Forecasting the exceptional rainfall events of summer 2007 and communication of key messages to Met Office customers

Transcription

1 Forecasting the exceptional rainfall events of summer 2007 and communication of key messages to Met Office customers Nick S. Grahame Paul Davies Met Office, Exeter, UK The summer of 2007 will remain in the memory for many years to come for those people who experienced the catastrophic flooding. Public expectation for a good summer may have been quite high following a warm and very dry April but a series of active frontal systems during May and June reduced those expectations. Meanwhile, the combined effects of increased groundwater, reduced evaporation and the arrest of falling water tables became more apparent in time and began to raise awareness of the potential flood risk as successive events occurred. Of course, the heavy rainfall event of 25 June across Yorkshire and Humberside, Derbyshire, Lincolnshire and Worcestershire was exceptional in itself but previously saturated ground is certain to have amplified the impact. The unsettled synoptic pattern continued through July with further large-scale frontal systems interspersed with localized heavy showers or thunderstorms. However, the heavy rainfall event of 20 July was an example where both mechanisms combined to produce exceptional amounts of rain and devastating impacts. This article will concentrate on the operational forecasting aspects in the Met Office, underpinned by its numerical modelling capability. However, the other important aspect is how the messages were communicated to customers of the UK National Met Service, including top-level Government personnel. days saw destabilization over the UK and localized heavy downpours developing from low-level convergence prior to more general bands of heavy rain as low pressure systems moved in from the Atlantic. These systems then tended to intensify over the UK within a general area of atmospheric ascent linked to the left-exit region of the displaced jet stream (Durran and Snellman, 1987). Numerical models provided good guidance on heavy rainfall on the north side of a complex and slow-moving area of low pressure affecting much of northern England between 13 and 15 June. Early warnings within the National Severe Weather Warning Service (NSWWS) are issued if thresholds are exceeded and the risk of widespread disruption is considered to be more than 60%. An early warning was not issued for this event because the ground was not saturated and the risk of widespread disruption was considered to be low. However, Met Office services for the Environment Agency (EA) highlighted the potential for large amounts of rain and the public were made aware via the local media. Targeted short-period flash warnings based on rainfall thresholds were issued nearer the time. Many sites in Yorkshire received at least a month s rainfall in 24 hours. Rainfall was particularly high along the eastern slopes of the Pennines as a result of orographic uplift in moist easterly winds. Whilst this rainfall event did not itself produce major flooding problems, it saturated the ground over much of northern England, creating conditions that amplified the impact of further heavy rainfall later in the month. With the unstable nature maintained over the UK, a repeated sequence of events began to occur with localized heavy downpours developing during the daytime away from slow moving frontal systems. A combination of empirical and theoretical techniques were used to try and identify in advance areas prone to exceeding the heavy rain criterion for issuing of flash messages with sufficient lead times for mitigating action to take place. In addition, the Met Office higher-resolution models (Lean et al., 2008) provided extra detail on localized low-level convergence and the 1.5 km horizontal resolution model (non-operational) was run on occasions in test mode for this purpose (Figure 1). 268 The synoptic evolution through June The first week of June produced some fine weather as an area of high pressure migrated across the country and then drifted northwards within a blocked pattern over north-west Europe. Meanwhile, a longwave upper trough was developing over the North Atlantic with the jet stream displaced further south than its climatological position (Blackburn et al., 2007). The following Figure 1. T+8h forecast of rainfall rates (mmhr-1) in Met Office 1.5 km model (verifying at 1200 UTC on 22 June 2007). Colour scale is in the bottom, right-hand corner. ( Crown Copyright Source: Met Office.) Continued on page 269

