As previously reported, the NERC-funded EMERGE Project has brought together scientists, researchers and early warning specialists from across Iceland, Norway and Scotland including the Scottish Environment Protection Agency. Following successful exploratory workshops in Oslo (May 2022) and Glasgow (March 2023), it was the Icelandic Meteorological Office (Veðurstofa Íslands) who hosted the final project event during the week of the 8th to 11th May.
The Icelandic Meteorological Office (IMO) are responsible for monitoring, informing, forecasting and providing warnings for natural hazards. In no other country that the project has been exposed to, can you observe such challenges around extreme and multiple natural hazards including the impacts of volcanic ash and extreme flooding. As witnessed on a guided tour of the Svínafellsjökull glacier and surrounding area, there is a real feeling of living on the edge of potentially extreme hazards.
The workshop was attended by staff and specialists from IMO, Iceland’s search and rescue team (ICE-SAR), University of Iceland, the Civil Protection Authority and the Icelandic national broadcasting authority, who were recording a podcast on natural hazards in the country.
As discussed, glacial floods (jökulhlaup) present the greatest risk to people in Iceland, with 1 to 3 per decade causing significant damage to bridges, roads and infrastructure. “Medium sized floods are those with a maximum discharge of several thousand cubic metres per second”, and, as illustrated by various speakers, the scale of the issue is being made greater, with mass tourism in the past decade increasing the risk exposure.
One point was made by both the IMO and the Civil Protection Authority (Almannavarnir), in that there is a fine balance between timely warnings, when you only have 45-60 minutes warning for a typical jökulhlaup, and false warnings… “always be open about communicating uncertainty” was the key message from the Almannavarnir representative Ágúst.
Reflecting on the common challenges and issues across the three countries, one question has been consistent… are these risks worse in remote regions given the lack of monitoring? Although all organisations are working towards improved observational networks where needed. Tourism, whilst not unique, is certainly a bigger challenge in Iceland and how to warn of immediate hazards with such a transient population. It’s interesting that the IMO have already embraced cell broadcasting as a regular form of communication given the recent Emergency Alerts trial in the UK. Also, whilst the use of the risk matrix is common across Iceland, Norway and Scotland, so is the challenge of ‘yellow fatigue’ and communicating uncertainty and low confidence events.
The EMERGE Project has developed successful collaboration and sharing of ideas, and is unique in focusing on remote regions and multiple hazards. There will be more to follow from the project, including a planned publication around project learning, including warning communication where at-risk customers have differing appetites for information and dealing with compound events.
In August this year, SEPA published its new Flood Warning Development Framework. The framework defines the direction of travel for flood forecasting and warning services in Scotland with the aims to maintain and improve existing, and deliver new, operational flood warning services; upgrade capabilities through development and innovation; and deliver enhanced digital communications and an engaged and customer focused service. So what does this Framework mean for surface water flood forecasting capabilities?
In previous articles (Quick as a Flash Part 1 and Part 2) we’ve commented on how science developments have supported new first guess forecasting tools to handle the uncertainty in predictions. Although, the development of such tools was only seen as the first step, at the time we said “if we are to truly get to grips with surface flooding, we are likely to need much better weather prediction models and many more alternative forecast scenarios to properly quantify the uncertainties. This will require substantially more super-computing capacity in the years ahead.”
Later developments have included the implementation of the UK’s first operational surface water flood forecasting system operating at a 24-hour lead time. The approach combined advances in the science of forecasting convective rainfall events in the Met Office with a new blended ensemble (MOGREPS-UK) and nowcast ensemble combined with the use of SEPA’s pluvial flood risk maps and made operational through the Grid-to-grid model within the FEWS platform.
As described, following the trial of this system “The lesson we learnt during the (Glasgow) Games is that the staffing requirements to operate such a service are high due to the need to continually monitor the forecast, given the variability between model runs, and the briefing requirements to help end users understand the uncertainty.”
As set out in the Framework, SEPA as part of our work with the Scottish Flood Forecasting Service, intend to develop new capabilities for surface water flood flooding for the public and infrastructure sectors such as transport. As referenced in the Framework, a recent Discovery report has set out a future road map for developing our approach to this challenge.
