Ready for winter – new flooding communications programme

The Scottish Flood Forecasting Service (SFFS) will have another outlet for its flood guidance activities this winter.

The Scottish Environment Protection Agency (SEPA) is launching a campaign to provide the public with advance notice of potential flooding. Through a newly created suite of winter marketing materials and a strategic approach to communicating, this campaign will be able to more accurately target communications activity to those at risk of flooding. It will also be able to provide up to 3 days’ notice of potential flooding, giving people more time to prepare and take action.

An example of one of the messages to be posted in response to a forecast of possible flooding.

In order for this campaign to be accurate and effective, SEPA’s Flooding Communications team will work with the SFFS, to monitor the likelihood and impacts of flooding. If significant or severe flooding is imminent, the campaign will be activated and messages will be broadcast to the areas at risk. Messages will be advertised on both national and local radio stations, on social media and online and will also be shared with key partners and flood action groups for further dissemination.

The communications activity will complement SEPA’s Floodline Service, which provides free messages to customers when SEPA issues a Flood Alert, Flood Warning or Severe Flood Warning in their area, and will encourage more people to sign up to the service.

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Summer river flooding in Scotland

It had been a fairly quiet few months for the operational side of the Scottish Flood Forecasting Service, before heavy rain struck much of the country at the start of June, causing some significant flooding in the north east, and minor flooding in many other locations. Forecasts consistently indicated low pressure and ‘wrap around’ feature bringing heavy rain to the north and north east in particular, though there was some variation from forecast to forecast as to the exact location and rainfall amount forecast, as shown in Figure 1.

Figure 1: Peak 24 hour rainfall totals forecast on Sun, Mon and Tues for Tues 6/ Weds 7 June

The Grid-to-Grid model also highlighted this uncertainty, though it was clear that all scenarios would lead to much higher than usual flows (particularly for the time of year) in some areas. See Figure 2.

Figure 2: Grid-to-Grid forecast on previous day and range of forecasts for River Narin

Figure 3: Area of Concern map for Tues 6 June Flood Guidance Statement

To reflect the uncertainty the Flood Guidance Statement indicated a low likelihood of significant flooding, with an Area of Concern map. See Figure 3.





In the event, over 100mm of rainfall in 24 hours fell in some areas, and some rivers exceeded flooding levels, particularly in the north east. A number of Flood Alerts and Flood Warnings were issued. Some properties were evacuated and the Inverness to Aberdeen railway line was flooded. Surface water flooding issues were also widespread.

Figure 4: Portsoy in Aberdeenshire (photo from The Scotsman)

Perhaps the most interesting aspect of this event is that even in summer, and following a prolonged dry spell, river flooding is always a possibility during a prolonged rainfall event. Whilst surface water flooding did occur, the main focus was on the rivers.

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Snowmelt forecasting capability within Grid-to-Grid

The formation and melting of snow can be a key component of river flows in the UK, particularly in upland areas. Different configurations of the countrywide Grid-to-Grid (G2G) model are used to deliver the snowmelt component, using different approaches for Scotland; and England and Wales. The Scotland configuration used by the forecasting service uses the G2G Snow Hydrology scheme with Numerical Weather Prediction (NWP) model inputs of precipitation and air temperature at screen-height to form and melt the snowpack. In contrast the Flood Forecasting Centre (FFC) in England and Wales uses NWP parameters to split precipitation into rain and snow; and the Joint UK Land Environment Simulator (JULES) to estimate snow melt as an external input to G2G.

We have recently taken delivery of a review of the two methods, in a project jointly commissioned by the SFFS and FFC and carried out by the Centre for Ecology and Hydrology (CEH). The report reviews the snow hydrology methods, describes the operation of the two methods, looks at ways to improve them, and carries out comparisons between the two methods. Some examples of the output are shown below.


