The <scp>UK</scp>’s wet and stormy winter of 2013/2014

Abstract

Winter 2013/2014 was exceptionally wet and stormy as a succession of deep Atlantic low pressure systems, associated with a powerful jet stream, affected the UK. A major storm-surge down North Sea coasts on 5/6 December 2013 was followed by a quieter week before a sequence of major storms from mid-December to early January brought strong winds and heavy rain. The weather then remained very unsettled before a further rapid sequence of storms from late-January to mid-February. This was the wettest winter in the UK's observational records, and the stormiest period of weather experienced for at least 20 years. There were widespread impacts, many associated with the cumulative effect of so many storms arriving in rapid succession. Strong winds felled trees and caused some structural damage; persistent rain resulted in prolonged flooding of the Somerset Levels and the Thames Valley; huge waves made conditions around the coastline of the south and west extremely dangerous and caused significant damage. The winter storms caused widespread disruption to transport networks and disrupted power supplies; there were several fatalities and over 7000 homes and businesses were flooded (Gov.uk., 2014). This article provides an outline of the weather events of winter 2013/2014, and compares some characteristics of this winter with other winters in the UK's historical records. A second article (Muchan et al., 2015) describes the hydrological and hydrogeological responses and impacts, while a third (Sibley et al., 2015) covers coastal flooding. Further papers review the operational performance of Met Office weather forecast models through the storms (Lewis et al., 2015) and links to larger-scale global drivers (Huntingford et al., 2014). The first major event was on 5/6 December, as an area of low pressure tracked to the north of Scotland, rapidly deepening as it approached southern Norway (Figure 1(a)). The low pressure and strong northwest winds to the rear of the storm resulted in a storm-surge down North Sea coasts, with parts of northwest England and North Wales also affected. Across Scotland, winds gusted at 60–70kn, with Altnaharra (Sutherland) recording 81kn, Edinburgh, Blackford Hill 71kn and the mountain station at Aonach Mòr (1130 m amsl) 123kn. The strong winds were accompanied by around 50 mm of rain across Highland Scotland, with blizzards across high ground, but of most significance was the storm-surge, which coincided with high spring tides. This was the largest storm-surge for 60 years, with total water levels comparable to – and in places in excess of – those of January 1953 (Sibley et al., 2015). Prichard (2013) gives more details of the 1953 event. Over 70 severe flood warnings were issued and several hundred properties on the coasts of eastern England and North Wales were inundated. However, hundreds of thousands of others were protected by flood defences including the Thames Barrier – which saw multiple closures through the winter (Environment Agency, 2014). Over 2000 km2 of agricultural land was protected, and there were no flood-related fatalities. In contrast, the North Sea floods of 31 January 1953 caused 300 deaths in England and 1800 on the near-continent. After a quieter period, a stormy spell from mid-December to early January saw major winter storms affect the UK on 18/19 December, 23–25 December, 26/27 December, 30/31 December, 3 January and 5 January. These storms were driven rapidly across the Atlantic by a powerful jet stream, with the central track generally across Scotland. The areas of low pressure were both very deep and unusually large. Figure 1(b) shows the storm of 23–25 December, with the area of low pressure dominating most of the North Atlantic. The central pressure, 927 hPa, was exceptionally low, with 936.4 hPa recorded at Stornoway (Western Isles), the lowest mean-sea-level-pressure (MSLP) since 1886 (Burt, 2007; 2014). Figure 2 shows MSLP time series through the winter at Stornoway and Valley (Anglesey). The initial downward spike is the storm-surge of 5/6 December, with the next set of spikes corresponding to the sequence of storms from mid-December to early January. During each of these events the pressure approached or fell below 950 hPa; readings as low as this are relatively rare for UK land stations. Each of these storms was associated with very strong winds, gusting at 60–70kn or higher across Scotland and exposed coastlines elsewhere across the British Isles; Edinburgh, Blackford Hill recorded gusts of 66kn on 14 December, 60kn on 18 December, 64kn on 20 December and 65kn on 24 December. Gusts of over 100kn were recorded across Scotland's mountains, with Cairngorm summit (1237 m amsl) recording 108kn on 19 December and 119kn on 27 December. Initially, the strongest winds were across Scotland, but the focus then shifted further south, with Aberdaron (Gwynedd) recording 95kn on 26 December and Needles Old Battery (Isle of Wight) 92kn on 3 January. These two stations are in particularly exposed locations; elsewhere winds gusted at 50–60kn or higher along the west and south coasts. Inland locations also saw gusts of 40–50kn or higher, for example Odiham, Hampshire recorded 62kn on 23 December and Wisley, Surrey 51kn on 3 January. At first, most of the weather impacts were associated with strong winds to the north of