale.

Typically, ridges and troughs migrate along this conveyor of west-to-east winds, but recent weeks have seen the stalling of this progression, so that the USA has been stuck beneath a strong upper ridge while the UK has been stuck under the influence of an upper trough. The result has been, for us, non-stop wet weather with a mixture of dynamic and convective rainfall bringing an abundance of costly flooding incidents. But why has this stalling occurred?

At Skeptical Science, we recently discussed a March 2012 paper by Jennifer Francis and Stephen Vavrus, published in Geophysical Research Letters. It is worth returning to this publication in the context of the current conditions. Francis and Vavrus looked at how these upper-level wave-patterns were responding to the well-documented changes taking place in the Arctic, where the warming trend is strongly enhanced in comparison to the rest of the planet, to the extent that it actually has a technical name - Arctic amplification. The warming has markedly reduced the thermal gradient between the Arctic and the tropics.

The work identified two key effects: firstly a reduction in zonality - the west to east movement - and secondly an increase in wave amplitude, meaning a tendency for ridges to push much further north and troughs to push much further south. Think of how a sluggish lowland river tends to develop meanders: if the jetstream slows down it, too, starts to meander a lot more. In combination, these two effects lead to some areas having prolonged periods of anticyclonic weather, bringing heat and increasing the risk of drought, and other areas having more prolonged periods of cyclonic weather, bringing excessive rainfalls and flooding.

Francis and Vavrus found the effects to be most evident in autumn and early winter, consistent with times of low sea-ice, but they also suggested that the same effect in summer could be caused by early snow-cover melt in high-latitude land areas. We have looked at long-reach teleconnection between the Arctic and Europe in early winter before: the extraordinary cold spell of December 2010 is often cited as an example of this process, with a block of high pressure over the Atlantic and prolonged northerlies bringing Arctic air south over the UK and elsewhere. Although forecast models suggest a return to a more mobile scenario soon, the question remains: was June-July 2012 an example of what we can expect in future when the system stalls in the warmer months?

The work of Francis and Vavrus and indeed other researchers in this field, for the research is ongoing, will be put to the test by what actually happens in the coming years. There is no reason to expect the Arctic amplification effect to cease (barring a miracle from policymakers), so that if they are right, we should see a continuing trend towards slow-moving, high-amplitude waves in the Northern Hemisphere. The weather-extremes caused by such patterns are problematic simply due to their prolonged nature. Drought, affecting many USA states, is one result. Saturated ground across parts of the UK (leading directly to rapid run-off from heavy rain and an increased flooding risk) is another. Both have large impacts on agriculture: in droughts, crop yields are poor, whilst in a saturated situation, plant diseases are more frequent and harvesting may be difficult or impossible. Both outcomes have major impacts upon food prices, so that if flooding hasn't hit you in the pocket then climbing food-prices will. Is this a taste of things to come? Because if it is, then global warming, left unmitigated, is going to be very expensive indeed.

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Comments 1 to 16:

  1. Great post. It does not make life much better living on Ynys Mon where we are starting to feel like a worn out sponge, but at least we know the dynamics behinds this awful weather, and it underlines the issue that climate change is not all about heat waves and drought.
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  2. Indeed, Gareth. Some respite though is on the cards as things have finally stirred into motion: high pressure moves east over us this weekend and following a few unsettled days early next week it reloads from the west. I can hear the sound of a thousand fishing-rods being dusted - but will the mackerel still be around?

    It has been a difficult summer in the garden: today I harvested the shallots before they rotted in the ground and in 95% of cases all foliage was died-off to ground-level. Many small ones, but I'll replant them next March for green onions late April through to July. But it just makes me wonder how I should plan for coming years, as a grower. I never realised that resilience-thinking was needed in grow-yer-own, but it seems that it is!
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  3. Great post, John.

