UK winter storms have produced the stormiest December since 1969, the wettest December in Scotland since 1910 and the highest storm surge since 1953. What has been causing this series of exceptional events and how heavy might the losses associated with it be?
Following Storm Xaver earlier in December, the UK has continued to experience prolonged periods of stormy and unsettled weather. This repeated cycle of fast moving, clustered storms has brought a constant stream of low pressure systems and heavy rainfall onto already saturated ground and swollen rivers. Seven deaths in England between 23 December and 5 January have been associated with the severe weather, with more than 300 properties flooded and 750,000 households left with no electricity supply.
A combination of low pressure, strong winds and spring high tides has resulted in wave overtopping and repeated flooding in many coastal areas, particularly in the South West of England and mid-Wales. With many river catchments and drainage systems already at capacity from previous weather systems, additional rainfall overwhelmed many areas bringing localised river and surface water flooding to many areas across the south of England, particularly Kent, Surrey and Sussex, and along the River Severn in Gloucestershire.
During Christmas Eve, high winds, with gusts in excess of 90 mph, brought falling trees down onto power lines, resulting in a peak power outage for 150,000 people in the South East of England
The number of maximum gusts exceeding 60 knots across the Met Office observational stations with an altitude less than 250m in December 2013, implies that this was the stormiest December on record since 1969. In Scotland and South East England it was also very wet, where rainfall was over twice the monthly average. This was not the case across all areas though, with some easterly areas experiencing less than half the December average.
December 2013 rainfall as a % of 1981-2010 average
During December the jet stream has been strong and sitting in the right place to guide storms to the UK. In addition the jet stream can add to the strength of storms and vice versa. This positive feedback can result in storms clustering together over a period of time.
The strength and position of the jet stream can be due to a number of factors - including the natural variability governing Atlantic weather. Another possible factor is the quasi-biennial oscillation (QBO) which, when in its westerly phase, can increase the westerlies in the jet stream. The QBO has been in its westerly phase since early 2013, and we expect it to decline over the next few months.
On 5 December a rapidly deepening low pressure system brought strong north-westerly winds to the northern half of the UK. Winds gusted at 70 - 80 knots across Highland Scotland, the Western Isles and Central Belt; with high altitude observation stations seeing maximum gusts over 100 knots. The rain radar from 0800 GMT (Figure 4) shows frontal systems pushing south, bringing heavy rain and snow across high ground. Concerns about coastal flooding grew, with 28 severe flood warnings and 140 flood warnings in place at 1120 GMT on 5 December.
Analysis chart 5 December 2013
Rain radar showing frontal systems pushing south
A deep area of low pressure brought heavy rain and very strong winds to much of the UK overnight 18-19 December. Wind gusts of over 60 knots were recorded in exposed coastal locations in the west, with some stations recording gusts exceeding 70 knots. Some high altitude stations recorded gusts of over 100 knots. The rain radar at 1800GMT on 18 December shows a frontal system bringing very heavy rain, accompanying the strong winds, to many western areas.
Analysis chart 19 December 2013
Frontal system bringing very heavy rain and strong winds, to many western areas.
The effects of the storms were highly localised, with some areas suffering from single or multiple types of flooding. For example, Aberystwyth experienced overtopping from 1.8m high waves along the promenade area on 3 January, with residents and students from Aberystwyth University halls being evacuated. In Kent, the River Beult burst its banks on Christmas Eve, flooding the village of Yalding. On 3 January, the village flooded again but to a lesser extent.
At this point, a realistic estimate of losses is not possible. The multi-peril nature of the recent storms will add to the complexity of assessing the impacts of these events. Inundation of farmland, business continuity and loss of business through travel disruption are thought to be major economic impacts of these storms and will likely be reflected in any claims. The Environment Agency estimated that approximately 1,200 properties in England flooded between 24 and 26 December. Between 2 January and 5 January, they estimate 220 properties were flooded. On 24 December, flooding caused electrical failures at Gatwick Airport as well as on railways lines from the airport into London, with landslips disrupting rail travel in Surrey. Railway lines on the Welsh and Cumbrian coasts were severely damaged or washed away during the 3 January storm.
UK claims following the St Jude’s Day storm and Xaver are in the region of 400 to 600 million pounds. This total is set to rise significantly when claims from these recent events are added. This represents a significant amount of damage but insurance companies are expected to be able to withstand the claims.