The Discovery report puts forward five recommendations. The first to ensure we have competent staff to enable confident decision making in uncertainty. Further recommendations propose coupled meteorological and hydrological approaches to forecasting, building on recent research towards improved surface water flood forecasts for Scotland. The report concludes that we should explore how we work in partnership with other agencies in supporting a climate resilient infrastructure and explores the topic of warning for surface water events. As suggested following the London floods of 2021, there could be justification for having stand-alone warning systems for pluvial flooding.
Following the Pitt review into the 2007 floods, the UK witnessed a step-change in hydrometeorological capabilities with significant advances in river and coastal forecasting. It’s not impossible to imagine that we now need a similar step-change in capabilities to ensure we are prepared for the future impacts of surface water flooding due to climate change.
The workshop was held at a time of growing concerns around water scarcity in Norway. Following a winter of between 30% and 50% of the typical precipitation and a spring with limited snowmelt runoff, many reservoirs are now very low. The result is a region-wide call for reduced water use and cascading hazards such as forest and grass fires in the south of the country.
The aim of EMERGE is to develop new long-term partnerships with international researchers to better understand the unique challenges natural hazards pose in remote settings in Iceland, Norway and Scotland; and there’s plenty all partners can learn from the Norwegian approach to multi-hazard early warning.
NVE operate Varsom a trusted multi-hazard warning system providing national and regional warnings for flood, landslides and avalanches. The service is operated in partnership with the Norwegian Meteorological Institute. Whilst the public-facing service operates a simple escalating green-yellow-amber-red risk status, a lot of the challenges include yellow warning fatigue, with the warnings ‘operating more in log scale rather than linear’.
A key challenge for remote areas in Norway is the lack of observations supporting early warning systems. Similar to Scotland, Norway suffers from poor radar observations across the upland areas and limited spatial coverage of rainfall observations in remote and upland areas. To supplement this, NVE and the Met Institute use citizen science observations such as the NetAtmo network of independent rainfall observers and crowd-sourcing observations of hazards through the Regobs (Register Observations) application.
Operating a service that supports both urban and remote regions in Norway is also a challenge. With that, in 2022 NVE launched a four-year project FlomRisk is collaboration with Met Norway, the Norwegian Road Authority, the Directorate for Civil Protection and the Mapping Authority. The project aims to improve the flood warning system in Norway by testing the implementation of a flood impact-based warning system in five pilot municipalities, giving more precise, local and useful information.
Hervé Colleuille, Head of the Forecast of Flood and Landslide Hazards in NVE was keen to use the EMERGE project as a knowledge sharing platform, “We are particularly interested to learn from the flood warning system operating by SEPA in Scotland. Norway has quite close geological and meteorological conditions, the same amount of population for an area four times larger. The Scottish well-functioning flood warning system is a model and a source of inspiration for us in Norway.”
In the penultimate account in a series of articles on how the mountains of Scotland influence our approach to monitoring and flood forecasting, Mike Reading from the Met Office writes about the challenges of mountain forecasting.
“In Scotland, we face some very challenging upland environments for flood forecasting, as explained in this article. Forecasting for mountain regions follows a similar process to any other forecast. This starts by looking at observational data, then analysing the initial situation. Computational processes are then applied to create forecast values. Finally, a conclusion is drawn from all the available information.
Observational data largely consists of measurements from weather stations, rain radar and satellite imagery. The analysis of observational data helps to verify model data and influences the final forecast. Observational data is also incorporated into computer models through a process called data assimilation, which acts to bring the model closer to reality at the start of each model run.
The meteorologist will take the available observational and model data and use these, along with their own experience, to create a forecast for precipitation accumulations in the mountains, whether this will be locked-up as snow or fall as rain, and whether there is likely to be melting of lying snow.
What are the challenges around forecasting in the mountains?