Comparison of current model configuration with a trial process in which the 1km grid cells are represented by a series of elevation zones. The differing amounts of snow melt in each zone are then recombined to calculate the grid cell run-off. It was found that the new method made little difference at the catchment scale – this example is for the River Muick at Invermuick.


Modelled flows for the River Spey at Boat o’ Brig using various snow module formulations.

The study confirmed that modelling snow processes generally improves performance, irrespective of the particular snow formulation used. A structured comparison of the two methods currently in use, alongside other formulations, shows the NWP-JULES method currently outperforming the G2G Snow Hydrology (G2GSH) module. This difference disappears when a revised parameter set for G2GSH based on a more extensive hydrometric data record is used. There is still substantial river flow uncertainty at the catchment level, however, with peaks being over- or under-predicted.

Comparison of streamflow modelled from different snowmelt methods. G2GSH is current Scotland methos. G2GSH-new is recommended new calibration. NWP-JULES is current England and Wales method.

Comparison of streamflow in Scotland modelled from different snowmelt methods. G2GSH is current Scotland methods. G2GSH-new is recommended new calibration. NWP-JULES is current England and Wales method.


As well as providing useful insights into how the model is performing, some recommendations were made. In the short term, new parameters for the G2G snow hydrology scheme could be implemented in time for next winter, to provide some immediate improvements. Longer term, more work is required on validation through snowpack monitoring and also on quantification of uncertainty, using NWP ensembles. The applicability of the NWP-JULES method used in England and Wales should also be considered.

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EFAS annual meeting in the Netherlands

Louise Parry recently attended the annual EFAS Partners meeting on behalf of SEPA and the forecasting service. She sends the following report.

In March this year the Annual European Flood Awareness System (EFAS) Partners meeting took place in De Bilt, near Utrecht, in the Netherlands. EFAS is a European Commission initiative set up by the Joint Research Centre (JRC) of the European Commission, to which SEPA (and the SFFS) became a partner in 2015. Access to EFAS is of operational benefit to the SFFS duty hydrologists as it is a complementary and added-value service to our national forecasting system, helping to increase preparedness for fluvial floods by providing a regional forecast up to 10 days in advance.

SFFS poster at EFAS

The annual meeting is an opportunity for the partners to feed back information on EFAS performance over the last year, as well as for the EFAS scientific community to update the partners on new developments in the EFAS service. The last 12 months have been relatively quiet in Scotland in regards to fluvial flooding (thankfully!), therefore there was little to be fed back to EFAS in terms of their performance. However, areas of the UK have suffered flooding from other sources, in particular from surface water (pluvial) flooding, which is an extreme challenge to the forecasting community due to the highly localised nature of the events in contrast to the resolution of the forecasting models. However, both the SFFS and our England and Wales cousins, the FFC, have models and tools which help the forecaster assess the risk of a surface water flood event occurring. Therefore, we (the SFFS) decided to collaborate with the FFC again this year, to produce a feedback poster with a twist, giving the requested feedback, but also detailing some of our approaches to surface water flood forecasting. This created the opportunity for scientists and forecasters from other nations to ask questions regarding our methodologies and prompt discussion on ideas for future developments. This exchange of experiences and ideas for development is what I believe to be one of the key benefits of the EFAS community and the annual meeting.

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Storm Frank On Tour

Operationally it has been a fairly quiet last 12 months or so for the forecasting service. The interest in the events of winter 2015-16 still continues, however, particularly the floods resulting from Storm Frank at the very end of 2015. Recently members of the forecasting service presented aspects of our work surrounding this event, at two different events in Edinburgh.

Karen Pinkerton explaining her poster on Communicating Storm Frank

On 15th March Karen Pinkerton presented a poster at the Royal Society of Edinburgh conference ‘How can we learn to live with floods?’. The poster looked at our forecasts in the run up to the flood events, and how the flood guidance produced by the service influenced the emergency response, in particular in Dumfries and Galloway. It emphasised the importance of early communication.





