Donegal Extreme Rainfall and Floods of August 2017

By Adam Pasik, ClimAtt Master’s Student, Department of Geography, UCC

The Weather of August 2017

August 2017 in Ireland was rather cold and dull, yet for the most part well within its normal scope of variability. All twenty five principal weather stations recorded mean temperatures somewhat below their 1981-2010 long term average (LTA) and the number of recorded sunshine hours was also below the LTA at most stations. Overall, August was unexceptional in terms of precipitation with monthly totals ranging from 75% to 185% of the LTA across the country, and only one day with gale force winds was recorded (Met Éireann, 2017a).
However, one event of localised extreme rainfall took place in the north western part of the country, causing extensive and severe flooding and landslides (Donegal Now, 2017; Maguire, 2017a).

Meteorological Background

In the early morning of the 13th of August the United States National Hurricane Center (NHC) issued its first public advisory notice on the tropical depression no. 8. The NHC continued to issue updates on the storm four times daily until the evening of the 17th of August, when the storm moved away and no longer endangered the East Coast of the United States (NHC, 2017).

The initial tropical depression developed east of the Bahamas and began to travel north-northwest towards the United States. It had an estimated minimum central pressure of 1,011mb and sustained winds of up to 35mph. With the decreasing pressure and strengthening winds, the depression evolved into a tropical storm, and received the name Gert on the evening of August 13th (NHC, 2017). Gert attained hurricane force in the early morning hours of August 15th, and began to veer northeast. Now travelling away from the continent, Gert continued to increase in strength and attained its maximum strength around 3am on August 17th, reaching 105mph in sustained winds with stronger gusts and a minimum central pressure of 967mb. From there on, the hurricane began to weaken quickly as it continued to move northeast into the colder waters of the North Atlantic, and was reduced to a post-tropical storm by the evening of August 17th (NHC, 2017).

The remnants of Gert became absorbed by another low pressure system travelling across the Atlantic, before making landfall in the northwest of Ireland in the afternoon of the 22nd of August. The low pressure weather system brought extremely heavy, although very localised, rainfall yet no significant winds. North Co. Donegal was most affected with an extremely high 77.2mm of rain being recorded at Malin Head weather station, most of which fell in the space of just 8 hours (Fleming, 2017). This was the second wettest day (and the wettest August day) recorded at Malin Head since 1955. The only wetter day recorded was December 5th 2015 with 80.6mm of rain. However, on that day the precipitation was more evenly spread over a 24 hour period (Met Éireann, 2017b). At Malin Head, August 2017 as a whole received 185% of the LTA rainfall, where the above mentioned event was responsible for 83% of this total (Met Éireann 2017c).

Impacts: Flooding in the Northwest

This downpour resulted in flash flooding in the eastern part of Co. Donegal, Co. Tyrone and Co. Derry/Londonderry. Flood waters caused severe structural damage to major roads and destroyed bridges (McClements, 2017; McClements et al., 2017). Many homes and businesses were damaged and local farmers reported losing farm animals to the flood waters (Highland Radio, 2017a). Tens of families registered as misplaced and worked with the local council to avail of temporary accommodation due to their homes being inundated (Maguire, 2017b). The city of Derry was virtually inaccessible by road and its airport had to temporarily shut down and cancel all flights (Highland Radio, 2017b).

Severe flood damage to the Old Mill bridge, Buncarna, Co. Donegal. Photograph courtesy of Pat Colhoun

Worst affected however was the Inishowen Peninsula in Co. Donegal, where the damages included collapsed bridges and some roads being simply washed away. Some 1,500km of the road network were affected by the disaster on the Peninsula alone, parts of which are expected to remain impassable for weeks (Maguire, 2017b).

Restoration works at the Cockhill Bridge, Buncarna, Co. Donegal.
Photograph courtesy of Pat Colhoun

There were power shortages following the rain, caused by the flooding as well as lightning strikes. The Electricity Supply Board (ESB) estimated that at the height of the storm some 25,000 dwellings were without power throughout the country. On Inishowen 1,600 homes were still without power the following afternoon. In many cases it was deemed unsafe to restore the power until the flood waters have receded (McNeice, 2017). Irish Water has announced several burst mains and damages to wastewater infrastructure due to flooding, causing shortages in freshwater supply on the Peninsula (McNeice, 2017). At least two instances of landslides were also reported occurring at Grainne’s Gap, near Muff, and a smaller one in Urris (Donegal Now, 2017; Maguire, 2017a).

Flood damage to the football pitch of Cockhill Celtic, Buncarna, Co. Donegal. Photograph courtesy of Pat Colhoun

With more than 100 people having to be rescued by the emergency services from their stranded cars or flooded properties, it is surprising that no serious injuries or deaths resulting from this event were recorded (McClements et al., 2017).