Accurate observational data is crucial to creating an accurate forecast. However, official weather observation stations in the mountains are rare, with only four in Scotland and one in England, and these stations only recording wind, temperature and dewpoint. This requires meteorologists to look at surrounding low-level observations then assess how they are likely to relate to conditions at higher altitudes, often a less than straightforward task. Rain radar is also less reliable in mountain regions as the topography often interferes with the radar beam. Algorithms attempt to mitigate against this to an extent by enhancing the rain radar signal over mountains to account for the effect of orographic uplift. However, these algorithms are not perfect and a lack of rain gauges in the mountains to verify rain radar means that there is always a degree of uncertainty over exactly how much precipitation is falling over the hills.
In addition to limited observational data, the complex terrain of mountains is impossible for models to recreate exactly. This is important because mountainous terrain will have a large impact on rainfall accumulations, enhancing it in some areas through orographic uplift and the Seeder-Feeder effect, and acting as a focus for thunderstorm development in the summer months. Other areas will experience much drier weather than they would otherwise experience in flat terrain. While all models will attempt to recreate these terrain effects, the extent to which they achieve this will depend largely on the model resolution.
Past upgrades to the Met Office supercomputer have vastly improved the accuracy with which the models represent the topography. The highest resolution operational model run by the Met Office, the UKV with a resolution of 1.5km, will usually do a good job of picking out areas of orographic enhancement, but even these high resolution models will fail to capture local features and the exact height of hills. This will result in the raw models often underestimating, and occasionally overestimating, precipitation accumulations in the mountains. Post processed model data can often do a better job than raw models due to taking into account local climatology and past observational data, but once again the sparsity of observations in the mountains limits the effectiveness of this post processing.
Another challenge that mountain environments pose to flood forecasting is snowmelt. While models have become much better at predicting whether precipitation will fall as rain, sleet or snow, what happens to snow after it reaches the ground is more challenging. In a mountain environment snow will rarely fall as an evenly distributed layer. Wind usually transports it resulting in relatively bare areas and deeper drifts where wind-blown snow has accumulated. In addition to this, the UK’s maritime climate often results in freeze-thaw processes that can transform loose powder snow into hard snow-ice. Both of these processes can make snow more resilient to melting. These factors are captured poorly, if at all, by the model, and need to be considered by the meteorologist.
One final challenge of mountain regions for flood forecasting is assessing how impactful any potential flooding will be. Due to orographic uplift, rainfall totals in mountainous regions are often considerably higher than in surrounding low-lying regions. Mountain regions tend to be far more resilient to these higher figures due to their sparse population and the frequency with which they experience high rainfall events. However, they will channel accumulations from across a large catchment area into a small number of watercourses at lower altitudes. The time period over which accumulations fall and how quickly this feeds into watercourses at lower levels must be considered. Wind direction also plays an important role; large rainfall events brought in by winds from an unusual direction can often be more impactful than those on prevailing winds.
All of the above results in the mountains being one of the most difficult regions to forecast for. Despite huge advances in model performance over recent years, a considerable amount of experience and local knowledge remains crucial to providing accurate guidance when it comes to forecasting what impact mountainous regions may have on flood forecasting. The Met Office forecasting team at Aberdeen, part of the Scottish Flood Forecasting Service, has been creating mountain forecasts since the early 1980s. This has built up valuable experience in predicting mountain weather and, working alongside the forecasting hydrologist, now has a very important role in forecasting its potential to lead to impactful flooding.”
In this article, we take a retrospective view on a decade of challenges and achievements for the flood forecasting service with some of the many people who have made it a success. A period when Bertha, Ciara, Desmond, Dennis and Frank all became popular household names in the forecasting community, what a ten years it has been…
Following major flooding in 2007, the Pitt Review made recommendations for hydrological and meteorological forecasting services to come together to provide technical capabilities of forecasting, modelling and warning against all sources of flooding and increasing the preparation lead times for emergency responders. Whilst the Flood Forecasting Centre for England and Wales was established in 2009, other European Centres such as the national forecasting centre in France (SCHAPI) was held up as a good example to follow having developed a programme of forecasting service improvements such as the Vigicrues service – aimed at improving vigilance to flood risk.
