Baby, it’s cold (or warm) outside

It’s been running hot and cold. For those in the east and much of the Midwest, average temperatures have been lower this winter than for five, ten, or twenty years previous. Even parts of Florida have been impolitely cold, while some areas of New England are colder than they’ve been since the 1980s.

Meanwhile, almost everywhere within the West, even as far east as Kansas and Nebraska, have enjoyed record warm temperatures. While many are understandably pleased with the absence of snow and cold, concerns are already being raised about the minimizing impact the warm weather is having on snowfall. The white stuff is vital to providing drinking water, irrigation, and soil moisture which reduces wildfire hazard. Without snowpack, there may be trouble ahead.

For insurers, though, the immediate concern is the freeze. Winter Storm Fern, which stretched across a huge region from northern Mexico to southwestern Canada in late January this year, put roughly half of Americans under a weather alert. Insured loss estimates are running as high as $7 billion.

Fern won’t be as costly as her older brothers Uri (2021) and Elliott (2022), at $18 billion and $8 billion respectively, but she came at a time when reinsurance programs attach at higher levels. Add the fact that no serious winter storms have occurred since them, giving carriers a multiyear respite, and Fern will come as a cold shock to many carriers with exposure, just as it has to residents who thought it was supposed to be getting warmer.

As for reinsurers, only one inwards program had been impacted by Storm Fern at the time of writing. Although winter storm is notoriously slow to report, the number is unlikely to rise much. The higher retentions imposed after Elliott and Uri reset the normal level of attachment points, which appears to have spared reinsurers. They were also insulated by Fern’s geographical spread. The large number of states in her path spread the loss over multiple reinsurance programs.

Events of Fern’s magnitude typically expected to be retained by cedants, but some, having assumed a significant quantum of attritional loss so early in the year, might feel enough of a sting to spark an additional peak-perils reinsurance purchase.

The East coast has been quiet for the past few years, in terms of winter storms at least. The intensification of storminess this season is due mostly to upper-atmosphere circulation patterns, most of which are driven by the jet stream. Fuelled by temperature contrasts and shaped by the Earth’s rotation, this narrow band of strong wind circulates around the planet at about the altitude of a passenger jet on a long haul. It blows from west to east along the line where cold Arctic air meets warmer environs from equatorial regions.

The demarcation line is wavey, and sometimes it’s wavier than others. During the winter of 2025-6, it’s been like tourists witnessing a passing cruise ship. The southern extreme of the jet stream has dipped southwards, blasting cooler air than usual into the eastern US, and sucking warmer, dry air into the west.

The phenomenon has prompted low-pressure systems to develop, which have brought the east its snow and unseasonable cold. One of those systems fired by the off-path jet stream was storm Fern, which delivered far more snow and ice than is typical, even to Texas, Louisiana, and Tennessee.

Behind every good wave of the jet stream is the Polar Vortex. It’s the great blob of low pressure that sits atop the North Pole (the South Pole has a vortex of its own, but that’s another day’s concern). In winter, the Polar Vortex covers more territory. The Vortex sometimes behaves abnormally, perhaps when influenced by even broader changes to atmospheric conditions, but mostly for no obvious single reason. When it does, it can disturb the jet stream, which may grab a chunk of cold Arctic air and hurl it southwards, perhaps into the US.

The jet stream’s wavy circulation means the effects of the polar blob are felt elsewhere, too. Great Britain has had rain every day in 2026 at the time of writing, which is unusual even for that precipitation-prone island, and has caused serious flooding and storminess. And as mentioned, the western US has been unseasonably warm, as the southward push of cold air in the east sucked up hot tropical air in the west. In both cases, it’s the wavy jet stream again.

For climate analysts trying to understand future temperatures and levels of storminess, one cold, icy winter says little about trends. Nor does it provide evidence that undermines the atmospheric warming trend. It’s possible to have a colder winter in a warmer world.

It’s not possible to say with confidence that winters, on average, are getting stormier in particular places. Nor is the opposite true. Reviews are mixed. For every meteorologist predicting more winter storms, another is pitching for fewer, more intense events.

One thing is clear, though. Climate researchers on both sides of the debate agree that the variability from year to year is increasing. The past few years were fairly low impact. This year has been relatively serious. The up-and-down variability of winter storm systems is becoming more persistent, with fewer “average” years. For insurers planning a reinsurance program, this makes it quite a bit more difficult to decide upon the right levels of attachment, retention, and limit.

Like everyone else, reinsurance buyers are bound to have a measure of “recency bias.” That happens when recent events change our perception of the longer-term experience. Winters in the US are getting warmer, but this year’s turn-of-the year cold snap can mask that fact, and change our thinking about trends.

It’s not really possible to say if increased storminess in winter or at any time has become a new normal in any particular place. Recency bias may make that feel like truth in North Carolina, which was battered by Hurricanes Helene, then hit by ice storms this January, only to have a “Bomb Cyclone” explode over the state in February. That event slammed down more than a foot of snow, creating one of the state’s worst winter storms this century. However, it’s much safer to chalk-up this 1-2-3 experience to variability volatility than to any permanent change in the prevailing weather.

We can be more confident about some of the new weather patterns that have emerged as the atmosphere warms. As mentioned, the impact is visible in the variability of winter storminess. It can be seen also in the reach, location, and duration of tropical cyclone seasons, and in the occurrence of severe convective storms. However, all these phenomena remain difficult to predict. That makes the job of modelers and insurers a bit more difficult at the catastrophic-event-end of the scale.

Everyone whose book could be impacted by US winter storms needs at least to recognise that their volatility has escalated. They should think about looking at different Year Loss Tables in different ways, and consider scenarios with clusters of events, even if their models are not built that way.

At the very least, they need to work with knowledgeable advisors who are ahead of the models, those looking not just at the past, but also at different interpretations of the present to determine the protection they need for the future.