December 2017 presented a volatile mix of winter storms and unseasonably mild conditions across the Northern Hemisphere, marking a month where Arctic air clashed with maritime extremes. This period captured significant meteorological attention as a potent series of North Atlantic cyclones delivered intense wind and flooding to Europe, while the eastern United States oscillated between record warmth and sudden arctic blasts. The interplay between the Atlantic jet stream and persistent blocking patterns dictated a narrative of sharp temperature contrasts and disruptive weather events that challenged seasonal expectations.
Global Atmospheric Patterns and Drivers
The dominant feature throughout December 2017 was a highly amplified and undulating jet stream, which facilitated frequent exchanges of air masses between polar and mid-latitude regions. A strong stratospheric warming event in late November and early December weakened the polar vortex, allowing cold air to surge southward into northern and central Europe, as well as portions of North America. This disruption was counterbalanced by a persistent high-pressure system over the North Pacific, which channeled storm systems directly into the British Isles and Scandinavia, fueling the development of what would become historic windstorms.
Major European Windstorms
The month was highlighted by the rapid succession of Windstorm Eleanor and Windstorm Fionn, which battered Western Europe within days of each other. Eleanor, named by Meteo France, made landfall on December 5th, bringing hurricane-force gusts exceeding 150 km/h to coastal France and Germany, causing widespread power outages and travel chaos. Just a week later, Windstorm Fionn impacted the UK and Ireland, with Storm Emma's remnants merging to create a secondary low that exacerbated flooding in the southwest of England.
Impact and Aftermath in the UK and France
UK: Transport for London suspended rail services, while coastal flooding in Cornwall and Devon prompted emergency responses.
France: Over 1.4 million households lost electricity, with cleanup efforts hampered by ongoing rain-saturated soil.
Insurance losses for the two events were estimated in the hundreds of millions of euros, underscoring the season's financial toll.
Temperature Extremes in North America
Across the continental United States, December 2017 was a study in contrasts, with the eastern half experiencing one of the most volatile Decembers on record. An early-season blast of Arctic air in the first week set the stage for a dramatic late-month heatwave that shattered records from the Midwest to the Northeast. Cities such as New York and Boston observed temperatures that were more typical of April, with Central Park in New York recording a reading of 70°F (21°C) on December 27th, a staggering 44 degrees above the monthly normal.
The Alaska Heatwave and Wildfire Risk
While the eastern US shivered, the state of Alaska endured a remarkable and concerning period of unseasonable warmth. Anchorage experienced temperatures that hovered near freezing for days, eliminating the typical December snowpack and raising concerns about ecological disruption and increased wildfire risk in the subsequent spring. This extreme juxtaposition illustrated the profound disconnect between regional climate patterns and reinforced the message of a planet experiencing increasingly erratic seasonal behavior.
Global Climate Context and Significance
Despite the cold outbreaks in the mid-latitudes, December 2017 contributed to a long-term warming trend, with the global temperature for the month ranking among the highest Decembers on record according to datasets maintained by NOAA and NASA. The reduced Arctic sea ice extent and the persistent warmth in the Arctic Ocean were believed to be factors influencing the jet stream's wavy pattern. This month served as a clear example that global warming does not equate to uniform warming, but rather to the amplification of extreme weather events and temperature swings.