2 Continued from page 268 While forecasters were concentrating on short-term, small-scale events, they were also aware of further larger-scale systems moving up from Biscay and developing in the left-exit of the upper flow. A signal was emerging for a more widespread and potentially disruptive heavy rainfall event at the end of the weekend and early part of the following week (24/25 June). Confidence in location and detail was not high enough to issue an early warning on 21 June but key customers were made aware of the evolving situation and potential impacts. Meanwhile, the Met Office Press Office issued a news release based on the unseasonable weather expected over the midsummer period. 1 By the following day (22 June), model solutions from a number of forecasting centres supported the idea of a developing low centre transferring either close to, or across, 1 pressoffice/2007/pr html the southern half of the UK by 25 June. There were still uncertainties in the exact track and speed of this development but forecasters judged that the risk of a potentially disruptive severe weather event was sufficient enough to issue an early warning through the NSWWS. This warning contained appropriate wording to provide some insight on the uncertainties to customers. BBC Weather Centre presenters started using the early warning on all output as soon as it was issued on 22 June, and it was posted onto and Ceefax at the same time. Flash warnings continued to be issued over the weekend with heavy downpours over Scotland and an intense convergence line that formed across north-east England on 23 June, aided in part by the sea breeze (Figure 2). The early warning was updated and finetuned over the weekend of 23/24 June as shorter period model guidance from the deterministic models and ensemble data showed sufficient convergence on the key elements. The mention of exceptional rainfall with 100 mm of rain or so was also included for the first time in the update issued on 24 June. There was a clear (and subsequently correct) focus on areas where disruption was considered highest, including parts of north-east England where ground was saturated from previous heavy rainfall events (Figure 3). The surface synoptic situation on Monday 25 June at 1200 UTC is shown in the companion paper by Hanna et al. (2008 their Figure 10, p. 258). A deep and slow-moving area of low pressure was centred over the English Channel, with its frontal systems lying across northern and central England for much of the day, causing a prolonged period of heavy rain and widespread flooding. Rainfall totals are discussed in more detail in Hanna et al. (2008). The Met Office website recorded almost 1.7 million page impressions on 25 June with over 1 million on 26/27 June. BBC Weather Centre presenters provided additional broadcast bulletins and interviews during and following the main event on 26 June. Forecasting summer 2007 rainfall events Key events in July Figure 2. Radar composite map at 1200 UTC on 23 June ( Crown Copyright Source: Met Office.) Figure 3. Met Office NSWWS Early Warning disruption map issued at 0958 UTC on 24 June (left) and actual precipitation June (right). Media attention began to die down as July started but there were still further localized flooding events reported during the first half of the month as low pressure remained dominant over the UK. By 16 July, some of the deterministic models were indicating an exceptional rainfall event on 20 July with the highest totals biased towards East Anglia and the south-east of England. The Met Office Executive Board was briefed on the possibility of this event, enabling relevant teams and partners to be prepared to take additional action as required. Met Office forecast guidance indicated the potential for an extreme event based on the available model evidence but also highlighted the uncertainties associated with the location of the highest rainfall totals. Variability in deterministic model solutions continued over the next couple of days but there were useful signals from the Met Office Global and Regional Ensemble Prediction System (MOGREPS Bowler et al., 2008) and ECMWF (European Centre for Medium-Range Weather Forecasts) Ensemble Prediction Systems to support significant rainfall totals that were likely to have an impact. This was particularly relevant given the vulnerability of the river catchment areas following previous rainfall events. ECMWF uses an Extreme Forecast Index (EFI) to identify significant deviations from model climatology (Lalaurette, 2003) and Figure 4 shows the EFI for 24-hour rainfall on 20 July based on its 18 July 0000 UTC ensemble run. An early warning was issued on 18 July for potential disruption from heavy rain on

3 Forecasting summer 2007 rainfall events Figure 4. ECMWF Extreme Forecast Index, highlighting marked deviations from climatology with respect to forecast 24-hour rainfall totals. The index shown here is for 20 July. ( ECMWF.) July in particular, with southern and eastern areas of the UK considered at highest risk at that stage. The potential for mm of rain was included but uncertainty in the exact location of the heaviest rain was also mentioned. Although this represented the best possible guidance in terms of a single solution, a selection of what if scenarios were provided to the EA, based on the individual members within MOGREPS (Figure 5). Over the following 24 hours, most model solutions (including higher-resolution Met Office models) began to suggest a relative westward shift in the expected rainfall maximum and provided more detail on rain rates. With increased confidence in the forecast evolution, a news release was issued, following consultation with the Highways Agency and EA Press Offices, to ensure a coherent statement. 2 The Met Office North Atlantic and European (NAE) forecast model provided good advice during the period leading up to the event, consistently indicating more than 100 mm of rain within 24 h on 20 July, and with an increasing focus on the southern Midlands as the most likely location for this rainfall maximum. Some other models provided conflicting solutions by suggesting that the highest totals would be further east but forecasters followed NAE advice. A convergence of model solutions towards NAE took place at lead times of 24 hours or less to provide forecasters with further confidence. Meanwhile, shorter-range output from the 18 July 1800 UTC run of MOGREPS was also particularly helpful in highlighting the potential for extreme rainfall totals (Figures 6(a) (b)). Based on this evidence and a knowledge of the vulnerability of catchment areas, an update to the early warning was issued 2 pressoffice/2007/pr html 270 Figure 5. Each frame shows the 24-hour rainfall accumulation through 20 July for each member in MOGREPS (based on low-resolution forecast 90 km horizontal resolution from 18 July 0000 UTC). ( Crown Copyright Source: Met Office.)

