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Deutscher Wetterdienst Business Area Climate and Environment Flooding in Eastern Central Europe in May 2010 Reasons, devolution and climatological assessment Peter Bissolli, Karsten Friedrich, Jörg Rapp, Markus Ziese Abstract After rainfalls for days and heavy thunderstorms in May and early June 2010, large parts of eastern central Europe were concerned by flooding. The main focus of the flooding was located in southern Poland, but also the Czech Republic, Slovakia, Hun- gary, Croatia, Bosnia and Herzegovina, Bulgaria and southern and eastern Germany were concerned. Precipitation occurred in several phases, with by far the highest reg- istrated amount in the middle of May. Reason of this heavy precipitation was a quasi- stationary upper air low in relation with a so called Vb-like weather situation, strong temperature contrasts, a high amount of water vapour and orographical effects. Due to dike building activity in past years, the extent of damage was limited, nevertheless, considerable damage in agriculture and also fatalities were to be complained. The flooding was not unusual for this area, but it belonged to the outstanding heavy pre- cipitation events concerning its intensity and extent, which occurred quite frequently in recent time. Synoptic development The main synoptic-scale reason for persistent and heavy rainfall in the territory of southern Poland and northern Carpathians was a stationary upper air low over Southeast Europe. This low was generated as a self-contained circulation pattern on 15 May 2010. At that time the southern part of a long-wave trough over Western Europe became an isolated low pressure area ("cut-off process"). As a consequence of this process an upper air low was formed over central Europe and Italy. It moved eastwards and reached Southeast Europe. There it re- mained stationary or moved only slowly for about one week, and was to be identified even on 23 May as a marginal low. The upper air low transported subtropical warm air from North Africa and the eastern Medi- terranean to the north. This air mass was led around this low over Russia and the Baltic states and was included in the low’s inner circulation. At the same time, strong cyclogenesis near surface was initiated over the Adriatic. This de- velopment was intensified due to lee effects south of the Alps - typical for so-called “Vb”- situations. The surface low was named YOLANDA by the Free University of Berlin. It moved slowly eastwards and became finally – like the upper air low - almost stationary over eastern Romania for some days (Fig. 1). Dynamic lows of the mid-latitudes, which take a more southern path across the Mediterra- nean or which are built even there and sweep north-eastwards, intensify due to the subtropi- cal air mass over that area. When this air mass is transported northwards, it causes wide- spread warm air advection (Fig. 2) with long persisting rainfalls on the west flank of the low. Furthermore, the water vapour content in the subtropical air is significantly higher than that of the surrounding air. The precipitable water amounted 20 - 30 kg/m² in the period between 14 and 18 May over Poland, while strong lifting occurred especially in the headwater areas of the rivers Oder and Vistula, which induced condensation and production of rain droplets (Fig. 3). 2 In addition, orographic effects appeared. At the west side of the low, the surface wind turned to north, so that additional lifting effects on the north side of the Sudeten and Carpathian mountains triggered particularly high rainfall intensities. Fig. 1: Surface analysis from 17 May 2010, 12:00 UTC (Source: DWD) Fig. 2: Temperature advection at 700 hPa on 17 May 2010, 12:00 UTC (Source: Satrep Online, EUMeTrain, ZAMG). 3 Fig. 3: Precipitating Clouds (PC) on 17 May 2010, 12:00 UTC (Source: Satrep Online, EUMeTrain, ZAMG). More rainfall activity took place, when a frontal zone reached Central Europe on Tuesday, 25 May 2010, and became quasi-stationary. In south-eastern Central Europe thundery rain fell again, while only some isolated showers occurred northwards. Further mid-latitude cyclones, which shifted from western to Eastern Europe, brought more precipitation to eastern Central Europe from time to time, partly even thundery, but overall much lower. Later on, an upper air cyclone was separated from a low pressure complex over northern Europe. This cyclone moved south-eastwards across the North Sea and Central Europe to the central Mediterranean. In early June, the headwater areas of the rivers Oder and Vistula came again under influence of the strong lifting region of this low. The precipitation maximum was shifted eastwards