DWD background report Winter storm season 2017/18 (01.09.2017–28.02.2018) – meteorological description of the winter storms SEBASTIAN, XAVIER, HERWART, ZUBIN, BURGLIND, FRIEDERIKE Due to the large temperature and thus energy differences between the polar regions and the warm-temperate climate zone during the northern hemisphere’s winter half-year, unstable wave formations typically arise on the frontal zone, with the potential to evolve into storm cyclones. According to the long-term climatological average, the high-pressure weather situations, usually dominant through to the last ten days of October, are succeeded by a cyclonic circulation regime. This then marks the start of autumn or winter storm season in central Europe. In 2017/18, the winter storm season began very early on 12 September when a polar air outburst originating in Greenland strengthened a short-wave trough to such an extent that a rapid drop in pressure (‘explosive cyclogenesis’, i.e. a fall in air pressure in a low’s centre of at least 1 hPa/h over a period of at least 24 hours1) in the corresponding surface pressure field transformed an initially unremarkable secondary low into the storm cyclone SEBASTIAN. SEBASTIAN brought hurricane-force gusts (≥ 12 Bft) mainly to the coastal areas and mountain tops in the low mountain ranges on 13 September, as well as daily precipitation totals of several tens of litres per square metre in places in southern Germany. A vigorous west wind drift again dominated over the European continent three weeks later. The secondary cyclogenesis (Shapiro-Keyser type, i.e. a special form of polar front cyclones in maritime areas2) of a central low, which had in the meantime arrived over Fennoscandia, brought the second autumn storm of the season. This storm, named XAVIER, mainly affected the north and east of Germany, bringing hurricane-force gusts (≥ 12 Bft) to the areas in and around Berlin and to the low mountain ranges in eastern Germany. The low pressure centre, which moved eastward in parallel to the coast, released several tens of litres of rain per square metre within 24 hours. During cyclonic north-westerly weather conditions over the North Atlantic and European region at the end of October 2017, a secondary trough, in combination with lee effects on the Norwegian mountains, induced the rapidly moving storm cyclone HERWART. Its wind field brought hurricane-force gusts (≥ 12 Bft) mainly to the low mountain ranges in eastern Germany and to the coastal areas; the North Sea and the Baltic Sea were both hit by storm surges. In the northerly airflow at its rear, HERWART’s frontal system headed south rapidly and reached the German Alpine foothills. Sometimes copious precipitation fell in the windward areas of the low mountain ranges. The rotating low ZUBIN already began building up in the first week of December over the warm waters of the Gulf of Mexico at the leading edge of a body of cold air that dominated almost the entire North American continent. At first, ZUBIN intensified as it moved at the front of the trough in a north-easterly direction up to the southern tip of Greenland. But then, it began to weaken, split up and came back together again only to divide again just south of Iceland and continue its track over the North Atlantic and the North Sea to finally influence especially the central and southern parts of Germany. ZUBIN was the only storm in the 2017/18 winter season in central Europe which did not arise from a secondary cyclogenesis, but affected the weather itself as a central low. Single hurricane-force gusts (≥ 12 Bft) 1 Bott, Andreas 2012: Synoptische Meteorologie, Methoden der Wetteranalyse und –prognose, Springer, Berlin und Heidelberg, p. 350 2 Bott, Andreas 2012: Synoptische Meteorologie, Methoden der Wetteranalyse und –prognose, Springer, Berlin und Heidelberg, p. 346 1 DWD background report occurred on exposed ridges and summits as well as in lowlands in southern Germany. The cold front passed rapidly and the amount of precipitation was consequently unimpressive in most cases. BURGLIND formed on New Year’s Day over the North Atlantic in the form of a flat wave disturbance on the southern edge of a dominant storm cyclone. While the semi-permanent centre itself remained motionless over the north-east Atlantic, BURGLIND intensified over the British Isles and, under the influence of a secondary trough in the middle troposphere, swung towards the Baltic Sea region where it gradually dissipated. BURGLIND affected large parts of Germany. The passing cold front brought heavy rain with thunderstorms in some places as well hurricane-force gusts (≥ 12 Bft) at speeds of over 200 km/h on exposed ridges and summits in the south-west of Germany. The highest wind speeds measured in the DWD measuring network occurred at the Feldberg station (Black Forest, 1486 m a.s.l.) at 217.4 km/h. This also was the highest wind speed measured during the entire 2017/18 winter storm season. The last storm cyclone of the winter half-year 2017/18, FRIEDERIKE, had a source similar to ZUBIN five weeks earlier. This time, however, a powerful trough resulting from a deep polar air mass from the Canadian Arctic dominated the eastern half of North America. In mid- January, a flat low-pressure system formed on the low’s leading edge over the warm Sargasso Sea and, driven by the upper-level jet, tracked in a north-easterly direction; from there, it then crossed the Atlantic to reach Europe with the west wind drift on the northern flank of the Azores High. FRIEDERIKE thus reached the edge of a central low near Iceland so that, due to a Shapiro-Keyser type explosive cyclogenesis, its core arriving from the British Isles crossed over northern Germany very quickly in an easterly direction. The strongest gusts occurred particularly in a broad swathe to the southwest of the low behind the cold front, probably caused by a ‘sting jet’ as the jet stream sank under the influence of dynamic restructuring and transferred its momentum to the lower air layers. Peak gusts reaching speeds of over 200 km/h swept across some ridges and summits mainly in central Germany, along with widespread hurricane-force gusts (≥ 12 Bft) in lowlands. The highest wind speeds for this storm and the second highest in the 2017/18 season at 203.4 km/h were observed on the Brocken (Harz, 1142 m a.s.l.). The synoptic analysis for the storm cyclones in the North Atlantic and European region during the winter half-year 2017/18 shows that only in the case of ZUBIN the vortex acting as guiding low-pressure area itself developed into a storm cyclone over central Europe. All the other storms resulted from secondary low developments, some of which (SEBASTIAN, XAVIER, HERWART and FRIEDERIKE) might, owing to their higher track speed, be regarded as ‘fast-moving’. Explosive cyclogenesis, defined as a fall in air pressure in a low’s centre of 1 mbar (hPa) or more over a period of at least 24 hours, occurred in two cases (SEBASTIAN and BURGLIND). Noteworthy is also that the low-pressure areas XAVIER and FRIEDERIKE featured some properties of a Shapiro-Keyser cyclone. 2 .