Tag Archives: European Winter Windstorm Season

Seasonal Forecast of European Windstorm Activity: Will a Stormier Atlantic Deliver Increased Losses?

Known indicators point to stormier conditions in the North Atlantic this winter. However, what this means for Europe windstorm losses is much less certain.

Our ability to understand and forecast variability of North Atlantic winter storminess continues to improve year-on-year. Research highlights in 2017 include:

  • A new, and skillful, empirical forecast model for winter climate in the North Atlantic revealed that sea ice concentrations in the Kara and Barents Seas are the main source of predictable winter climate variations over the past three decades. Interestingly, a separate 2017 study supports earlier forecasts of either a slowing or reversal of the sea ice reductions in the Barents and Kara Seas between now and 2020, implying an uptick in storminess over the next few years.
  • An innovative tool to analyze sources of predictability in a numerical forecast model revealed strong links between tropical climate anomalies and winter climate in the North Atlantic in that model.

Twelve months ago, the forecasting indicators for the windstorm season broadly pointed to a 2016/17 season characterized by below average storminess — a forecast borne out by subsequent observations. We have already had a fairly active start to the 2017/18 season, with Windstorms Xavier, Herwart, and ex-Hurricane Ophelia causing local damage, but what is the outlook for the rest of the season?

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What Is In Store For Europe Windstorm Activity This Winter

From tropical volcanoes to Arctic sea-ice, recent research has discovered a variety of sources of predictability for European winter wind climate. Based on this research, what are the indicators for winter storm damage this season?

The most notable forcings of winds this winter – the solar cycle and the Arctic sea-ice extents – are forcing in opposite directions. We are unsure which forcing will dominate, and the varying amplitude of these drivers over time confuses the situation further: the current solar cycle is much weaker than the past few, and big reductions in sea-ice extent have occurred over the past 20 or so years, as shown in the graph below.

Figure: Standardized anomalies of Arctic sea-ice extent over the past 50 years. (Source: NSIDC)

There are two additional sources of uncertainty, which further undermine predictive skill. First, researchers examine strength of time-mean westerly winds over 3-4 months, whereas storm damage is usually caused by a few, rare days of very strong wind. Second, storms are a chaotic weather process – a chance clash of very cold and warm air – which may happen even when climate drivers of storm activity suggest otherwise.

RMS has performed some preliminary research using storm damages, rather than time-mean westerlies, and we obtain a different picture for East Pacific El Niños. Most of them have elevated storm damage in the earlier half of the storm season (before mid-January) and less later on. Of special note are the two storms Lower Saxony in November 1972 and 87J in October 1987: the biggest autumn storms in the past few decades happened during East Pacific El Niños. The possibility that East Pacific El Niños alter the seasonality of storms, and perhaps raise the chances of very severe autumn storms, highlights potential gaps in our knowledge that compromise predictions.

We have progressed to the stage that reliable, informative forecasts could be issued on some occasions. For instance, large parts of Europe would be advised to prepare for more storm claims in the second winter after an explosive, sulphur-rich, tropical volcano. Especially if a Central Pacific La Niña is occurring [vi] and we are near the solar cycle peak.

However, the storm drivers this coming winter have mixed signals and we dare not issue a forecast. It will be interesting to see if there is more damage before rather than after mid-January, and whatever the outcome, we will have one more data point to improve forecasts of winter storm damage in the future.

Given the uncertainty in windstorm activity levels, any sophisticated catastrophe model should give the user the possibility of exploring different views around storm variability, such as the updated RMS Europe Windstorm Model, released in April this year.

[i] Fischer, E. et al. “European Climate Response to Tropical Volcanic Eruptions over the Last Half Millennium.” Geophys. Res. Lett. Geophysical Research Letters, 2007, .
[ii] Brugnara, Y., et al. “Influence of the Sunspot Cycle on the Northern Hemisphere Wintertime Circulation from Long Upper-air Data Sets.” Atmospheric Chemistry and Physics Atmos. Chem. Phys., 2013.
[iii] Graf, Hans-F., and Davide Zanchettin. “Central Pacific El Niño, the “subtropical Bridge,” and Eurasian Climate.” J. Geophys. Res. Journal of Geophysical Research, 2013.
[iv] Baldwin, M. P., et al. “The Quasi-Biennial Oscillation.” Reviews of Geophysics, 2001.
[v] Budikova, Dagmar. “Role of Arctic Sea Ice in Global Atmospheric Circulation: A Review.” Global and Planetary Change, 2009.
[vi] Zhang, Wenjun, et al. “Impacts of Two Types of La Niña on the NAO during Boreal Winter.” Climate Dynamics, 2014.

What to expect this 2014-2015 Europe Winter Windstorm Season

When it rains in Sulawesi it blows a gale in Surrey, some 12,000 miles away? While these occurrences may sound distinct and uncorrelated, the wet weather in Indonesia is likely to have played some role in the persistent stormy weather experienced across northern Europe last winter.

Weather events are clearly connected in different parts of the world. The events of last winter are discussed in RMS’ 2013-2014 Winter Storms in Europe report, which provides an in-depth analysis of the main 2013-2014 winter storm events and why it is difficult to predict European windstorm hazard due to many factors, including the influence of distant climate anomalies from across the globe.

Can we predict seasonal windstorm activity during the 2014-2015 Europe winter windstorm season?

As we enter the 2014-2015 Europe winter windstorm season, (re)insurers are wondering what to expect.

Many consider current weather forecasting tools beyond a week to be as useful as the unique “weather forecasting stone” that I came across on a recent vacation.

I am not so cynical; while weather forecasting models may have missed storms in the past and the outputs of long-range forecasts still contain uncertainty, they have progressed significantly in recent years.

In addition, our understanding of climatic drivers that strongly influence our weather, such as the North Atlantic Oscillation (NAO), El Niño Southern Oscillation (ENSO), and the Quasi-Biennial Oscillation (QBO) is constantly improving. As we learn more about these phenomena, forecasts will improve, as will our ability to identify trends and likely outcomes.

What can we expect this season?

The Indian dipole is an oscillation in sea surface temperatures between the East and West Indian Ocean. It has trended positively since the beginning of the year to a neutral phase and is forecast to remain neutral into 2015. Indonesia is historically wet during a negative phase, so we are unlikely to observe the same pattern that was characteristic of winter 2013-2014.

Current forecasts indicate that we will observe a weak central El Niño this winter. Historically speaking this has led to colder winter temperatures over northern Europe, with a blocking system drawing cooler temperatures from the north and northeast.

The influence of ENSO on the jet stream is less well-defined but potentially indicates that storms will be steered along a more southerly track. Lastly, the QBO is currently in a strong easterly phase, which tends to weaken the polar vortex as well as westerlies over the Atlantic.

Big losses can occur during low-activity seasons

Climatic features like NAO, ENSO, and QBO are indicators of potential trends in activity. While they provide some insight, (re)insurers are unlikely to use them to inform their underwriting strategy.

And, knowing that a season may have low overall winter storm activity does not remove the risk of having a significant windstorm event. For example, Windstorm Klaus occurred during a period of low winter storm activity in 2009 and devastated large parts of southern Europe, causing $3.4 billion in insured losses.

Given this uncertainty around what could occur, catastrophe models remain the best tool available for the (re)insurance industry to evaluate risk and prepare for potential impacts. While they don’t aim to forecast exactly what will happen this winter, they help us understand potential worst-case scenarios, and inform appropriate strategies to manage the exposure.