Meteorological Causes of Flashflood in Western Slovakia on 7 June 2011

1. Meteorological Causes of Flashflood in Western Slovakiaon 7 June 2011 M.Benko, M.Matiašová, N.Polčák, P.Valová Forecast and Warning Department SlovakHydrometeorologicalInstitute, Bratislava Slovakia

3. Píla 7/6/2011 – weather situation • ground pressure field – low pressureover Western and Central Europewith central cycloneover Ireland • shallow pressure field over Central Europe about 1010 hPa • upper level atmosphere(500 hPa) – deeping and spreading troughover Western Europe • Slovakia was at its front side, warm air mass from south-west Mean sea level pressure, 12 UTC Absolute topography 500 hPa, 12 UTC

4. Píla 7/6/2011 – weather situation warm and moist air mass over Slovakia area 850 hPa temperature about 12 – 14 °C relative humidity at 925 hPa 80 %, resp.85-94 % (ECMWF, resp. ALADIN model) Relative humidity at 925 hPa , ECMWF12 UTC Air temperature at 850 hPa, ALADIN 12 UTC Relative humidity at 925 hPa, ALADIN 12 UTC

5. Píla 7/6/2011 – weather situation vertical profile of relative humidity for Bratislava shows moist air at low troposphere (up to 3 km)

6. Píla 7/6/2011 – weather situation • vertical wind shear as the strong convection characteristic • vertical wind profile for Bratislava Vertical wind shear 7/6/2011 at 12 UTC (925 hPa level – black, 500 hPa level – red) Vertical wind speed and direction – forecasted for 7/6/2011 15 UTC - Bratislava

7. Píla 7/6/2011 – weather situation • 24 hour rainfall amount for interested area (black ellipse stands for Modra area) 7, resp. 9 mm • probability of rainfall of total amount over 10 mm - about 21 % • the higher rainfall amounts were predicted for mountainous regions of Central Slovakia, the real rainfall amounts were 20-30 mm 24 hour rainfall amount – calculated by ALADIN model 24 hour rainfall amount – calculated by ECMWF model

8. Píla 7/6/2011 – stability indexes KI >30[K]– suitable conditions for convection at 12 UTC

9. Píla 7/6/2011 – stability indexes CAPE -1000 up to 2200[J/kg]– suitable conditions for convection at 15UTC

10. Píla 7/6/2011 – stability indexes CIN<15[J/kg] – suitable conditions for Cu, Cb cloudsat 12 UTC

11. Píla 7/6/2011 – stability indexes SREH 100 [m2s-2] – very low probability of severe thunderstorm at 12 UTC

12. Píla 7/6/2011 – stability indexes EHI ~ 1 [m4s-4]- very low probability of severe thunderstorm accompanied bytornado at 15 UTC

13. Píla 7/6/2011 – the storm development • the warm air mass over Slovakia area from 30/5/2011 • the occurance of thunderstorms from 30/5/2011to 8/6/2011 in Slovakia at most meteorological stations • Tmax over western Slovakiawas 25-29 °C at 6/6/2012 and7/6/2012 • temperature of convection was calculated 24°C at 7/6/2011 from morning ALADIN run • the mentioned temperature was reached at many stations of SW Slovakia already at 9 UTC

14. Píla 7/6/2011 – the storm development A creation of the first convective cells visible on meteorological radar data over Malé Karpaty area still at 11.00 UTC

15. Píla 7/6/2011 – the storm development The storm cells growing over the Malé Karpaty ridge (black ellipse) and storm new cells growing southeast of Bratislava (red elipse)

16. Píla 7/6/2011 – the storm development High radar reflectivity of storm cell over Píla village area(black ellipse)

17. Píla 7/6/2011 – the storm development The animation of radar pictures

18. Píla 7/6/2011 – the storm development

19. Píla 7/6/2011 – the storm development The storm cells from meteorological satellite MSG in IR band - tops of the clouds highlighted.

