When Did Windstorms Become So Wet?

Looking back to the start of the European windstorm season, my colleague Brian Owens pondered whether the insurance industry would experience a windfall or windy fall? Well, a week into February, I think all observers would agree that this has been a very active season.

As the industry continues to count the cost of the succession of systems that have assaulted our shores, it is apparent that the accumulated losses over the season will make this a year from which much can be learned.

The storms impacting northern Europe have frequently brought damaging winds to coastal areas, occasionally exceeding 90mph in the most exposed areas.

However, the driving jet stream has typically been very strong to the west but tapered off in the northeast Atlantic. This has caused systems to explosively deepen and mature before they reached the U.K. and Ireland, but then decay as they approached these shores. Consequently, the long storm paths have prompted higher waves and storm surges, but the latter decay, even for extremely deep cyclones, has meant less damaging winds. This has thus far spared Atlantic-facing countries from extreme wind losses.

But as the season has developed, the main story hasn’t been storm gusts. Anyone living in or visiting the U.K. this winter can testify that it has been exceedingly wet. Not just from excessive rainfall, but from repeated coastal inundations from storm surges combined with high tides as well. Consequently inland and coastal flooding has been significant, dominating our attention.

UK precipitation compared to long-term average; dark blue > 200% of average.

U.K. precipitation compared to long-term average; dark blue > 200% of average

The persistent rainfall since December has caused river catchments such as the River Severn and Somerset Levels to swell, particularly across southern England and Wales. Groundwater reservoirs and soils are also saturated, leading to pluvial and groundwater flooding.

However, perhaps most interesting this season has been the surge-driven coastal flooding. Storm surges occur when strong winds force the underlying water toward the coast. As the surge develops, water levels are influenced by the shape of the coastline and tidal interactions, both of which can act to amplify surge heights and resulting coastal flooding.

While property damage has not yet reached the scale of prior major flood incidents in the U.K., this series of events highlights the importance of evaluating the complete flood cycle, from the initiating precipitation and antecedent conditions to the final mode of flooding, as seen during the 2012 U.K. flooding.

With tidal ranges as large as 15 m in the U.K., the timing of the surge is vital for determining the scale of the hazard. Surges that impact a region at high (spring) tide pose the most risk for flooding. The storms impacting northern Europe this winter have consistently coincided with some of the highest tides of the year.

Level of surge (green), relative to actual (blue) and predicted (red) storm-tide

Level of surge (green), relative to actual (blue) and predicted (red) storm-tide

Beginning with Windstorm Xaver in December, the U.K. east coast and coastal locations in Germany were given their sternest test since the devastating 1953 and 1962 events. Fortunately coastal defenses have been improved since those historical floods and the subsequent flooding was not significant.

Numerous systems have continued to arrive through January, with southeast England and Wales, Ireland and northern France particularly affected. As recently as last week, Windstorms Petra and Ruth brought yet more coastal damage and flooding, and the risk of more flooding remains high this week.

As with the wind and inland flood impacts of each individual storm, the coastal damage may not be viewed significant in isolation. Consequently specific storms from this season may not stick in the memory, like 87J has. But the accumulating damage and cost of this continuous series of events has made this a season to remember.

It has also posed a question around how we as an industry evaluate our wind and flood risk. Do we evaluate these perils in isolation or do we consider the correlation these perils have in winter months. A question that may become more prominent as the future of flood insurance in the U.K. evolves.

Senior Manager, Model Product Strategy, RMS
As a member of RMS' model solutions team, Adrian works to guide more informed usage of catastrophe models and enhance understanding of model uncertainty. This requires interaction with the market, as well as other important stakeholders such as regulators and rating agencies, to help RMS develop tools that capture the evolving needs of the risk management industry. Based in London, his primary focus is on supporting the RMS European modeling suite and in facilitating client understanding around the open modeling capabilities of RMS(one). Adrian holds a BS in meteorology and oceanography from the University of East Anglia and an MS in engineering in the coastal environment from the University of Southampton.

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  1. Pingback: Understanding the Potential Impact of the Next Catastrophic European Flood | The RMS Blog

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