Then on 21st March, Louise Parry and John Mitchell gave a talk to the Royal Meteorological Society entitled ‘Forecasting Storm Frank’. This very much focussed on the science behind forecasting both the meteorology and the hydrology.

Louise Parry and John Mitchell at Royal Met Soc

Each presentation illustrated a different side of the critical work done by the service during flooding emergencies – both the science and the service, neither of which could be effective without the other.


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Hydrologists make the water go round – BHS international conference

I recently attended the 2016 international conference of the British Hydrological Society, at Cranfield University. There were a number of broad themes, including flood risk and water resource management, with most of the first day being given over to presentations on the UK floods of December 2015 and January 2016. Topics included real time modelling, post flood modelling analysis and discussion of return periods, in particular whether it is plausible to have several floods of return period 100 year+ over a single decade, or whether in fact such clustering is part of the natural long term cycle.

The majority of presentations looked at the floods from the perspective of northern England, so my talk on the impacts in Scotland of the flooding from storms Desmond and  Frank and the work of the Scottish Flood Forecasting Service, provided a different viewpoint. SEPA’s Nigel Goody (below), past president of BHS, also looked at the destruction wrought in Scotland, in particular on our hydrometric capabilities, providing a warning about the huge uncertainties involved in measuring these sorts of events.

The full programme of the conference can be found at this location.

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High Impact Weather and Climate Conference

High Impact Weather and Climate was the subject of the conference held by Royal Meteorological Society and National Centre for Atmospheric Science at the University of Manchester last week. Members of the forecasting service attended, along with many colleagues from the Met Office, universities and other practitioners. The three days were split into three broad topics – Observing, Predicting and Responding, and included key note speeches and a number of workshops, as well as poster sessions. The over-riding theme was that of impacts – how do we know when they are happening, how do we forecast them, and what is the best way to communicate this to those who need to know?

Liz Bentley from RMetS introduces the conference

Liz Bentley from RMetS introduces the conference

The observations talks and workshops centred around the seemingly simple question – how do we know what is happening? High quality observation networks are sparse, so the point was made that crowd sourcing data, whether on weather or impacts, is a potential way forward, with an acceptance that the quality of individual observations may be less than ideal. Initiatives using smartphone technology, the Met Office WOW  site, and SEPA’s Report A Flood  are examples of this. The nature of observations was also questioned; particularly at the extremes, modelled parameters may be just as valid as ‘observed’ – Met Office Chief Scientist Prof Julia Slingo made the interesting point that an observation is just one particular version of reality.

(L) Mike Cranston discusses the Flood Guidance Statement during the 'Perfecting the Weather Warnings' workshop. (R) Louise Parry discusses the SFFS poster with Steve Cole from CEH.

(L) Mike Cranston discusses the Flood Guidance Statement during the ‘Perfecting the Weather Warnings’ workshop. (R) Louise Parry discusses the SFFS poster with Steve Cole from CEH.

Workshops such as those focussing on the Hazard Impact Model  and the Flooding from Intense Rainfall  projects presented the latest thinking in impacts modelling – the latter drawing on the work done for the Glasgow surface water flooding model  in 2014, as illustrated in the SFFS poster.

On the response side, the challenges ahead were presented in an excellent talk by Virginia Murray centred on the Sendai Framework for Disaster Risk Reduction , the ultimate aim of which is provide access to early warnings for all people by 2030. Currently 80% of developing countries have only basic or no warnings systems in place. Sally Priest, from the Flood Hazard Research Centre , made the point that just sending people a warning doesn’t necessarily lead to them taking action – people don’t always understand the warning, trust the authority, or act rationally. Care is particularly needed when communicating probabilistic warnings, the subject of a workshop involving the forecasting service, with partners at the Met Office and Flood Forecasting Centre. We played the Game of Making Decisions Under Uncertainty (developed by Micha Werner, to be available at the link soon) in which delegates were invited to make play the role of business owners and make cost-loss decisions based on probabilistic forecasts of flooding, and also discussed the situations in which low probability high impact warnings should take precedent over higher probability low impact situations. Even within the forecasting community there are many varying opinions on this topic.