at this time, and thus more over the catchment area of the Vistula. On 2 June 2010, the low was active particularly in southern Germany, especially near the Alps caused by orography. The situation was defused concerning the precipitation after the 4 June 2010 due to the broad high pressure ridge extended to eastern Central Europe behind the low. Development of Precipitation Events Inside the warm air, the area between the Adriatic Sea and Poland were influenced by wide- spread rain starting in the middle of May. The highest amounts of precipitation fell in the headwaters of the rivers Odra and Vistula between 16 and 19 May 2010. The centre of pre- cipitation was situated in the border region of Poland and the Czech Republic in the Western Beskids not far from the border triangle with Slovakia. More than 100 mm precipitation in 24 hours were measured in the morning of 17 May (Fig. 4). The highest amounts were about 180 mm. The following day brought precipitation totals around 80 mm with a maximum of 115 mm. Further 50 mm fell on the third day. At the stations Lysa Hora (catchment of Odra River) and Bielsko-Biała (catchment of Vistula River), about 320 mm of precipitation were 4 measured in the mentioned period (Fig. 5). The area of precipitation was small and very sta- tionary, as it is shown in Fig. 6 by totals over several days. In the same period only 22 mm fell south of the Western Beskids. Due to further precipitation at the beginning of the month of May, the soil moisture was above normal. At the station Kasprowym Wierchu (1991 m AMSL), 72 cm of snow were measured on 18 May 2010. On 25 May, only remnants of snow were to be seen at the station. A portion of precipitation in the mountains fell as snow during 16 and 17 May 2010. Further abundant precipitation were measured between 15 and 22 May 2010 in the region of High Tatra (100 - 180 mm), in the eastern part of Slovakia (about 90 mm), in the border re- gion of Slovakia and Hungary (100 - 120 mm) and in Croatia and Bosnia (70 - 120 mm) (Fig. 6). The highest precipitation in Croatia and Bosnia was measured between 15 and 16 May 2010. The totals within the period of 15 to 22 May 2010 amounted to up to more than 150% of the monthly long-term mean 1951 - 2000 (Fig. 7). Up to three times of the monthly long- term mean were measured in the above mentioned period at the stations Lysa Hora and Bielsko-Biała. More heavy precipitation was observed within the border triangle of Poland, Czech Republic and Slovakia at the beginning of June. The above mentioned mountain station Kasprowym Wierchu registered 70 mm of precipitation in the morning of 2 June 2010 for the last 24 hours and two days later again 77 mm. Even more than 80 mm were measured at other stations within the near periphery (Zakopane in Poland and Mochovce in Slovakia) on 2 June 2010 and again about 60 mm on 4 June 2010. Regionally 160 mm of precipitation were recorded on the first three days of June in the northern part of High Tatra. In between, mainly during the night from 2 to 3 June 2010, southern Germany, in particular the outland of the Alps, was hit by heavy rainfall. 100 mm of precipitation with a maximum of over 200 mm cumulated from 1 to 3 of June 2010 within 48 hours in this area. Fig. 4: Spatial distribution of daily precipitation on 17 May 2010, 06 UTC, based on radar meas- urements and synoptic observations (Source: hydro.chmi.cz). 5 180 160 Lysa Hora - Czech Republic 140 Bielsko-Biała - Poland 120 100 80 precipitation [mm] 60 40 20 0 1/05 4/05 7/05 10/05 13/05 16/05 19/05 22/05 25/05 28/05 31/05 3/06 6/06 Fig. 5: Precipitation in the catchment of Vistula and Odra for May and the beginning of June 2010. Fig. 6: Precipitation totals in eastern Central Europe calculated from synoptic weather reports from 15 to 22 May 2010 (gridded 1°x1° lat./lon.; source: Global Precipitation Climatology Cen- tre, DWD). 6 Fig. 7: Relative deviation of gridded precipitation totals (15 - 22 May) to the long-term mean 1951 - 2000 (entire month of May; source: Global Precipitation Climatology Centre, DWD). Hydrological effects Due to the high amounts of precipitation there were extreme flood events in the catchments of Oder, Vistula (Fig. 8), Tisza and other tributaries of river Danube. Oder The water level was 1.6 m below the top water level of the summer flood 1997 at gauge Mie- donia within the border region of Poland and Czech Republic (LUA, 2010). The region around Opole was widely inundated. The flood crest reached the border to Germany after about 10 days (Fig. 9). The top water level in Ratzdorf at the confluence of Oder and Neisse remained approximately 60 cm below the record of 1997.

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