Extremely rare series of thunderstorms

During a two-week period in May and June 2016, large parts of Germany were repeatedly affected by severe thunderstorms causing major losses and even several fatalities. Due to extreme rain totals exceeding 100 mm within a few hours, water levels in many small creeks rose dramatically and led to flash floods inundating some towns entirely. The severe weather episode set in on 29 May, when a flood wave with a height of up to 3.5 m damaged 80 buildings in Braunsbach (federal state of Baden-Württemberg), of which five were completely lost. Only three days later, an extreme flash flood struck the town of Simbach (federal state of Bavaria), where the water level increased from 20 cm to more than 5 m within only 12 h. The Ahr valley in the federal state of Rhineland-Palatinate experienced heavy rain events even on multiple days leading to the worst flood along the river Ahr ever reported. According to extreme value analyses with respect to 24-hour and 7-day rain totals, return periods exceeded 200 years in several regions with some local maxima even reaching markedly higher values. In Germany, overall economic losses accounted for EUR 2.6 billion.

Prior to the storm episode, moist and warm subtropical air masses were advected towards Germany, where insolation caused vertical temperature gradients to steepen leading to increasingly unstable conditions. Subsequently, a shallow surface low stretching over western and southern Germany went along with widespread lifting. The remarkable length of the storm episode can be attributed to an Omega-blocking event, which is characterized by a strong high-pressure ridge flanked by two low-pressure troughs representing a very persistent large-scale weather situation. Moreover, the pressure gradient and the resulting wind speed both at ground level and in the lower troposphere were very weak allowing for thunderstorms to become almost stationary and thus yielding locally extreme rain accumulations.

At the Institute of Meteorology and Climate Research (IMK-TRO) and the Center for Disaster Management and Risk Reduction Technology (CEDIM) at Karlsruhe Institute of Technology, scientists analyzed the severe weather episode from the meteorological point of view (Piper et al., 2016). One objective of their study was to assess the persistence of thunderstorm-favoring conditions with respect to a 55-year reference period. They showed that clusters of 10 consecutive days exhibiting extreme precipitation anywhere in Germany have occurred only three times during the reference period (probability p = 0.3%; extreme precipitation is defined as exceedance of the 99.9% quantile; REGNIE data provided by the German Weather Service DWD). Similar results were obtained regarding compound events with low stability and weak mid-troposphere flow. A 13-day cluster, as observed in May and June 2016, has been found only twice during the 55-year reference period. Furthermore, the scientists developed an indirect, multivariate technique and applied it to the output data of a reanalysis model (CoastDatII provided by Helmholtz-Zentrum Geesthacht). This method allows determining the tendency for thunderstorms to occur for each day since 1960. Applied to the storm episode 2016, it yields a cluster of 11 consecutive convective days. Evaluating the entire reference period statistically results in a probability of less than 1% for such a thunderstorm cluster.

With a view to the role of climate change, the authors point out that the storm episode 2016 did not go along with large-scale moisture transports or positive temperature anomalies. Apparently, the potential relation of temperature and stability, as observed in previous studies (e.g. Mohr and Kunz, 2013), does not apply to this case. Consequently, climate change might have affected the storm episode 2016 in an indirect way only leading to an overall intensification of water cycle.

Piper D, Kunz M, Ehmele F, Mohr M, Mühr B, Kron A und Daniell J E (2016): Exceptional sequence of severe thunderstorms and related flash floods in May and June 2016 in Germany. Part I: Meteorological background. Nat. Haz. Earth Syst. Sci., 16, 2835-2850. doi:10.5194/nhess-16-2835-2016
Mühr B, Daniell J, Ehmele F, Kron A, Dittrich A, Kunz M (2016): Hochwasser/Überschwemmung Süddeutschland Mai/Juni 2016. CEDIM FDA Report, 13 pp.
Mohr S und Kunz M (2013): Recent trends and variabilities of convective parameters relevant for hail events in Germany and Europe, Atmos. Res., 123, 211-228. doi: 10.1016/j.atmosres.2012.05.016 http://dx.doi.org/10.1016/j.atmosres.2012.05.016