The other benefit of the conference was the opportunity to meet and discuss topics of interest with colleagues from other organisations, progress existing projects and lay the groundwork for future collaborations.

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Challenges and innovation towards the prediction of natural hazards

The 2016 European Geosciences Union General Assembly saw us co-convene our second PICO (Presenting Interactive Content) session on operational forecasting and warning systems for natural hazards. Along with several other sessions in hydrological forecasting there were plenty of take home points regarding the challenges and innovation of natural hazard early warning.

PICO session

Delegates at the PICO session on operational forecasting and warning systems for natural hazards at the 2016 General Assembly of the EGU. 13,650 scientists from 109 countries attended the event during the six days between 17 and 22 April.

There is a growing effort in the area of real-time mapping of the consequences of hazards to support emergency response. In the Philippines, Project NOAH is responsible for the development of a storm surge advisory service with the purpose of warning for expected inundation of storm surges using hazard mapping (Santiago) – see image. A similar concept being adopted in the Chapere watershed in Bolivia involves river forecasts linked to real-time inundation scenarios to allow civil protection authorities to estimate the impact on the population (Rossi). For approaches in forecasting of surface water flooding in the UK the hazard footprint is combined with an indication of the likelihood to provide an overall indication of risk (Cole) – as seen with our approach to the Commonwealth Games pilot – which this is now being adopted on a countrywide scale in England and Wales.

Project NOAH

Project NOAH in the Philippines is a science research and development initiative to aid disaster prevention and mitigation efforts through the use of innovative information services.

A method of staged decision-making based on probabilistic forecasting was presented highlighting that a cost-loss approach was possible (Booister). The work, partly based on research in Scotland with the flood forecasting service, opens up possibilities for more flexible decision making compared to more traditional deterministic approaches.


Real-time flood monitoring using social media.

The use of crowd sourced information to improve predictions is another area of innovation. As reported last year initiatives such as Floodtags or WeSenseIt have provided excellent examples of such approaches. This year an approach to relying on indigenous knowledge for flood and drought disaster forecasting in Ghana was presented, with the system using local co-ordination linked with satellite-based warnings (Jungermann). Coupling flood forecasting and social media was also covered in the context of the European Flood Awareness System (EFAS) and the use of the monitoring application Global Flood News (Kalas).

Challenges still remain with flash flood prediction although some innovative approaches are being trialled. On a global-scale developers of the Global Flood Awareness System (GloFAS) are setting up a system using 4-day ensemble surface runoff forecasts with a flash flood index based on climatology (Baugh). On a regional-scale high resolution convective-permitting quantitative precipitation forecasts are being used to support a new Vigicrues flash flood service in France (Dermargne).

Surface water flood forecasting

PICO presentation on real-time forecasting of surface water flooding hazard and impact

The emergence of multi hazard platforms are proving beneficial to responders with one such case in Southeastern France – RHYTMME – providing a warning and mapping system for floods, landslides and debris flows (Fouchier). Risk management strategies are now supporting multi-hazard approaches and are particularly beneficial where hazards interact both spatially and temporally (Gill) as seen with Storms Desmond (UK) and Synne (Norway) with landslide and flooding impacts being supported through national hazard assessments (Boje).

This is a unique international forum for sharing ideas on the prediction and early warning of natural hazards and we look forward to further sharing of research and operational system development at the EGU in 2017.

Posted in EGU, Flood, Forecasting, Natural Hazards | 1 Comment

SFFS at the EFAS Annual Meeting, April 2016

20160406_190755The flood forecasting service in Scotland is a partner of the European Flood Awareness System (EFAS); a EU Joint Research Centre (JRC) service which provides probabilistic flood forecast information to more than 48 member organisations across Europe based on the ECMWF deterministic and probabilistic meteorological forecasts and the LISFLOOD distributed hydrological model. Last week (6-7th April), scientists and operational forecasters from across Europe gathered in Seville for the 11th Annual EFAS Partners meeting.