4 (a) Forecasting summer 2007 rainfall events (b) Figure 6. (a) Probability of rainfall accumulations on 20 July exceeding 25 mm (based on higher resolution forecasts 24 km horizontal resolution from MOGREPS run on 18 July at 1800 UTC); (b) Individual members showing rainfall accumulations for 20 July. ( Crown Copyright Source: Met Office.) on 19 July with greatest risk of disruption focused on southern and central parts of England and the Welsh borders (Figure 7). Hourly totals of 30 50mm were also communicated to the EA and other customer groups as an escalation of activity occurred. The surface synoptic situation on Friday 20 July at 1200 UTC is shown in the companion paper by Prior and Beswick (2008 their Figure 2, p. 261). A slow-moving depression was centred over south-east England, with a plume of very warm, moist and unstable air associated with the occlusion and characterized by 850 mbar wet-bulb potential temperature (wbpt) of C. The highest hourly rainfall totals were associated with this feature and 42.8 mm was recorded between 1000 UTC and 1100 UTC at Haywards Heath (West Sussex). Note the trough extending to the north-west of the low centre that represented a more subtle element of this synoptic situation. The feature was linked to an additional plume of 850 mbar wbpt but values were relatively lower at C. However, its location coincided with strong positive vorticity advection at upper levels (relative to the occlusion) and dynamic ascent released instability to produce intense and persistent rainfall along the feature (Figure 8). In retrospect, the complexity of the situation may explain why the models varied in the run up to the event. Widespread flash flooding occurred on the morning of 20 July across south-east England, and later in the day across the 271

5 Forecasting summer 2007 rainfall events 272 Figure 7. Met Office NSWWS Early Warning Issued at 0907 UTC on 19 July (left) and actual precipitation July (right). Figure 8. Surface pattern (isobars in black), 250 mbar pattern (white contours) and relative vorticity (coloured areas red/purple is positive, blue is negative) at 1200 UTC on 20 July ( Crown Copyright Source: Met Office.) Midlands as the system moved north-westwards, causing widespread disruption to the motorway and rail networks. River flooding followed over the subsequent days along the Severn and its tributaries in Gloucestershire, Worcestershire, Herefordshire, Shropshire, and along the Thames and its tributaries in Wiltshire, Oxfordshire, Berkshire and Surrey. Flooding was also reported in Telford and Wrekin, Staffordshire, Warwickshire and Birmingham. Some examples of specific communication channels used to raise awareness of both events Environment Agency Prior to the event on 24/25 June, communications between the Met Office and EA were escalated to the highest levels as concern grew about the risk of extreme rainfall in areas where ground was already close to saturation. Close liaison continued thereafter to exploit the skill-sets of each organisation, to manage the situation jointly and to develop a consistent strategy. This improved the effectiveness of subsequent warnings issued by both organisations (e.g. the early warning on 27 June to cover the risk of further disruption from slow-moving bands of rain over the following weekend contained links to EA flood warnings). Meanwhile, the EA flood-line service included advice for the public to call the Met Office Customer Centre for weather information. A significant increase in call volume was noted as people responded to this advice. Activities relating to the flooding in July built on the experiences of the June event. Public Weather Service advisors The Public Weather Service (PWS) in the Met Office has a national network of advisors whose role is to assist the users of PWS forecasts and warnings, and to help in the interpretation of forecast information in terms of potential impact on life, property and infrastructure. The PWS advisors played a significant role during June and July, using their key contact networks, including representing the Met Office at Cabinet Office Briefing Room (COBR) and Police Command Centres as well as providing television, radio and newspaper briefings. PWS advisors attended the Fire and Rescue Service s National Flood Centre in Worcester over the weekend of 30 June/1 July and again on 21 and 23 July. Continuous updates on temperature and weather were also provided to the Cabinet Office, Health Protection Agency and EA to help with the planning of clean-up operations following the flooding events. Government Ministerial briefing notes were provided to the Ministry of Defence (MOD) and prepared for the Department of Communities and Local Government (DCLG) and Department for Environment, Food and Rural Affairs (Defra) on request. A personalized briefing was also provided to Hilary Benn MP (Secretary of State, Defra), by the Met Office Operations and Customer Service Director. Highways Agency An embedded Met Office forecaster service was provided to the Highways Agency from 29 June to 6 July, to provide on-site advice as the threat of disruptive rainstorms persisted. Network Rail An enhancement of services was provided to Network Rail before, during and after both events, including discussions on the additional capabilities provided by the higher resolution models. The remainder of the summer As a response to the poor summer weather and the episodes of widespread flooding in both June and July, the Met Office provided a media briefing to journalists on 26 July. The aims of the briefing were to keep the media informed and to provide a single point of focus for the Met Office response over the continuing poor weather and associated flooding. One aspect covered the fact that the jet stream across the Atlantic was displaced further south than normal and this was a contributory factor to the extreme rainfall. Figure 9 shows the mean 250 mbar flow for July 2007 with the jet core more powerful and further south than in an average July (Blackburn et al., 2007). Localized downpours still occurred during August with further frontal episodes. However, there were more settled periods of weather with perhaps the most welcome of these coinciding with the late August Bank Holiday. The Met Office Press Office