the UK, causing widespread disruption to transport networks and loss of power supplies. However, the storm of 23/24 December brought 50–70 mm of rainfall in 24 h across a swathe from Dorset to Kent – around two-thirds of the whole-month average rain for December falling on already saturated ground, shifting the emphasis to both pluvial and fluvial flooding. Leatherhead, Surrey, was badly affected as the River Mole burst its banks and flights at Gatwick Airport were severely disrupted in the busy run-up to and over the Christmas period (BBC News, 2013). Towards Christmas, the persistence of the late-December storms, each bringing around 20–30 mm of rain, led to growing concerns over larger-scale fluvial flooding (Muchan et al., 2015). Flows increased steeply in many major river basins and by 27 December fluvial flood warnings extended to all regions of Great Britain. In early January, floodwaters rose to inundate or isolate several villages in the Somerset Levels, and coastal flooding also became a major concern, particularly for exposed locations in southwest England and South Wales. From 3 to 6 January, two further storms brought strong winds and long-period swell waves which coincided with high spring tides. Figure 1(c) shows the storm of 5 January 2014 dominating the North Atlantic. Exceptionally high waves affected coastal communities in west Wales and southwest England. The historic promenade in Aberystwyth was badly damaged by waves, while flood risk from high tides in estuaries was exacerbated by high run-off from rivers. After the first sequence of storms, the weather remained persistently unsettled and very wet across most of the UK through January, as low pressure dominated, although it was less stormy. Individual daily rainfall totals were unexceptional, but there were only a handful of dry days in the month and rainfall accumulations continued to mount, particularly across southern England. Towards the end of the month and well into February a second sequence of major winter storms again affected the UK, with events on 25/26 January, 31 January/1 February, 4/5 February, 8/9 February, 12 February and 14/15 February. These storms were again associated with unusually large and deep areas of low pressure – the central pressure typically dropping to around 950 hPa. Figure 1(d–f) shows the storms of 4 February, 12 February and 14 February respectively, each with remarkably similar pressure patterns. The drop of MSLP for each of these storms can be seen in the downward spikes in Figure 2. This second sequence of storms tracked typically across Wales and central England, so impacts extended to the south coast. In Figure 2 the MSLP at Stornoway was lower than Valley for the first sequence of storms, but this is reversed for several storms in the second sequence, indicating the lower-latitude storm-tracks. The ongoing wet and stormy weather prolonged and exacerbated existing flooding problems. Flows in the Thames and Severn, which had increased rapidly in late December, were sustained at very high levels until mid-March (Muchan et al., 2015). There was extensive and prolonged flooding in the Thames catchment, while large areas of the Somerset Levels also remained inundated until early March (see aerial photograph of Somerset Levels flooding, Figure S1). The strongest winds were from the storm of 12 February, when the Met Office issued a red warning for wind – the first issued for wind by the Met Office National Severe Weather Warning Service since 3 January 2012.1 Maximum recorded gust speeds included 94kn at Aberdaron (Gwynedd), 83kn at Lake Vyrnwy (Powys) and 81kn at Capel Curig (Gwynedd). These are close to record values for these stations; Aberdaron recorded an hourly mean wind speed to 1700 utc of 68kn. Around 100 000 homes and businesses were without power, and there was some structural damage reported. This was one of the most significant storms to affect Wales and northwest England in recent decades. The other storms in the sequence also resulted in some very strong winds, with gusts of 80kn at St Mary's Airport, Isles of Scilly on 4 February, 80kn at Needles Old Battery on 8 February, and 95kn again at Needles on 14 February. Inland locations again repeatedly saw gusts of 40–50kn or higher; Heathrow recorded 52kn on 25 January and South Farnborough, Hampshire 64kn on 14 February. As well as inland flooding, there were numerous impacts from the storms around the coastlines of west Wales and southern England. Strong winds, high tides and tidal surges acting in combination led to huge waves battering the coastline. The wavelength of the swell was particularly long, with individual waves building up large amounts of speed and energy, and reaching record heights; for example, on 12 February, Kinsale Energy Gas Platform off southern Ireland recorded a maximum wave height of 25 m (Met Eireann, 2014). Huge waves overtopped coastal flood defences, and many coastal communities in Cornwall, Devon and Dorset experienced flooding and damage to infrastructure, buildings and sea defences. The South West Mainline railway was severely damaged at Dawlish, Devon during the storm of 4/5 February, severing a key transport link for many weeks (see photographs of waves at Porthleven, Cornwall, and the Dawlish railway line on the inside front cover of this issue). The second half of February remained very unsettled, with further