The annual meeting is an opportunity for the users and developers of the EFAS models and data services to gather and exchange information on the service. Discussion included the set up and operation of the EFAS Meteorological and Hydrological Data Collection Centres in Germany (EFAS MDCC) and Seville (EFAS HDCC), respectively, and of developments to the EFAS hydrological model, including the new flash flood layers, which we received notifications from during the December and January flood events (from storm Desmond to the River Don event on the 7th January, 2016). In addition, EFAS introduced their new Rapid Risk Assessment layer which is due to become operation in the summer, 2016, and shares some similarities to the risk assessment products of the SFFS and FFC, and the introduction of a series of webinars to be delivered to partners over the course of the coming years.  Of particular interest were the presentations on the Copernicus risk mapping products and by the European Space Agency (ESA). The Copernicus datasets and products can be used to produce near real-time imagery, maps and shapefiles of flood extents during an event, and were used in England and Ireland over the December 2015 stormy period. The products could be used for other natural/man-made disasters as well. ESA were there to promote the wider use of satellite data products in flood forecasting models such as temperature and soil moisture datasets, which can be available on a range of timesteps including daily or even sub-daily in some cases. Both of these satellite products: mapping and satellite derived spatial datasets of climatological and hydrological parameters, are something which could warrant further investigation into the potential use of to improve the models and services provided by both SEPA and the SFFS.

SFFS- FFC presentation on Storm Desmond and Frank

SFFS- FFC presentation on Storm Desmond and Frank


Example slide from joint SFFS-FFC presentation showing the EFAS forecast and and SFFS flood guidance information & forecasts for Storm Frank

The highlight of the conference however was the presentation by myself and Rob Cowling from the England and Wales FFC- and yes I admit to being biased! We, along with Jim Casey from the Ireland Office of Public Works (who did a separate presentation), had been asked to give presentations on the recent Winter floods in the UK and Ireland in December. Due to limitations on time, and to show cross-organisation co-operation Rob and I decided to present together, enabling efficiency in delivering context to the flooding through a background introduction including of our operational models, the UK climate and hydrological regimes in general, before detailing the impacts and our forecasting performance during the Storm Desmond and Frank events. The presentation was well received with many questions asked about our forecasting systems and use of EFAS during the events.


The beautiful office location and Ops Room of the Guadalquivir river authority, Seville.



Old friends and colleagues on the roof of Seville cathedral

In truth though, the highlight of the conference was the city of Seville itself which warrants more time for exploration that I had. The organisers of the meeting deserve congratulations on a well organised and very informative and productive meeting, but also our thanks in ensuring we had some exposure to the delights of the city. They organised a trip to the cathedral (the 3rd largest Gothic cathedral in the world) and its rooftops, before a fabulous dinner on the Wednesday night, plus a trip to the monitoring and control room of the Guadalquivir river authority who have the pleasure of working in a spectacular location (see picture). It’s fair to say we all left feeling a little envious of their office location, although they are missing the fair mountains I can see from my office window (through rain!).

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Storm Frank-Short Range Forecast Performance

Figure 1

Figure 1: Weather chart for 0000 GMT on 30th December shows storm Frank located to the NW of Scotland.

On the 30th December, 2015, new maximum peak levels were recorded on the Rivers Cree, Dee and upper Tweed due to rainfall which fell overnight from the 29th to the 30th December associated with Storm Frank; an unusually deep area of low pressure which brought weather fronts across Scotland (see figure 1). Around 370 properties were flooded in Ballater, NE Scotland, alone, but the devastation was wide spread across Scotland (see figure 2). In the previous article “Storm Frank – how was the medium range flood guidance?” we discussed the development and 2- 5 day forecast of Storm Frank and its potential impacts, whereas the aim of the current article is to discuss the variations in, and factors affecting, the forecast performance over the 24 hours prior to the event peak flows to give an overview assessment of model performance.