6 Figure 9. Mean 250 mbar flow for July 2007 (kn). ( Crown Copyright Source: Met Office.) Figure 10. MSLP anomalies (mbar) for the three-month summer period (June, July, August 2007). ( Crown Copyright Source: Met Office.) was finally in a position to issue a news release to communicate some good news for a change. The mean sea-level pressure (MSLP) anomaly for June, July and August is shown in Figure 10. Note the extensive negative anomalies extending from Scandinavia, across the UK and towards the Azores area. Blackburn et al. (2008) discuss the largerscale influences in more detail. Conclusions The high-impact heavy-rainfall events of summer 2007 provided the Met Office with the opportunity to demonstrate the ability to provide advance warnings but also additional capabilities such as risk assessment, what if scenarios, impact statements by working with professional partners and enhanced support to Government and Emergency Planners (Cabinet Office, 2007). Forecasting of the rainfall event on 20 July was perhaps the most detailed and accurate ever achieved by the Met Office for a highprofile, severe weather event. To be able to accurately forecast 100 mm of rain at county level with 24 to 36 hours lead time is an indication of the process wide improvements that have recently been made. In addition, the on demand high resolution (1.5 km) forecast model was run for parts of the affected area to give more detailed forecast information at shorter lead times. Significant use was also made of multimodel probability forecasting techniques. Over recent years, Ensemble Prediction Systems have become a vital tool in helping forecasters to assess risk and confidence on the occurrence of extreme events, and they proved particularly useful on this occasion. Acknowledgements The authors would like to thank Adrian Semple for preparing some of the graphical material. Also thanks to an anonymous reviewer for valuable comments that have improved the clarity of the paper. References Blackburn M, Hoskins B, Slingo J Notes on the meteorological context of the UK flooding in June and July NCS-Climate and Department of Meteorology Walker Institute for Climate System Research: University of Reading; published online at: Blackburn M, Methven J, Roberts N Large-scale aspects of the UK floods in Summer Weather, 63: Bowler NE, Arribas A, Mylne KR, Robertson KB, Beare SE The MOGREPS short-range ensemble prediction system. Q. J. R. Meteorol. Soc. (In press). Cabinet Office The Pitt Review: Learning lessons from the 2007 floods Durran DR, Snellman LW The diagnosis of synoptic-scale vertical motion in an operational environment. Wea. Forecasting 2: Hanna E, Mayes J, Beswick M, Prior J, Wood L An analysis of the extreme rainfall in Yorkshire, June 2007, and its rarity. Weather, 63: Lalaurette F Early detection of abnormal weather conditions using a probabilistic extreme forecast index. Q. J. R. Meteorol. Soc. 129: Lean HW, Clark PA, Dixon M, Fitch A, Forbes R, Halliwell C, Roberts NM Characteristics of high resolution NWP models for forecasting convection over the UK. Mon. Wea. Rev. (submitted) Prior J, Beswick M The exceptional rainfall of 20 July Weather, 63: Correspondence to: Nick Grahame, Met Office, FitzRoy Road, Exeter, EX1 3PB nick.grahame@metoffice.gov.uk Crown Copyright, 2008, published with the permission of the Controller of HMSO and the Queen s Printer for Scotland. DOI: /wea.317 Forecasting summer 2007 rainfall events 273