Winter Storm

A Peril of Many Faces

Winter storm is not a single hazard but a family of related cold-season perils that together constitute a major and often underappreciated source of insured losses. Unlike the dramatic single-event losses of hurricanes or earthquakes, winter storm losses often accumulate across many sequential events through a season — each moderate in isolation but collectively significant. And unlike tropical cyclones, which are constrained to particular ocean basins and seasons, winter storms can affect almost every populated region of the temperate world from October through April.

The insurance industry encounters winter storm risk across two broadly distinct categories: European windstorms (extratropical cyclones bringing severe winds and sometimes snow) and U.S. winter storms (a more complex hazard set including blizzards, ice storms, freezing rain, and extreme cold events). Each has different physical drivers, different damage mechanisms, and different modelling approaches.

European Windstorms

European windstorms — also called extratropical cyclones or mid-latitude cyclones — are intense low-pressure systems that derive their energy from the temperature contrast between cold polar air and warm tropical air. They are fundamentally different from tropical cyclones in their physics but share the characteristic of producing very high wind speeds over large geographic areas.

Formation and Characteristics

European windstorms typically form in the North Atlantic Ocean, where the polar jet stream — a fast-moving ribbon of air at high altitude — creates conditions for rapid cyclone development. Cold polar air from the Arctic meets warm subtropical air from the south along a boundary called the polar front. Where disturbances develop along this front, they can rapidly deepen — a process called explosive cyclogenesis or "bombogenesis" — with central pressure dropping 24 millibars or more in 24 hours. The resulting storms can produce sustained winds of 100–150 km/h across swaths of northwestern Europe, with gusts exceeding 200 km/h in exposed locations.

Key differences from tropical cyclones include:

• Size: European windstorms are typically much larger than tropical cyclones — a single storm can simultaneously produce damaging winds across the UK, France, Germany, Belgium, and the Netherlands
• Track predictability: While tropical cyclone tracks can be uncertain, European windstorm paths follow climatological patterns that are relatively well understood, though individual storm behaviour remains difficult to forecast beyond 5–7 days
• Season: European windstorm season runs from approximately October to March, peaking in December–February
• Damage mechanism: Primarily wind damage to buildings and infrastructure, with less contribution from storm surge than in tropical cyclone events (though North Sea storm surge is significant for low-lying Netherlands and Germany coastal regions)

Benchmark European Windstorm Events

Several historic events define the European windstorm risk landscape and serve as benchmark scenarios for model validation:

• Storm Daria (January 1990): One of the costliest European windstorms ever, causing insured losses of approximately USD 8 billion across France, UK, Germany, and Belgium. Peak gusts exceeded 200 km/h in France.
• Storms Lothar and Martin (December 1999): Two storms in quick succession — Lothar struck on December 26, Martin on December 28 — causing combined insured losses exceeding USD 12 billion. Lothar was described as a "once in 200 year" event for France. The sequential nature of these storms demonstrated the importance of modelling aggregate annual losses, not just single events.
• Storm Kyrill (January 2007): Caused USD 7 billion in insured losses across 12 European countries, demonstrating the multi-national accumulation problem facing reinsurers.
• Storm Ciaran (November 2023): Struck France, Italy, and Switzerland with exceptional intensity, causing significant losses and prompting discussions about whether climate change is altering European windstorm frequency or severity.

U.S. Winter Storms — A Multi-Hazard Peril

U.S. winter storm encompasses several distinct hazard types that can occur individually or in combination during a single event:

Blizzards and Heavy Snow

Blizzards combine heavy snowfall, strong winds (sustained winds of at least 56 km/h), and low visibility (less than 400 metres) for a sustained period. Insurance losses from blizzards arise primarily from:

• Roof collapse: Accumulated snow loads can exceed the design capacity of roofs, particularly flat or low-pitched commercial roofs. Snow load of 1 metre depth weighs approximately 100–500 kg/m² depending on snow density — wet, dense snow being far heavier than light, dry snow. Commercial buildings with large flat roof areas are particularly vulnerable.
• Building envelope damage: High winds drive snow and ice into building openings, causing water intrusion and ice dam formation.
• Business interruption: Transport network disruption, power outages, and inability of staff to access workplaces cause significant business interruption losses beyond physical property damage.

Ice Storms and Freezing Rain

Ice storms are among the most damaging and underappreciated winter weather events. They occur when rain falls through a shallow layer of freezing air near the surface and freezes on contact with cold surfaces — roads, trees, power lines, and buildings. The resulting glaze ice accumulation can cause enormous damage:

• Power infrastructure: Ice accumulation on transmission lines adds extreme weight — a single span of heavily iced transmission wire can weigh 10–15 times its normal weight — causing lines, poles, and towers to collapse. Ice storm power outages can affect millions of customers for days to weeks.
• Tree damage: Trees retain their full leaf area in winter storms (unlike deciduous trees in summer), and ice accumulation causes widespread branch failure and toppling of whole trees, which in turn damage buildings, vehicles, and power lines below.
• Transportation: Glaze ice on roads is far more hazardous than snow — nearly frictionless and difficult to treat with salt — causing widespread accidents and network shutdown.