    My wife and I considered moving to Wales about a year ago, but I feared that big ocean west of Wales and what climate change might make it do. So we're still living south of the heart of Europe, where I fear the Mediterranean drought. ;-)

    I wink, but it's serious.
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  4. In Ireland we have similar concerns to Wales.

    There have been a couple of unprecedented flooding episodes in the last couple of years. Some areas of Cork city have been flooded twice and insurance rates are coming under pressure. This year, the Irish Met Office raised the average temperature used in their forecasts by 0.5C, and also precipitation expectations (while of course saying "this may be due to natural variation, not climate change").

    Though local councils were supposed to take account of climate change for 10 years or so, many new housing estates were built on flood plains during the housing "boom".

    As a personal anecdote, the nice leafy housing estate where I live and the adjacent one are divided by a small stream which runs down a culvert. While it seems insignificant, the local council did a study which showed that a 1-in-100 year flood could overtop the culvert and inundate about 1/3 of our estate and 2/3 of the adjacent one. Flood walls upstream are projected, but some downstream estates are worse affected, and they may get precedence in the work.

    It would all cost a few million euros, but our situation is multiplied by many around the country. Money has been set aside by central government, but inevitably there will be winners and losers. While my house would not be affected by a flood (by sheer luck, we bought a house on higher ground), it will no doubt cost people in insurance, and possibly render tham homeless after a flood.

    Just one way climate change will impact ordinary people as "1-in-100 year" floods become more common.
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  5. Thanks, Neven!

    I regard sea-level rise as a slower threat - and much of Wales is of high enough elevation that the overall shape of the country will be little changed if all the polar caps went - but the downside is the loss of much fertile growing land.

    Shoyemore - interesing point WRT flood insurance. There are some properties I know of locally that have flooded in recent years and one has been on the market three years with no takers - I suspect it's because nobody will insure the place any more. The house in question is hundreds of years old....
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  6. PS should explain that in the UK it is almost impossible to get a mortgage on a building that they won't cover against flooding. Hence this could become a big problem over the coming years.
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  7. Nice post, John. It is a natural counterpart to Rob's post on the US Midwest, but I'm stating the obvious.

    I'm baking here in Kansas, and have reduced my gardening efforts to simply trying to keep the rhubarb and asparagus alive.

    The current drought and heat here have made climate change more topical in the local paper, and I have come across comments to the effect that you guys are getting a lot of rain; therefore, it is just natural weather variability. I try to point out that less precipitation in some areas, and more precipitation in others is a long-standing prediction. Hence, the term 'climate change'.

    There is always debate about whether or not any event in particular is a result of climate change. I'm not sure what the alternate hypothesis is; that you can significantly change the energy balance of the planet without affecting weather?

    Side note: My father is an ex-Brit and he went to university in Wales. Cheers
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  8. Housing in flood risk areas should be insured by the government. This would ensure new developments are built outside flood risk zones and existing at risk properties are given adequate protection.

    Unfortunately flooding soon drops off the political radar. This was demonstrated recently by the big cuts to the Environment Agency budget of nearly 30% despite numerous reports stating spending need to increase every year just to maintain the existing level of protection.

    The design process is also lacking. I work as a surface water and drainage modeller and we are only required to protect properties against a 1in40 year rainfall event. The design storms we use are based on the Flood Evaluation Handbook, which only uses data up to 2000 I believe. Climate Change is not factored into the design process. Hard engineering options, such as up sizing of pipe or large storage tanks, are preferred to sustainable options for maintainence reason.

    Responsibility is also an issue. Currently the EA is responsible for river flooding and the water companies are responsible for urban drains. Things like culverted water course and engineered open channels in urban regions seemed to be a bit of a grey area. What is needed is an integrated approach with complete stakeholder participation ideally on the catchment scale. Things are starting to head in this direction with councils being given the overall management role. However tight council budgets have meant that flooding is not seen as a priority.
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  9. That is spot on John. Great post. The intense cold spells we've seen in recent winters were result of the same 'stalling' no doubt. The future of the british isles looks like one of very wet and very dry seasons.

