European windstorms are powerful extratropical cyclones which form as cyclonic windstorms associated with areas of low atmospheric pressure. They can occur throughout the year, but are most frequent between October and March, with peak intensity in the winter months. Deep areas of low pressure are common over the North Atlantic, and occasionally start as nor'easters off the New England coast. They frequently track across the North Atlantic Ocean towards the north of Scotland and into the Norwegian Sea, which generally minimizes the impact to inland areas; however, if the track is further south, it may cause adverse weather conditions across Central Europe, Northern Europe and especially Western Europe. The countries most commonly affected include the United Kingdom, Ireland, the Netherlands, Norway, Germany, the Faroe Islands and Iceland.
The strong wind phenomena intrinsic to European windstorms, that give rise to "damage footprints" at the surface, can be placed into three categories, namely the "warm jet", the "cold jet" and the "sting jet". These phenomena vary in terms of physical mechanisms, atmospheric structure, spatial extent, duration, severity level, predictability and location relative to cyclone and fronts.
On average, these storms cause economic damage of around €1.9 billion per year and insurance losses of €1.4 billion per year (1990–1998). They cause the highest amount of natural catastrophe insurance loss in Europe.
Cyclogenesis
North Atlantic Oscillation
The state of the North Atlantic Oscillation relates strongly to the frequency, intensity, and tracks of European windstorms. An enhanced number of storms have been noted over the North Atlantic region during positive NAO phases (compared to negative NAO phases) and is due to larger areas of suitable growth conditions. The occurrence of extreme North Atlantic cyclones is aligned with the NAO state during the cyclones' development phase. The strongest storms are embedded within, and form in large scale atmospheric flow. It should be kept in mind that, on the other hand, the cyclones themselves play a major role in steering the NAO phase. Aggregate European windstorm losses show a strong dependence on NAO, with losses increasing/decreasing 10-15% at all return periods.
Connection to North American cold spells
A connection between wintertime cold air outbreaks in North America and European windstorms has been hypothesized in the last years. Cold spells over Central Canada and Eastern US appear to be associated with more frequent windstorms and flash floods over Iberia, whereas cold spells over Eastern Canada show a connection to windstorms over Northern Europe and the British Isles. The reason behind those teleconnections is not fully clear yet, but changes in the behavior of the Polar jet stream are likely to be at least related to this effect.
Clustering
Temporal clustering of windstorm events has also been noted, with eight consecutive storms hitting Europe during the winter of 1989/90. Cyclones Lothar and Martin in 1999 were separated by only 36 hours. Cyclone Kyrill in 2007 followed only four days after Cyclone Per. In November 2011, Cyclone Berit moved across Northern Europe, and just a day later another storm, named Yoda, hit the same area.
Nomenclature
Naming of individual storms
Up to the second half of the 19th century, European windstorms were named after the person who spotted them. Usually, they would be named either by the year, the date, the Saint's day of their occurrence or any other way that made them commonly known.
However, a storm may still be named differently in different countries. For instance, the Norwegian weather service also names independently notable storms that affect Norway, which can result in multiple names being used in different countries they affect, such as:
1999 storm "Anatol" in Germany, is known as the "December hurricane" or "Adam" in Denmark and as "Carola" in Sweden. 2011 storm "Dagmar" in Norway and Sweden is known as "Patrick" in Germany and "Tapani" in Finland. 2013 St. Jude storm in the English media, is known as Christian in German and French (following the Free University of Berlin's Adopt-a-Vortex program) it was named Simone by the Swedish Meteorological and Hydrological Institute, and as the October storm in Danish and Dutch, it was later given the name Allan by the Danish Meteorological Institute following the political decision to name strong storms which affect Denmark.