The January 1998 North American Ice Storm affected Quebec, Ontario, New Brunswick, and parts of New England, leaving 4 million customers without power for up to 5 weeks and causing USD 5 billion in total losses. It remains one of the most damaging winter events in North American history and a benchmark scenario for ice storm cat modelling.

Freeze Events and Burst Pipes

Freeze events — periods of sustained extreme cold — cause losses through a mechanism that is distinct from direct wind or snow damage: the freezing of water in pipes, sprinkler systems, and HVAC equipment. When water freezes it expands by approximately 9%, generating pressure that can burst pipes and fittings. The resulting water damage upon thaw can be catastrophic, particularly in:

• Buildings in mild climates where pipes are not insulated or are routed through unheated spaces
• Vacant or unoccupied buildings where heating systems have been reduced
• Commercial properties with extensive sprinkler systems in unheated spaces
• Manufactured homes and mobile homes with exposed plumbing

The Texas freeze of February 2021 was the most dramatic demonstration of freeze vulnerability in recent history. Texas homes are built for a mild climate — pipes frequently run through uninsulated exterior walls or attic spaces, and many homes lack adequate insulation. When temperatures dropped 30–40°C below normal for several days, millions of pipes froze and burst. The subsequent thaw produced cascading water damage across millions of properties simultaneously — creating a demand surge that drove up repair costs and extended repair times across the entire state.

The Winter Storm Modelling Framework

European Windstorm Modelling

European windstorm models closely resemble hurricane models in structure. Stochastic track catalogs are generated from reanalysis atmospheric data (such as the ERA5 dataset from the European Centre for Medium-Range Weather Forecasts), producing thousands of synthetic storm tracks across the North Atlantic and Europe. Wind fields are generated for each synthetic storm, accounting for storm translation speed, size, and the interaction with topography. Vulnerability functions relate peak gust wind speed to damage ratios for European building types.

A key challenge specific to European windstorm modelling is the diversity of building stock across different countries and the differing insurance market structures — the same physical wind speed produces different insured losses in the UK (where most property is insured), France (where take-up is high), and Poland (where insurance penetration is much lower). Multi-country models must account for these market-specific factors in their financial modules.

U.S. Winter Storm Modelling

U.S. winter storm models are structurally more complex than European windstorm models because they must represent multiple distinct hazard types. The primary components include:

• Snow accumulation modelling: Using historical snowfall data and meteorological reanalysis to generate stochastic snowfall fields, with snow load calculations for roof collapse risk
• Ice accumulation modelling: Representing the meteorological conditions (temperature inversion, freezing rain) that produce glaze ice, and its accumulation on surfaces
• Freeze temperature modelling: Capturing the distribution of minimum temperatures, duration of freezing conditions, and spatial extent of cold air outbreaks
• Wind modelling: Blizzard-force winds combined with snow cause different damage than either hazard alone

Vulnerability functions for U.S. winter storm are particularly challenging to develop because the relationship between a physical hazard (e.g. minimum temperature) and damage (burst pipes) depends critically on building characteristics and practices that vary significantly by region — specifically, whether plumbing is adequately insulated and protected. A temperature of -10°C causes very different damage rates in Minnesota (where buildings are designed for it) versus Texas (where they are not).

Climate Change and Winter Storms

The relationship between climate change and winter storm risk is complex and, in some ways, counter-intuitive:

• European windstorms: Climate models project possible increases in North Atlantic storm track intensity, though with significant model uncertainty. Some research suggests the most severe European windstorms may become more frequent even as the overall number of storms decreases.
• U.S. winter storms: The Arctic Amplification hypothesis — the observation that the Arctic is warming faster than lower latitudes — suggests that the polar jet stream is becoming wavier and more prone to large southward dips (known as Polar Vortex disruptions). These dips can bring Arctic air far south of its normal range, producing cold outbreaks in regions with little historical experience of extreme cold. This mechanism is the proposed explanation for the increasing frequency of severe cold outbreaks in the southern and central United States — including the 2019, 2021, and 2023 Texas cold events.
• Changing snow climatology: While overall snowfall is projected to decrease in most regions as temperatures rise, the intensity of individual heavy snow events may increase because warmer air can hold more moisture, producing heavier precipitation when temperatures do drop below freezing.