periods of rain, but fortunately by mid-February the worst of the storms were over. With westerly Atlantic weather dominating almost throughout, winter 2013/2014 was mild; the UK winter mean temperature was the fifth highest in a series from 1910, with a marked absence of cold spells (Kendon et al., 2015). There were few air frosts, especially in February with none at all at many inland parts of southeast England. There was also little or no snow during the winter, especially at lower elevations and for the southern half of the UK. By contrast, the Scottish mountains saw exceptionally deep and prolonged cover at higher elevations, with persistent snowfalls at higher levels through the storms from mid-December to mid-February lying un-melted until late February or March. At Cairngorm Summit (1237 m amsl), the temperature hovered around −3 °C, rarely falling below −5 °C or above 0 °C from 18 December until 8 March, whereas nearby Aviemore (228 m amsl) was consistently around 6 °C warmer through this period. This consistent temperature difference was linked to the persistent Atlantic weather type and absence of temperature inversions. During settled spells of weather in winter it is not uncommon for inversions to develop, leaving cold air trapped at lower levels, with mountain tops above the inversion in warmer air. However, during unsettled spells of weather lapse rates are steeper and, at lower levels, less likely to be complicated by inversions, leaving Cairngorm Summit consistently several degrees colder than Aviemore. Snow cover built up to very great depths, with cornices and avalanches being significant hazards for hill-walkers and climbers; several very large avalanches occurred during rapid thaws in late February (SAIS, 2014 provides many images of Scottish snow cover – see photograph of snow cover on Carn Liath, on page 47 of this issue). Sunshine totals were well below average across the north and west of the UK, with less than 50% of average sunshine hours in some areas. Eskdalemuir (Dumfriesshire) recorded only 45 h of sunshine for the winter, on average 0.5 h per day. However, it was a sunny winter across southern and eastern England with sunshine hours exceeding the average by more than 20% quite widely (NCIC, 2014). There was an absence of gloomy days with anticyclonic stratus cloud, and frequent periods of sunshine between the mobile weather systems. The storm of 12 February was unusually severe but, though very significant, the other storms were individually not exceptional for a typical UK winter; more severe storms have occurred relatively recently – for example two severe storms affected Scotland on 8 December 2011 and 3 January 2012.2 The winter was unusual for the relatively low-latitude track of the storm systems, but especially for the number of major storms to affect the UK. One measure of duration, extent and severity of storms is the number of UK stations each day recording maximum gust speeds greater than 60kn. This analysis suggests that winter 2013/2014 was the stormiest winter for around 20 years, with December 2013 probably within the top 10 stormiest months in the last 50 years. February 2014 was also very stormy, but January 2014 was more exceptional for rainfall totals rather than wind. An alternative measure of the frequency of severe storms to affect the UK is the Jenkinson Gale Index (G > 50) based on reanalysis-derived surface pressure fields (Jones et al., 1993). This provides a more homogeneous time series of storminess from 1871 to date, although observations are not directly of gust speeds. An analysis by Cornes (2014) indicates that the number of severe storms to affect the UK during the winter, based on this index, was 10, with the next highest in this series being 6 in winter 1909/1910, and only 1–2 such storms in an average winter. In a separate study, Matthews et al. (2014) found that this was the stormiest winter for the UK and Ireland in a 143-year series. We have compared monthly and seasonal areal rainfall totals to other winters in the last 100 years, based on 5 km gridded datasets from 1910 (Perry and Hollis, 2005), while the historic England and Wales precipitation (EWP) series from 1766 (Alexander and Jones, 2001) provides a longer-term perspective. Figure 3 shows monthly and seasonal rainfall anomalies, while Table 1 provides a selection of areal statistics. In December, the wettest areas were across Scotland and southeast England, reflecting the track of the low pressure systems, initially across Scotland and then across more southern parts. Some sheltered parts of eastern England, such as Lincolnshire, received below average rainfall. Most of Highland Scotland received twice the average rainfall, and this was the wettest calendar month for Scotland on record. However, in contrast to southeast England, there were few reported impacts (apart from landslides), and flooding was limited due to this being a climatologically wet area of mountainous topography, sparsely populated with few developments in the floodplain. In January, the wettest weather was across southern England and eastern Scotland, with two to three times the long-term average rainfall – this was the wettest calendar month on record for the region of England SE and February again saw two to three times the average rainfall across many parts of the UK, including and for the regions of England SE