Severe Xynthia over southwestern and western Europe

A severe storm named “Xynthia” affected , Spain, , , parts of south-east England, , the , , and . Strong gusts on 27-28 February 2010 caused extended damage on traffic routes, electrical power outage, destruction due to flooding at the French Atlantic coast and more than 60 losses of lives. Most of the damage was in France and western Germany. The track of this storm and its rapid development were outstanding, but the magnitude of the gusts was comparable to other violent in the past.

Synoptical development and weather conditions Xynthia arose from an initially shallow low pressure system which formed over the subtropical sea area south of the Azores Islands on Friday, 26 February 2010. The southward flow of colder air masses in the upper air caused the deepening of a broad trough over the central and eastern North Atlantic. A shortwave trough within this broader system and a high temperature difference between extremely warm air over Africa and colder air over the eastern Atlantic caused a strong of Xynthia. On Saturday, February 27, the moved northeastwards over Portugal and the Bay of Biscay to the westernmost areas of France and intensified very rapidly to a core pressure of about 967 hPa around midnight which means a deepening of about 20 hPa within 24 hours (Fig. 1-3). During the following three days it began weakening and moved further northeastwards along the coastline of northern France and the , and then it crossed the southern to southern Finland until March 3. There it had a core pressure of 990 hPa with further slow dislocation and weakening. This is quite an unusual track of such a cyclone; in most cases such storms develop further northwards over the Atlantic and then move eastwards over western and central Europe.

Fig. 1: DWD sea level pressure analysis 27 February 2010, 12 UTC.

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Fig. 2: Meteosat 9 satellite image 28 February 2010, 00 UTC.

Fig. 3: Track of cyclone Xynthia. 3

At the southeast flank of the storm, very warm subtropical air flowed to Spain and later to Central Europe. Temperatures rose above 20°C in many places in Spain on Friday and Saturday, locally even above 25°C. Sunday morning brought temperatures around 15°C to southern Germany and Switzerland due to foehn north of the . The warm air mass extended over large parts of central Europe until Sunday noon, separated from the far colder air north and east of it (Fig. 4). After passage of the storm, colder subpolar air flowed to central Europe on the rear flank of the cyclone.

Fig. 4: Meteosat 9 satellite image, surface pressure (red isolines) and 2m air temperatures (numbers) on 28 February 2010, 12 UTC. Source: DWD

Wind gusts were highest at the southeast flank of Xynthia, particularly after cold front passage. The Canary Islands saw gusts up to around 200 km/h over mountains in the night from Friday to Saturday. Gusts up to more than 200 km/h were also measured in northern Spain on Saturday. During the night from Saturday to Sunday, still gusts at hurricane force (120-130 km/h) were recorded at many stations in France (Fig. 5). On Sunday morning, gusts of the same magnitude affected northern France, Luxembourg, Belgium, western and southwestern Germany, and Switzerland; near the Alps due to foehn locally even more than 140 km/h. The duration of the storm was about 7 hours in places of western Germany; that was much of the daytime on Sunday. The highest measured wind gust in Germany (DWD stations) was 180 km/h on Sunday evening on the Mountain which is a very exposed mountain station. But also on other lower mountain stations, local gusts of 140 up to more than 160 km/h were reached. Widespread stormy gusts, but no longer at hurricane force still occurred on Monday morning. Northern coastal areas of Germany did not have very strong gusts due to their vicinity to the low pressure core. In Austria, gusts were 4 generally lower than in other parts of central Europe, though quite high. 80-100 km/h were recorded in some places, on high mountains in the Alps up to 145 km/h. 100 km/h were also exceeded in Belgium and the Netherlands. The highest reported gust in whole western Europe was 238 km/h on the Pic du Midi in the French Pyrenees (2877m above sea level) on 27 February.

Fig. 5: Meteosat 9 satellite image, surface pressure (red isolines) and wind gusts in km/h (numbers) reported on 28 February 2010, 06 UTC. Source: DWD

The storm was also accompanied by some higher amounts of rain at the warm front of Xynthia, moving northeastwards. 24h rainfall totals in western Germany, Belgium and the Netherlands amounted up to around 20 mm in places, locally more than 30 mm. In eastern and Finland, Xynthia brought some fresh snow starting on Monday. Especially at the French Atlantic coast, the high wind speeds caused a temporary sea level increase of up to around 1.50 m (La Rochelle). The arrival time of Xynthia in this area, in the night from February 27 to 28, just coincided with a spring , resulting in extended flooding.