Alan Motion (Former Head of Strategic Relationships for Devolved Administrations at the Met Office) explains the strategic drivers behind the service back in 2010: “To achieve an operational flood forecasting service in Scotland the first vital step was to bring key stakeholders on-board, especially within the prospective delivery partners and the relevant areas of Scottish Government. Gaining an understanding of the practicalities and politics involved was identified as a key factor even before we got to the drawing board. Reaching out to both national and international colleagues was vital to the success of that quest. The challenge then was in assessing the knowledge gained against the resources we felt may be available to us to support delivery of a science-led, largely user-defined, service for Scotland.”
As Alan says, establishing the service was at the time, “…full of challenges and the usual ups and downs such projects experience but very fulfilling.”
The virtual centre of expertise, building on existing operational teams at SEPA and the Met Office in Perth and Aberdeen, went live on the 9th March 2011. However, this was not without various challenges as Richard Maxey (SFFS Co-ordinator at SEPA) explains: “When I joined SEPA at the end of August 2010, apart from the concept and the agreement to resource and launch the service, there was very little actually in place. With the launch a mere six months away, we had to develop a Flood Guidance Statement, and work out how we would deliver it operationally …nothing will match the frenzy of activity that led to the initial launch of the service but by February 2011 we had a product and a process.”
Collaboration has remained key to the success of the forecasting service. As Pete Buchanan (Former SFFS Co-ordinator at the Met Office) explains: “Working with multidisciplinary colleagues in the respective Met Office and SEPA organisations demonstrated how much could be achieved in a relatively short timescale. The commissioning and early consolidation of the SFFS provided an effective, efficient and often innovative approach to flood forecasting in Scotland.”
Underpinning the service with a science-led approach to forecasting across Scotland has been crucial, especially in the early stages when systems were limited. New coastal forecasting systems have been implemented and a Heavy Rainfall Alerting system to support surface water flooding introduced. One of the most significant achievements and game-changer for forecasting capabilities for the whole of Scotland was the introduction of the five-day countrywide river forecast. The approach was to couple high-resolution numerical weather prediction models with a 1km gridded hydrological model, to forecast the potential for flooding for the whole of Scotland. Richard explains: “These early days were accompanied by a rapid uptake of forecasting products, such as the Grid-to-Grid model and a rejigged daily weather forecast from the Met Office more suited to the new ways of working.”
Along with the science and system challenges, there were some very early operational challenges. The Dumfriesshire Floods of December 2013 was one of the first significant events, which resulted in an Amber Flood Guidance Statement. This was followed in 2014 by the remnants of ex-tropical storm Bertha that caused significant flooding across Morayshire. In both cases, the use of the hydro-meteorological modelling in G2G proved important in the early identification of the specific flood risks. The then Environment Minister, Paul Wheelhouse, commented following the Dumfriesshire flooding “I was extremely impressed by how well prepared all the authorities were – aided by the timely and accurate flood forecasts from the Scottish Flood Forecasting Service”.
Focusing on the impacts of flooding has increasingly been the focus of the daily decision making process, with the meteorologists and hydrologists always asking the question, how bad is it likely to be and where are the impacts. As a meteorologist for the service and former BBC weather reporter Peter Sloss explained in an article in 2014 … “(we have seen) a huge improvement with few surprises during the evolution of a weather event. Always fine-tuning the details of the weather forecast up to and during the event and communicating this message to all concerned. Also… the meteorologist is much more focussed on the impacts of the rainfall rather than just the amount of rain expected.”