Figure 2: (a)The Flood Guidance Statement (FGS) maps issued ahead of the event on the 29th-30th December, and a map (b) of public reported incidents of flooding associated with storm Frank.

Antecedent Conditions

There had been significant snowfall over the high ground of the South West and North East over the Christmas period (see figure 3a; MODIS image for the 27th December,2015). The 27th-28th December had been relatively dry but there was some rainfall overnight on the 28th into the morning of the 29th, some of which will have fallen as snow. However, by the 29th December, much of the snow cover had melted particularly at lower elevations (figure 3b). Grid-to-Grid (G2G), the gridded hydrological model which we use to forecast river flows nationally, has a snow hydrology component, which estimates snowfall, accumulation and melt based on precipitation, air temperature and basic snow pack principles, and it had simulated the observed pattern of melt.

Figure 3: MODIS imagery (courtesy of NEODAAS/University of Dundee) for (a) 1300hrs 27th December, 2015, and (b) 1300hrs 29th December, 2015. NB- in the second image the ground extent of snow cover is obscured by cloud cover, however, the reduction in snowcover can be seen by comparing the images for the north of Scotland.

 Storm Frank- Rainfall


Figure 4: 24 hour rainfall totals recorded during storm Frank from 0900hrs on 29th December, 2015.

The main bulk of the rainfall associated with Storm Frank occurred between 1800 hrs (+3 hours for NE Scotland) on the 29th December and 0900hrs (+3 hours for the NE) on the 30th December. Maximum recorded rainfall for the event was 130.6mm which fell in 15.5 hours at Lower Black Laggan raingauge in the southwest (Dumfries and Galloway), whereas in the Upper Dee catchment maximum rainfall was recorded at Glenmuick at 95.6mm in 15 hours. In general, 50 to 100mm of rain fell across the Southern Uplands and much of Highland Scotland (figure 4).

Sample forecast Locations

Table 1 is a sample of gauging stations located near to locations where impacts associated with flooding (see figure 2) occurred and/or new maximum recorded levels were observed (indicated by the values highlighted in red in column E). Columns F, G and H indicates the performance of the three G2G forecasts run in the 24 hours prior to the event peaks by comparing the percentage difference of the forecast peak to the observed and the difference in the estimated time of peak. For the difference in timing a negative value indicates that the forecast peak was predicted to occur earlier than it did in reality and the values are fractions of an hour (therefore a value of -0.25 indicates the forecast peak was estimated to occur 15 minutes earlier than in reality). Forecasts 1 and 2 (F1 and F2) were run at 13:45 and 18:30 on the 29/12/ 2015, and F3 at 07:30 on 30/12/2015. These forecasts use deterministic rainfall forecasts from the Met Office UKV runs at 0900hr, 1200hr and 0000hr.


Table 1: Current best estimate of observed Q and time of peak on the 30th December, 2015, recorded in WISKI at several locations across Scotland, and the performance of the three G2G forecasts at these locations prior to the event peak. Forecasts 1 and 2 (F1 and F2) were run at 13:45 and 18:30 on the 29/12/ 2015, and F3 at 07:30 on 30/12/2015. Difference of the forecast peaks and timings to the observed at given as percentage difference for flow, and fraction of an hour for time. *value for observed peak is based on the current rating and could be subject to review.

The values highlighted in yellow in columns F-H show at which forecast the previous maximum flow (column D) was forecast to be exceeded. For the 8 sites (red highlight) in table 1 where new peaks were recorded, F1 predicted the previous maximum flow being exceeded for 6 of the sites, and by F3 all of the 8 occurrences were predicted.

In general, the forecasts performed well for predicting the timing of the peak flows, and for indicating the severity of the event at least 24 hours ahead (refer to previous blog article for longer lead time identification of the severity of Storm Frank). For most sites the forecasted peak occurred ahead of the observed time (red text in table 1), which is the preferable direction of timing errors, and the forecast which performed the worst in terms of timing was forecast 2. For the upper Dee gauges which are used for assessing risk for Ballater, the estimate of time for the peaks were in the range of 3 hours too early to 15 minutes late.