and and England and this was the February in the series through the winter were due to the persistence of the wet rather than individual event. This is by Figure which shows daily rainfall totals for The only exceptionally wet day was 23 December, with but there were days in a period from December to 14 February with 10 mm or almost three times the winter average of The was of the winter average rainfall and, of the average for this of central southern England, parts of south Wales and eastern Scotland received half a of rainfall or more in over two areas were wet from mid-December to early January with Highland and recording and mm for the period from December to 10 January, 23–25 mm per while two higher rain at over 300 m in the Lake recorded over 2000 mm of rain for the winter. Figure shows UK rainfall totals for winter Most of southern England, parts of northwest England, and southern and eastern Scotland received over twice the winter average rainfall, with over in a swathe from Hampshire to but almost all parts of the UK were than This was the wettest winter in the UK series from 1910 with of and also the wettest winter in the series from 1766 with of The exceptional of winter 2013/2014 is in the UK time series (Figure Winter was the wettest winter in the first half of this with and both but 2013/2014 is the wettest by a of 60 mm – of the England SE and the winter rainfall total was the wettest in the series from 1910 by a huge – – We have the characteristics of the seasonal mean to a set of within historical records. This the et al., for winter et al., and an weather type based on the by et al. The wettest winters in the series are and all of which have in excess of periods that or have higher and is climatologically than winter, but this analysis to the winter of December to February. Figure 6 shows the frequency of the weather by et al. within each winter from 1871 to The 10 wettest winters in since 1871 are two wet winter those from a of such as and and those by westerly such as and With the of anticyclonic days are The winter of 2013/2014 for both and westerly days in the December 2013 saw westerly days with storms across Scotland, while there was a in this track for the storms of January and February. Figure shows winter average winds at hPa to an of 10 as a for the jet for wet winters by westerly weather those by weather winter 2013/2014 and the long-term average The westerly winters have a North Atlantic jet over the UK and south of its (Figure These winters to a more contrast in rainfall anomalies, particularly in the half of the UK, as in Figure for By contrast, the winters see the jet further south and the storms typically the south of the UK (Figure In this there is also a in the North to heavy rain to the of the UK as in the example of in Figure the winter of 2013/2014 periods of storminess of the mean winter (Figure is to the exceptionally wet of and than more recent in and – although for 2013/2014 the jet is also across most of the Atlantic. We that analysis of 2013/2014 winter these historical Matthews et al. also analysis on the and the with winter However, it is that not all historical winters with a of or westerly were wet for the UK. This is the UK's rainfall is very to the of the jet and or in more rainfall for the UK. The UK's exceptional wet and stormy winter was linked to low on the North – for example for January 2014 were below although not for the eastern 2014). The contrast between cold air south across the and the Atlantic are likely to have for the persistence and unusual of the North Atlantic jet in Figure and conditions for of storm systems. This in have to the of the in the which to high sea surface and a of precipitation in the (Huntingford et al., 2014). However, the of the low, for example, is not for the other wet winters over the last that are There be other global that wet and stormy weather for the UK, and in of the global there be in historical for such et al. (2014) also the of the and the for sea extent and to affect the UK's Further is to drivers of UK winters and, due to rare and high the of on the risk of such seasonal The UK's wet and stormy winter of 2013/2014 be for its persistent storminess and rainfall, with the of the jet of to the of the winter. also be for the multiple of the impacts – particularly the fluvial and coastal flooding – which led to not of to those but also for to of weather impacts – for example long-term flood defences and The widespread impacts resulted in sustained and these impacts were also across Ireland and The winter again led to of the UK is more weather than in the but it is also to the of the UK's there have other very stormy periods in the recent example to mid-December there have also other recent winters of – as by the winter of which saw prolonged anticyclonic weather patterns. is the for records, so that close to such winters can be networks are not only for weather but for weather events in the of these long-term records, and these networks to be to impacts from the severe weather are such as the are also to be an with which to events into that not be with These on of and, in many of of historical global records. to and (Met and for The is not for the or of by the than be to the corresponding for the Snow cover on Carn February lying snow at higher elevations with very little snow at The mountains are at with the at Scottish