During the ten years, there has been no greater flooding impact than during the winter of 2015 and 2016 with a succession of storms including Desmond and Frank. Storm Frank alone was severe with 101 Flood Warnings issued across Scotland, two Severe Flood Warnings and over 1,000 properties flooded. Karen MacDonald was a flood forecaster during this spell of flooding and recalls her experience: “Storm Frank was one of the first flood events that I was primary forecaster for. I had only been on the (forecasting) rota for 6 months so I was keen to get some weather to test my new skills but I remember seeing the forecast coming in and knew that this was an exceptionally big one! Across the country, most of Scotland’s major rivers were flooding so it was challenging to keep monitoring them all simultaneously.“
That winter of 2015/16 witnessed the issuing of the first Red Flood Guidance Statements and really tested the capabilities of the service. As Karen says: “Although this was an incredibly demanding flood event to deal with, I am glad that I was able to contribute to the response and am exceptionally proud of what we managed to achieve. Despite the widespread significant flood impacts, our Flood Guidance Statement and messages were able to provide the information to the responders on the ground that needed it to assist the public. That is why we do the work that we do, to help others and to make a difference. So as a new forecaster I was able to see first hand of the benefits that our service provides and appreciate the value of my new skill.”
As Karen says, improving the vigilance and preparedness to flood events has always been a primary aim of the service. Following Storm Frank, Mark Thomson, Principal Resilience Adviser at Dumfries and Galloway Council said, “It’s not just about the Forecasts/Alerts, it’s about the added value that the experts can offer when you want more detail regarding potentially vulnerable areas. It is fair to say that the SFFS outputs are the most important forecasts we receive on a daily basis, given the vulnerabilities we have! The key piece of information was … predicting record breaking levels on the Whitesands, that could exceed those experienced in 2009, 2013 and potentially as far back as 1982.”
As for forecasting science challenges, there is none greater than surface water and the ability to predict the timing, location and impacts of convective events. In the build up to the 2014 Commonwealth Games and understanding of the potential for disruption due to surface water flooding as seen in 2002, research was commissioned into how to develop a city-wide forecasting system. As Linda Speight (former SFFS Senior Hydrometeorologist) explains: “Developing the surface water flood forecasts for the Glasgow Commonwealth Games was an exciting opportunity to see what was possible with the emerging scientific capabilities for forecasting intense rainfall.”
“The conversations between decision makers, meteorologists, hydrologists and model developers gave us a unique perspective on balancing end user needs with science and practical limitations. It truly was a co-designed product. Involving everyone early on meant there was a shared understanding of the uncertainties in forecasting the timing and location of surface water flooding and hence realistic expectations of the output. Although there were still some interesting communication challenges for the SFFS during the Games!”
The global pandemic has provided challenges for many operational services. The forecasting service has had to adapt and work in different ways whilst still maintaining the critical role of providing the early forecasts and guidance to responders. As Jess Pryer (SFFS Co-ordinator at the Met Office) says: “The last twelve months have been a testing period for the Scottish Flood Forecasting Service, however despite this, neither organisation has stood still and we have embraced the challenges of COVID restrictions, exploiting technology and adapting our working practices a lot more quickly than we may have done.”
Cordelia Menmuir (SEPA’s Flood Forecasting and Warning Manager) recognises the significant importance of the forecasting staff that drive the service: “At SEPA we have always maintained that it is our knowledgeable staff, their experience and commitment to do the best they can to protect the people of Scotland that underpins the Flood Forecasting Service. Never has this been more apparent than over the last 12 months.I am immensely proud of what has been achieved. If we are to be ready to face the next challenge that gets thrown in our direction it is essential that we learn from our experiences and continue to build back better and stronger than before.”
Likewise, Jess reflects on what we have experienced: “I want to learn and build on these changes going forward to help the partnership evolve, collaborating more effectively in the future using each organisations expertise to develop and improve flood forecasting for Scotland.”
So what may the next decade bring? One of the main science priorities remains addressing the ‘Grand Challenges‘ of surface water flood forecasting with work progressing to develop this capability further. We witnessed some extreme rainfall in August 2020 that acts as a stark reminder of the potentially devastating impacts of these types of events under a changing climate. We are also expanding the reach of the forecasting service as a new 3-day flood forecast product is set to launch later in 2021 to the public, a big leap from the partner-only service currently provided.
Author acknowledgements – huge thanks to everyone that has contributed and supported the service over the past decade. A selection – of the many – words from a handful of people involved have been used in the article but appreciate there are so many others that have made the service a success- many thanks to you all!
In the second of a series of articles on how the mountains of Scotland influence our approach to monitoring and flood forecasting, Dr. Andrew Black writes about the role of hydrometric observations.