Analogies for attribution of extreme events

Unlike other research fields where the work is performed with tangible samples and materials, attribution studies of extreme weather events don’t offer that possibility, which make them sometimes quite difficult to understand.

The analyses carried out by meteorologists and climatologists usually involve huge amounts of data which seem abstract until you make sense of them, identify what you are looking for, and present the results in a visual and comprehensible way.

Through the review of the literature of attribution studies of extreme weather events, it is possible to find analogies to make the basis and methodology of theses studies easier to understand by comparing them with possible everyday  activities or actions.

Maybe the first and most well known analogy is the one of Professor Myles Allen of the loaded dice (you can watch the seminar Loading the dice: climate change and extreme weather in our former post). If you repeatedly roll several dice, it can happen that initially everything seems normal, nothing extraordinary. However, if you look twice, more carefully, you will soon realize that the number six seems to occur frequently. Of course, this could be by chance, but it could also be associated to other factor/s which would explain this apparently extraordinary number of sixes. But in order to discover that, you would have to roll the dice quite a few times to get to know the chances of obtaining so many sixes from all the rolls of the dice. So, in attribution of extreme events, it is also needed to run the model over and over again in order to obtain multiple results (ensembles) that can be analyzed in order to be able to draw some conclusions.

Another analogy appeared in the Bulletin of the American Meteorological Society (BAMS) of 2014 (Herring et al., 2014), whose vision of how this new research field works is reflected in a video entitled Steroids, baseball, and climate change. Before you watch it, here is the outline. Imagine a baseball player who starts taking steroids. After that, he hits an average of 20% more home runs than before and so probably you would attribute this amazing improvement to the steroids. But was this the only factor which changed during that period? Maybe, he was able to spend more hours training, hired a new trainer, or changed his diet. But if all those factors didn’t change, we can say that the steroids are the responsible for that increase in the probability of him hitting more home runs.

(©UCAR. Video by Noah Besser, produced by UCAR Communications for AtmosNews: NCAR & UCAR Science. This video is freely available for media and nonprofit use.)

The BAMS issue of 2013 (Peterson et al., 2013) echoed another analogy from the University Corporation for Atmospheric Research (UCAR, 2012). In this case they use a car instead of a baseball player. If we go every day to work by car, but we increase the speed of our journey, the greater the speed, the more the likelihood of us having an accident. However, if this event unfortunately happens, it may not be due to the speed. It could be related to bad weather, another driver speaking by phone, an obstacle in the road… In this case atmospheric greenhouse gases are analogous to the speed. We can study the odds of having such an accident at a particular speed but also taking into account some of the other factors. That’s to say, maybe the speed was a key factor in the accident, but some other aspect could have played a role too (this is akin to natural climatic variability).

In these two cases is clear that greenhouse gases could have being playing a role (steroids, speed), but that there are many other factors which could be influencing the result to some extent. However, understanding how greenhouse gases affect the likelihood or magnitude of an extreme weather event is a key step in the decision making process and the adaptation and mitigation measures for the long term, which should be still effective in 50 years’ time or more.

The third and last analogy is that of the cookies. This more recent analogy is based on the book Attribution of Extreme Weather Events in the Context of Climate Change from The National Academies of Sciences, Engineering and Medicine, about which we have already talked in our post Getting to know attribution studies: basic concepts and references. Dave Titley, Committee Chair of the Board on Atmospheric Science and Climate, gave a presentation entitled: Attribution of Extreme Weather Events in the Context of Climate Change within the framework of the Report Release Briefing on the 11th March 2016. You can read and download Dave Titley’s  presentation in the following link:Also, here is the fragment of Dave’s talk so you can understand better the slides:

One of the slides it is called A Banking Analogy. In this case one cookie is the event, similar to the home run or the accident by car in the other cases. All the factors that are involved in this case are the ingredients of the recipe, and they may or may not be responsible to some extent for the output, which can be different according with the proportions of the different constituents involved: quantities of the ingredients, temperature of the oven,… This analogy is fully explained in the following video:

Even though climate models may be intangible, I hope you can have a better understanding of attribution science of extreme weather events. So, here is a video where you can apply what you have learned so far and put all the pieces together to become aware of the importance of this methodology and the outcomes it can offer us in order to be prepared for a changing climate.

 This video is presented by the Climate & Development Knowledge Network (CDKN), who has an initiative to raise risk awareness in the developing countries. This Raising Risk Awareness initiative uses “state-of-the-art science to help Asian and African societies to understand the role of climate change in extreme weather events and prepare for future ones“.

If you feel now that you can be a part of the development of this exciting new field of research and want to contribute to the objective of ClimAtt, you can do it through the World Weather Attribution (WWA) project. Please, just click on the following image:

This project depends upon enthusiastic volunteers, citizen scientists who contribute to improvements in the science of attribution of extreme weather events and make a positive impact in societies around the world by offering some computing time.