20. Píla 7/6/2011 – the storm development Thestorm cell in the highest growth stage. In the image of meteorological satellite at 14:30 UTC in the VIS band it is possible to distinguish small overshooting tops of Cb clouds.

21. Píla 7/6/2011 – the storm development Total 6-hour rainfall calculated by INCA model and SYNOP analysis for 12.00 - 18.00 UTC

22. Píla 7/6/2011 – forecasts and warnings • high low level humidity and warm air mass over Slovakia gave very good conditions for growing of convective clouds • weak vertical wind shear and not high stability index values did not indicate the severe thunderstorms • slight flow is the reason of slow progress of storm cells and therefore the cells stayed longer over the region and the higher rainfall was occured • with regard to the above mentioned situation meteorologists on duty expected the occurrence of storms and in text forecasts the storms were forecasted for western Slovakia (forecasts issued on 6.6.2011 and 7.6.2011) "showers and thunderstorms locally, occasionally with hail“

23. Píla 7/6/2011 – forecasts and warnings • The first warning of storms valid for the affected area was issued on 6/6/2011 at 18:35 UTC, it was the first level storm warning valid up to 7/6/2011 18:00 UTC: "In the districts of Bratislava, Pezinok, Trnava, Hlohovec, Piešťany, Trnava, Galanta, Šaľa, Dunajská Streda isolated strong storms with hail are expected, which can cause a potential danger for human activities. The storms can be accompanied by short intensive downpours, and total amounts of precipitation may reach 20 to 40 mm and wind gusts of 17-25 m/s. " The warning was repeated four timesa night and morning. • due to intensive development of storms the warning was increased on 7/6/2012 to the second level of storm warning at 14:44 UTC "In the districts of Bratislava, Pezinok, Trnava, Hlohovec, Piešťany, Trnava, Galanta, Šaľa, Dunajská Streda are expected isolated strong storms with higher risk to human activity. The storms can be accompanied by a flashflood rainfall with totals of 40 to 60 mm and wind gusts of 25 m/s. The intensity of the accompanying phenomena in a given time of year is above the average and the damage probability is high."

24. Píla 7/6/2011 – forecasts and warnings • The warning was repeated on 7/6/2012 at 15:08, at 16:49 UTC the second level warning was extended to the south of Central Slovakia and Stredné Pohronie (Central Hron river basin) • At 17:17 UTC the guess of total rainfalls were increased for western Slovakia to 40-70 mm and at 18:26 this warning was repeated. The validity of warnings was up to 19:30 UTC. • At 22:02 UTC it was issued a new first level storm warning with regard to the storms, moving from eastern Austria towards the Malé Karpaty Mountains. However, these ones didn´t reach a high intensity of rainfalls.

25. Píla 7/6/2011 – forecasts and warnings • The meteorological station Modra – Piesky observed the intensive thunderstorms with heavy rain, associated with hails, at interval 13:00 - 16:15 UTC • Total 24-hour rainfalls in the affected area and the vicinity observed by following rainfall gauge stations: Modra - Piesky 104 mm, Častá 61 mm, Smolenice 42 mm, Modra 31 mm, Pernek 27 mm, Sološnica 26 mm, Pezinok 25 mm, Limbach 19 mm. • Flashflood on 7/6/2011 was caused by heavy rainfalls associated with multicellular storm system.Weak flow caused slow movement of storm cells and so intensive rainfalls lasted over the affected region for longer time. • The situation of 7/6/2011 is an example of a typical flashflood where favourable combination of all weather conditions, relief shape and relief orientation results in dangerous meteorological phenomenon with large property damage and threat to human lives. • Flashfloods in Slovakia regionoccur (during the summer) every year. • A key element for on-time detection of possible dangerous phenomena and for warning issuingare the meteorological, hydrological, radar and satellite measurements in real time are

26. Píla 7/6/2011 – record of Gidra water level

27. Píla 7/6/2011 – the flashflood damage

28. Thanks for your attention 