“Continually rising global temperatures over the coming century might easily be associated with a lessening in the importance of snowmelt in many parts of the world. However, the ‘Beast from the East’ of 2018 and the wintry conditions as I write this in early 2021 show that snow and its subsequent melt very much remain part of Scottish climate for the time being, especially in mountain and upland landscapes, from where Scotland’s largest river systems flow. It is not uncommon for large floods in Scottish rivers to be caused partly by snowmelt (Figure 1).
This article seeks to examine the potential benefits and pitfalls of using tipping bucket rain gauge (TBR) data from higher elevations for flood forecasting purposes during snowy conditions.
Observations during snowfall
Observations may be direct or indirect. The most direct observations can be obtained from instruments designed specifically for monitoring snow water equivalent or snow pack depth (Figure 2). Weighing gauges and acoustic distance measuring equipment is relatively rare and, even when they are available, they provide information only for one particular point, and may reflect local influences such as aspect combined with snow blow after initial snowfall. Webcams such as those hosted by Traffic Scotland and Winterhighland provide visual assessments which can be useful for assessing conditions, especially the former which include infrared capability for night-time.
By way of indirect observations of snowfall, TBRs are more commonly available, so there are advantages in terms of geographical coverage, but users of the data from these gauges must beware of possible problems arising from under-catch and the timing of melt. A helpful early sign of snowfall is the occurrence of rainfall at low altitudes while adjacent higher-altitude TBRs record minimal or zero precipitation. Reliable air temperature data helps in interpretation. A good example is shown in Figure 3: intense rainfall at low elevation was confirmed by eye-witnesses, while adjacent gauges on higher ground registered no tips.
Observations during melt events
Problems may arise with use of TBR data during snowmelt events owing to possible under-catch of snow, due to aerodynamic effects and also the timing of recorded gauge tips. However, some gauges may provide an insight into imminent high runoff on at least some occasions. The same Feshie snowmelt event of December 2018 is illustrated in Figure 4 showing the lag between TBR accumulations at a range of elevations from 260 – 900 m OD and river levels at a similarly wide range of elevations.
While it may be interesting to examine the lag time in various ways between TBR accumulations and river level responses, one gauge in Figure 4 stands out – an ARG100 gauge at 520 m which was situated on the east side of a small hummock in the lee of prevailing winds – not normally best practice for gauge siting (and since decommissioned). This gauge began to record at the same time as other gauges, but had recorded 33.2 mm in the first 12 hours of the event, by which time none of the other 5 gauges had recorded more than 9.2 mm. Similar tendencies are found in other snowmelt events, suggesting the 520 m gauge may be able to offer some ability to indicate the onset of a major snowmelt event: the gauge possibly benefits from being especially exposed, allowing faster melt than in other gauges, while also giving a crude indication of the amount of snow in the catchment.
It must be more typical to regard protruding rain gauges as a problem leading to under-recording of snowfall amounts and unduly early recording of melt compared with the majority of the snowpack. Figure 5 illustrates a set of gauges in melting snow at the Talla gauge comparison site at 425 m OD. At the time of the visit, none of the above-ground gauges contained any snow, save for the gauge buried in the pit, with its rim at ground level. The above-ground gauges clearly provided a misleading characterisation of the melt process – of course, the gauges are rain gauges not snowmelt gauges.
The Feshie network of TBRs captures data from an unusually large range of altitudes, from 260 – 900 m OD, and provides an abundance of data with which to examine snowmelt events. This brief review illustrates how, by extending from the valley bottom up to some of the highest ground, it is possible to obtain a much fuller, if inevitably imperfect, portrayal of snow-related fluxes from the Cairngorms, which would be of value to a flood forecasting duty officer.
TBRs protruding above the ground surface may be unrepresentative of the general timing of melt in a snowpack, but might on some occasions be of value in giving early warning and an approximate snow water equivalent for an imminent snowmelt flood.”
Research sponsors and collaborators at Wildland Limited, Cairngorm Gliding Club, Borders Forest Trust, Newcastle University/Environmental Measurements Ltd and Michael Pollock.