Figure 6_v2

Figure 5: Forecast and observed discharge and catchment 3 hour rainfall totals for the River Dee at Polhollick. The observed precipitation is composite (gauge and radar) rainfall from G2G, the forecast precipitation are from runs at 0900hr, 1500hr and 0000hr for G2G simulations at 13:55 and 18:35 on the 29-12-15 and 07:30 on the 30-12-15.

In regards to forecast performance for the peak flows, the severity and spatial extent of the impacts were generally predicted with sufficient lead time (+24 hours) to co-ordinate emergency response. However, in terms of whether the peak flows were over- or under- estimated there is an approximate geographical split across Scotland. For the gauges located in the NE, such as those in the River Dee catchment upstream of Ballater, Forecasts 1 and 2 underestimated the peak discharge of the fluvial event, with forecast 2 performing the most poorly in terms of timings and underestimation of the peak flows (see example in figure 5). The performance of forecast 2 can in part be attributed to the underestimation of rainfall in the forecasts for these catchments, as forecast 2 generally predicted the least amount of precipitation. However, forecast 1 overpredicted rainfall for these smaller and higher elevation catchments, but the flow at the gauge locations still underestimated peak flows.


Figure 6: Forecast and observed discharge and catchment average 3 hour rainfall totals for the River Cree at Newton Stewart. The observed precipitation is composite (gauge and radar) rainfall from UKV runs at 0900hr, 1500hr and 0000hrs for G2G simulations at 13:55 and 18:35 on the 29-15-15 and 07:30 on the 30-12-15.

Forecast 1 generally overpredicted the amount of rainfall across Scotland, but whereas forecast 2 underpredicted the rainfall for the Dee catchments, it overpredicted for the more southern gauges (and the River Dee at Park). The rainfall totals were however, still less than for forecast 1. The estimates of peak flows were generally improved over the southern catchments compared to those in the NE. Figure 6 shows the variations in predicted precipitation and river flow for the different forecasts for the River Cree at Newton Stewart in Dumfries and Galloway.


Based on the weather and national river forecasting results on the morning of the 29th December, additional simulations of G2G were run as new NWP model runs became available to the forecaster. In general the combined rainfall and river forecasts at gauge locations across Scotland indicated that this event would cause significant, and possibly severe, impacts at several locations across Scotland with sufficient lead times to enable co-ordination and planning of emergency response. For 6 of the 8 sites analysed in this blog which recorded new maximum peak river levels, the forecast run at 1345hrs on the 29th December estimated that the previous maximum recorded levels would be exceeded. However, the performance of the forecasts varies across Scotland, and is shown to be very sensitive to the rainfall forecast and model structural uncertainties particularly for the smaller catchments. It’s important to remember that the models analysed here are national scale models being used to predict local impacts and the detailing of rainfall intensity at the scale of small catchments is challenging, despite the relatively high resolution of the hydrological model (1km). The spatial variation in rainfall can be significant even in a situation of widespread (or dynamically forced) rainfall, and the highest resolution of the NWP models is 1.5km. Local rainfall totals and intensity can be focussed in certain catchments due to marginal changes in low-level airflow, with the potential of slow moving ‘mountain-waves’.

As science and technology improves, the predictions of rainfall and river flows will better represent local effects and thus the spatial variations. The evolution of such science will require careful assessment of the capability and application of such data to hydrological modelling in near real time as is required for flood forecasting. However, in the mean time, the performance of the forecasting tools and models which are available to us have been tested and proven this winter, during some of the worst events on record, to be invaluable in providing good and timely indications of the likely flood impacts.

Posted in Forecasting, G2G, Hydrometeorology, Risk communication, Weather prediction | 2 Comments