Damage In many parts of the most concerned areas, traffic routes were severely affected. Railways had to be closed over large areas and many flights were cancelled or delayed. Roads were closed due to trees which fell down. Luckily the storm occurred during a week-end and thus did not affect much daily traffic during working hours. Although widespread weather warnings were disseminated via the media, more than 60 losses of lives were deplorable in Europe; this was more than for the past storms Kyrill (2007) and Lothar (1999). Power and telephone 5 lines were also damaged in many places. On Sunday morning (February 28), 800,000 households in France were without electricity. Trees had been uprooted and roofs had been untiled, particularly in Spain. Flooding at the French Atlantic coast let dikes break down and affected oyster-breeding. Most of the fatalities caused by the storm occurred by drowning due to the flooding. Insurance costs of damage are estimated to around 1 Billion Euro in Germany and 2 Billion Euro in France (preliminary estimations of Deutsche Rückversicherung AG). This is clearly less than the costs of cyclone Kyrill in 2007.

Climatological assessment The storm was unusual concerning its track and outstanding (but not exceptional) concerning its rapid development on its track. It is not the “typical” Atlantic cyclone which moves quickly from the Atlantic to Europe and weakens over land. The track of Xynthia caused quite a large area in western and central Europe to be affected by the storm. The unusually strong flow of warm air was quite effective in that time of season and contributed to the higher wind speeds substantially. Such high temperatures as occurred during those days are generally not exceptional in late winter, but they are rare in combination with high wind speeds during that season and over the affected areas. The magnitude of the wind gusts was not exceptional compared with other storms in the past. Storm Klaus in January 2009 affected mainly northern Spain, Italy and southern France, but not such a large area like Xynthia, though local wind speeds were comparable or even higher. Storm Kyrill (2007) also had high wind gusts over Germany which were comparable. Storm Lothar in December 1999 affected large parts of France and southern Germany, and wind gusts were mostly higher and also exceeded 200 km/h locally. A couple of violent storms crossed central Europe in 1990 mainly during spring season, also with comparable strength. The highest known gust for Xynthia reported from the French Pyrenees at 238 km/h was clearly outstanding, but it was a local value. Another very high gust of 228 km/h measured in Orduña in northern Spain was a new local record in Europe for more than 100 years. Some recent studies suggested an increase of the number and strength of severe storms in the Northeast Atlantic European region for some decades during the second half of the 20th century and a northward shift of the storm tracks, but this might be a decadal-scale fluctuation which also occurred before. Losses due to storms show a strong increase, but when the losses are normalised (which means adapted to recent societal conditions), no more upward trends have been found at least for the period 1970-2008 (Barredo 2010, http://www.nat-hazards-earth-syst-sci.net/10/97/2010/nhess-10-97-2010.pdf). It can therefore be presumed that global warming had no major effect on the development of Xynthia, except generally higher temperatures compared to the past. The major effect instead could be expected from the constellation of the atmospheric circulation pattern which was outstanding this winter season. The North Atlantic Circulation Index (NAO) was extremely low this winter and negative during nearly the whole season, see NOAA-CPC data (http://www.cpc.noaa.gov/products/precip/CWlink/pna/nao.shtml). A negative NAO means a prevailing meridional circulation and a suppression of westerlies which cause more southerly storm tracks and an increased storm activity in southern Europe, and this was supported by seasonal warming and the warm air flow from the south. Thus, this winter provided excellent conditions for a storm with such an unusual track, especially towards the end of the season in the course of the seasonal cycle. There is, however, no significant trend of the NAO to be detected in nearly 200 years according to data of the Climate Research Unit, University of East Anglia, (http://www.cru.uea.ac.uk/~timo/datapages/naoi.htm), which means that there is no indication that such situations become more frequent and intense with time.

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National assessments of Xynthia (in national language):

Austria: http://www.zamg.ac.at/aktuell/index.php?seite=1&artikel=ZAMG_2010-03-01GMT14:04

Belgium: http://www.meteo.be/meteo/view/fr/66980-Recents.html?view=4068613

France: http://france.meteofrance.com/france/actu/actu?portlet_id=50150&document_id=22089

Germany: http://www.dwd.de/bvbw/appmanager/bvbw/dwdwwwDesktop?_nfpb=true&_windowLabel=T 25000132381160058587421&_urlType=action&_pageLabel=_dwdwww_wetter_warnungen_ Thema_des_Tages

Netherlands: http://www.knmi.nl/cms/content/77992/storm_xynthia_trof_vooral_frankrijk_27-_28_februari

Spain: http://www.aemet.es/es/noticias/2010/03/coclogenesisexplosivafebrero2010

Switzerland: http://www.meteoschweiz.admin.ch/web/de/wetter/wetterereignisse/foehnsturm.html

Last update: 15 March 2010 Author: Dr. Peter Bissolli, DWD