In the first of a series of articles on how the mountains of Scotland influence our approach to monitoring and flood forecasting, Mike Kendon writes about their role on rainfall.
“Much of Scotland’s landscape is dominated by its mountains. Although relatively modest in height, reaching typically around 1000 metres in elevation, they nevertheless have a dramatic influence on rainfall.
Due to its northerly latitude, Scotland is located close to the North Atlantic jet stream and therefore lies in the path of low pressure systems and associated rain-bearing fronts. As the westerly airflow is forced to rise over this barrier, low-level clouds are formed over the mountains. When precipitation from higher level ‘seeder’ clouds fall through the lower level ‘feeder’ cloud, the droplets collide and coalesce. This mechanism, known as the ‘seeder-feeder’ effect, is responsible for greatly enhanced orographic precipitation.
As a consequence, the West Highlands receive rainfall in abundance. Annual average rainfall may exceed 3500mm per year, and there may be a ‘day of rain’ (exceeding 1mm) on average almost two days out of every three. However, this orographic effect is highly localised and rainfall rapidly diminishes from west to east across Scotland, so much so that parts of the east coast (such as Angus, Fife and East Lothian) receive less than 700mm – a five-fold reduction – and these areas are among the driest parts of the UK (figure 1).
Scotland’s rainfall is not evenly distributed through the year. The wettest months are from October to March, associated with the normal seasonal southerly shift in the Atlantic storm track – and it is not uncommon for 50mm to fall widely across the West Highlands in a day – locally totals sometimes exceed 100mm. The driest and more settled months tend to occur in late spring and early summer.
The seasonal influence on rainfall tends to reduce moving eastward, as this area increasingly falls within the rain-shadow of the mountains. In eastern Scotland, there may be a greater proportion of convective, rather than frontal rainfall, and rainfall totals on the east coast are more evenly distributed through the year. Figure 2 shows monthly average rainfall totals moving west to east – Glendessary, West Highlands (annual average 3508mm), Gairlochy, in the Great Glen (2168mm), Dalwhinnie, Central Highlands (1304mm), Braemar, Grampians (932mm) and Inverbervie, east coast (683mm).
The broad west-east contrast shows the most obvious influence of Scotland’s mountains on rainfall, but in reality the picture is much more complex. For example, in an easterly airstream the pattern may be reversed, with the wettest weather across the Grampians and the West Highlands falling into the rain-shadow. Scotland’s complex coastline also influences its rainfall patterns; for example coastal fringes of the West Highlands and Islands may be much drier than mountainous areas a few miles inland. Across upland areas, much of the precipitation falls as snow during the winter months, and lying snow often lasts well into the summer months. Over the mountain summits, snow may fall (if not lie) at almost any time of year.
Understanding Scotland’s rainfall distribution is important for many sectors – e.g. water resources, flood risk management – agriculture, ecology and hydro-power. A network of several hundred rain-gauges across the country provides data to help us monitor rainfall across Scotland, and understand trends, variability and extremes. Annual precipitation is, in general, dominated by a large annual variability about a relatively stable long-term mean, but with an increase in the latest few decades (Figure 3). Understanding these patterns in rainfall and how these may be changing as our climate changes, remains an important area of ongoing research for the Met Office and other research institutions. But for Scotland in particular, what makes monitoring precipitation so fascinating is the complexity of the climate across relatively short distances, and this complexity is in large part due to the influence of the mountains.”
Hollis, D, McCarthy, MP, Kendon, M, Legg, T, Simpson, I. HadUK‐Grid—A new UK dataset of gridded climate observations. Geosci Data J. 2019; 6: 151– 159. https://doi.org/10.1002/gdj3.78
Kendon, M., McCarthy, M., Jevrejeva, S., Matthews, A., Sparks, T. and Garforth, J. (2020), State of the UK Climate 2019. Int J Climatol, 40: 1-69. https://doi.org/10.1002/joc.6726
With a couple of weeks to go before the EGU abstract deadline (13th Jan. 2021, 13.00 CET), it’s time to consider submitting that abstract! If you working in the field of operational forecasting and warning for hazards then consider this unique international session EGU session.