    I do have an issue with the increase in atmospheric water vapour caused by GW and recent expansion of Polar Mesospheric clouds (NLC's in lower latitudes) The issue itself being that at those heights solar UV busts apart the water molecule into its constituent atoms and the lightweight hydrogen atom bubbles up to the top of the atmosphere and escapes. Although for the size of a human there seems to be a lot of water on Earth, in proportion to the volume of the planet there is very little water indeed. Hydrogen is rare on earth, the main storages being H2O and CH4, therefore, like in Venus, when it all flies away water will be a thing of the past. This of course will not occur during our lifetimes, but it will certainly happen if GW is not slowed or stopped. As you mentioned 1 degree is about 7% increase of WV and at the same time, as the atmosphere warms, it expands leaving more room for higher humidity content. No matter which way you look at it is not looking good in the long term. We broke the sky :(
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  10. shoyemore:
    Hopefully, folks understand what happens downstream when you build stronger flood walls upstream.
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  11. JohnMashey

    The stream (called a "river" on the maps) heads off towards the Boyne (one of Ireland's major waterways, see Battle of same!), but on the way has flooded back gardens when it is in spate.

    The local plan is to cut out some meanders and deepen the bed so it continues unvexed to the Boyne. In fact, downstream folks have experienced far more bother than we have.

    Ok, an innocuous little example, but multiply it by what may be hundreds of cases of housing too close to floodplains or seashores, and tens of thousands across the world, and you have one way climate change will "come home" to people in short order.

    Incidentally, Matthew Kahn in Climatopolis argues that Governments should not intervene in cases like this and householders in floodplains, or near seashores, should be forced to accept their losses, or pay their own way. To be honest, I have not read the book, just heard the author on video, and I cannot find the link.

    Kahn's argument is for a completely free-market approach to climate change.
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  12. I regard sea-level rise as a slower threat

    I wasn't referring to SLR, but to the storms that the Atlantic gives birth to. That's why I hide behind the Alps (not that it's working).
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  13. Like I said, it isn't working: Deadly mudslides sweep through Austria. More from the local media.

    Apparently we are witnessing a one-in-30-years storm here in Styria, except of course that the last one wasn't 30 years ago...
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  14. Found the Matthew Kahn video

    The Free Market as a Solution to Climate Change

    Kahn sees the insurance industry as a key "sender of a price signal" about housing. Governments should not help communities at risk because they will only attract more inhabitants to those areas.

    These ideas are refreshing. But one wonders, if inaction continues, how feasible is the "free market" if tens of thousands, if not millions, are simultaneously affected?
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  15. I am not convinced at all that this flood in Wales can be or should be attributed to Anthropogenic effects. The Wye and the Severn valleys are both fed by the orographically enhanced rainfall that flows down into the valleys. The highest river level on the river Severn was recorded in 1947 when it reached a level of 5.8 metres. No flood since has ever reached this level. All this with added developments on flood plains and more run off. I believe the Wales floods along with Boscastle and Cockermouth were isolated events. Yes, in the future that MAY change. It is not even empirical evidence that you show here. Thailand had terrible flooding in 2011, but the British CCC determined that this flooding was not abnormal and the severity was caused by other factors. I think caution needs to be observed here. I am also not convinced that the thermal conveyor is being disrupted by 'Arctic amplification' as has been suggested. Our climate is very diverse and ever changing and I think it would be wrong, in the absence of a long term pattern, to draw this conclusion. I know hindsight is a wonderful thing, but without it we could be going down the wrong avenues.
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  16. Sorry - missed this (#15) - yes I'm well aware that the Cambrian Mountains see orographic enhancement on a routine basis. Was the 1947 high related to snowmelt? Locals in Talybont have long told me that the 1963 flood had that as a big factor.

    Not sure what you are saying with "I am also not convinced that the thermal conveyor is being disrupted by 'Arctic amplification' as has been suggested." That is not what has been suggested above.
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