This interactive session aims to bridge the gap between science and practice in operational forecasting for different water-related natural hazards. Operational (early) warning systems are the result of progress and innovations in the science of forecasting. New opportunities have risen in physically based modelling, coupling meteorological and hydrological forecasts, ensemble forecasting and real time control. Often, the sharing of knowledge and experience about developments are limited to the particular field (e.g. flood forecasting or landslide warnings) for which the operational system is used.
The focus of this session will be on bringing the expertise from different fields together as well as exploring differences, similarities, problems and solutions between forecasting systems for varying natural hazards. Real-world case studies of system implementations – configured at local, regional and national scales – will be presented, including trans-boundary issues. An operational warning system can include, for example, monitoring of data, analysing data, making forecasts, giving warning signals and suggesting response measures.
Contributions are welcome from both scientists and practitioners who are involved in developing operational forecasting and/or management systems for water-related natural or man-made hazards, such as flood, drought, tsunami, landslide, hurricane, hydropower, pollution etc.
This week sees the launch of two new Flood Warning schemes by SEPA. One is a scheme addressing fluvial flooding in Aberfoyle, in the southern Highlands. The other, discussed here, is for the Outer Hebrides (also known as Eilean Siar in Gaelic). The scheme has been developed over the past year or so and presented a number of forecasting challenges.
The Outer Hebrides consist of around 60 islands, of which 15 are inhabited. Many of the islands are linked by causeways. The coastline is complex, with many inlets, small islands and sand bars. The weather can be extreme (for the UK), with the west coast fully exposed to Atlantic storms. Travel around the islands is often severely restricted due to flooding of causeways by high water levels or wave action.
A forecasting system was developed for the islands in a project supported by coastal modellers at JBA Consulting, for use in SEPA’s Delft FEWS flood forecasting platform. It involves still water level (SWL) calculations for 44 forecast points, and automated look up tables to calculate overtopping rates, these rates being generated from offline modelling. In partnership with the Met Office, tidal surge, wind and wave short term forecasts are added to long term astronomical tide series to determine the values at the forecast points, which are compared to warning thresholds derived during the project.
The scheme was launched on 10 November 2020 and will deliver flood warnings to emergency responders and members of the public. The current forecasting outlook suggests it’s likely to produce forecasts of exceedance over the coming days.
Messages will be issued via SEPA’s Floodline service, as appropriate. Forecasts will also inform the coastal risk assessment for the forecasting service’s Flood Guidance Statement.
Launch of this new system is another milestone in our aim of working with the Met Office to extend flood warning lead times and understanding uncertainties in forecasts, through the use of ensemble forecasting.
As we’ve previously reported on here, many parts of the North and West Highlands of Scotland can be spared major flooding impacts even when significant rainfall (100 to 200 mm) can fall.
In October 2018, although the town of Oban was impacted by localised flooding, many areas were spared more property flooding. Likewise in March 2015, a hydrologically significant event across much of the Highlands again only led to minor property flooding. However, it’s often the road and rail infrastructure that is most significantly impacted by these types of events as we recently observed.
A frontal wave affecting parts of western Scotland overnight on the 12 and 13 September brought with it strong winds and orographically enhanced rainfall across the mountains. Quite widely 50 to 60 mm of rain was recorded with peaks of 80 to 120 mm recorded across Skye and Lochaber. The forecast (see below) was for widespread minor flooding to occur, but the guidance also suggesting a very low likelihood of more significant impacts.
During the Sunday morning reports started to materialise of major disruption across the transport network. Impacts included flooding and disruption on the Crainlarich to Fort William rail line at Bridge of Orchy and flooding of main trunk roads including the A890 and A82 in Skye and Lochaber and the A8 in Renfrewshire.
Attempting to link our forecasting capabilities with the transport receptors is the focus of a new area of work for us. In partnership with Transport Scotland, we’re now exploring ways of developing an impact-based forecasting approach for the Trunk Road network in Scotland.