Storm Boris triggers havoc across Europe
Storm Boris has brought widespread catastrophic flooding to central and eastern Europe, with almost two million people affected across eight countries (blue News, 2024). At least 24 people are known to have died over the course of the event (BBC, 2024a), and dozens of communities were cut off from the outside world by flood waters, landslides, and power outages.
Overview and Timeline
On 12 September 2024, Storm Boris hit Austria, Hungary, Romania, the Czech Republic, Poland and Slovakia, bringing extreme winds and fierce rains. States of emergency were declared in several areas including Bratislava, Slovakia (Guardian, 2024a) and the counties of Galati and Vaslui in Romania (Net Weather, 2024). Four deaths were reported in Romania (later updated to seven), whilst another person died near to the Polish-Czech border, which was closed at the height of the flooding (Le Monde, 2024). In addition, at least three people died in Austria, including a firefighter who perished when trying to tackle the flooding (Guardian, 2024a). Other impacts of the event include the following.
Austria
Lower Austria state bore the brunt of the devastation, with 12 dams being destroyed and 26 communities still cut off after four days of rain (Guardian, 2024b). Meanwhile, there was major disruption across the transport network with rail services suspended in the east of the country and Vienna’s metro system shut down (Guardian, 2024a; Le Monde, 2024).
Hungary
In Budapest, the mayor offered residents a million sandbags to protect against flooding (BBC, 2024b) as the waters of the Danube reached the steps of parliament, whilst in the nearby town of Szentendre, residents resorted to moving around using canoes (Euractiv, 2024).
Romania
More than 5,000 homes were severely damaged by floodwaters in the counties of Galati and Vaslui, resulting in hundreds of people needing to be hosted in temporary shelters (Romania Insider, 2024). Nearly 300 people had to be rescued from their homes, with extra emergency services being brought in to support the affected areas. 25,000 homes were also left without power in areas not directly impacted by the flooding.
Czech Republic
Industrial sites which had to shut down due to floods included BorsodChem chemical plant in Ostrava and Veolia Energie in Trebovice (DW, 2024). Fifteen thousand people were evacuated, with the Czech-Polish border being particularly badly affected (BBC, 2024d), and several people were still missing days after the worst of the event (DW, 2024).
Poland
Government estimates suggest that flooding caused damage and destruction to more than 18,000 properties across Poland (Euractiv, 2024). Forty thousand residents were evacuated from the town of Nysa (BBC, 2024d) and a further 1,600 in Klodzko (Guardian, 2024a). In Glucholazy, the main bridge collapsed, and the streets were left covered in mud (BBC, 2024d).
Italy
A week after the slow-moving storm began its progress across Europe, it struck the Emilia Romagna region in Italy, which was still recovering from the 2023 floods. Here, around 1,000 people were evacuated from their homes and firefighters carried out more than 500 rescue operations, as heavy rain triggered devastating flooding and landslides (Guardian, 2024c).
Meteorological characteristics
As with Storm Orinoco, which brought extreme rainfall to Germany earlier this year, the heavy rainfall during Storm Boris was caused by a Vb depression (World Weather Attribution, 2024). These form when cold Arctic air flows southwards and clashes with warm air flowing northwards from the Mediterranean. This time, the temperature contrast between the two air masses was enhanced by record-breaking sea surface temperatures in the Mediterranean, which fuelled the extraordinary rainfall totals (BBC, 2024a). Storm Boris was also particularly slow moving because the depression was anchored between two areas of high pressure (BBC, 2024a).
Vb depressions are rare with only around 5% of Central European cyclones being associated with this system (Hofstätter and Blöschl, 2019). However, despite this being the second weather system of this kind to affect the region in the space of a few months, there have been no significant changes in the number of Vb depressions since the 1950s (World Weather Attribution, 2024).
Rainfall and river gauge information (return period analysis)
Extraordinary rainfall totals were recorded in numerous locations. Lower Austria was affected particularly badly, with some towns receiving upwards of 300mm of rain (two to four times the average totals for September) in less than four days (Geosphere Austria, 2024). In the county of Galati, eastern Romania, three months’ worth of rainfall fell in just 12 hours (Romania Insider, 2024), whilst in Jesenik in the north-east Czech Republic, more than 470mm of rain (five times the monthly average) fell inside five days (BBC, 2024b). Falconara, Italy, also saw more than 200mm rainfall over the course of a couple of days, compared to a September average of less than 70mm (BBC, 2024a).
JBA conducted Extreme Value Analysis on historic daily rainfall climate data timeseries from NOAA (2024). The 43-year time series was fitted with a generalised Pareto distribution to produce an exceedance probability curve (Figure 3). The curve is shown by the solid orange line and the dashed lines represent the 95th percentiles, with observed rainfall values for each location indicated by the blue line. Kłodzko in Poland experienced a peak daily rainfall of 53mm on 13 September, which suggests a 10-year rainfall event. Falconara in Italy received 94mm of rainfall on 18 September which corresponds to a return period of 35 years.
We have also conducted Extreme Value Analysis on historic streamflow data to estimate the river flood return period of Storm Boris (Figure 4). The exceedance probability of streamflow on the Danube river was calculated using daily mean watercourse discharge data from the Global Runoff Data Centre (GRDC, 2024) and DanubeHIS (2024). A streamflow of 9,514 cubic meters per second in Vienna, Austria, on 15 September corresponds to a return period of 140 years. Further downstream on the Danube at Budapest, streamflow of 8,202 cubic meters per second on 21 September corresponds to a return period of 20 years.
Notable historical events
Storm Boris has had devastating impacts, drawing comparisons to the catastrophic European floods of 2002. As heavy rainfall from Boris persisted from September 12 to 16, 2024, it inundated regions in Poland, Romania, Slovakia, Austria, Czech Republic, Italy, and Germany, leading to widespread destruction and prompting mass evacuations (Climate Centre, 2024). Despite the higher intensity and larger scale, the number of fatalities is lower than in previous events because the emergency management systems have been reinforced and largely worked well (World Weather Attribution, 2024). Nevertheless, the flooding looks set to be the worst in the region since the 2002 Europe flooding, when parts of large cities like Dresden, Vienna and Prague were submerged (Climate Centre, 2024).
Climate change
As our planet warms, the frequency and intensity of extreme weather events is changing. In Central Europe, heavy four-day rainfall events like Storm Boris have been calculated to be twice as likely and 20% more intense than they were in the pre-industrial era (World Weather Attribution, 2024). According to that analysis, almost all the increase in likelihood and around a third of the increase in intensity is attributable to human causes.
Looking ahead, if we reach 2°C of global warming, climate models predict a further 5% increase in intensity and 50% increase in likelihood compared to the present day (World Weather Attribution, 2024).
This highlights the need for forward looking flood risk information such as JBA’s Global Climate Change Flood Data and Global Climate Change Flood Model, which can help users to assess the change in flood risk at any location worldwide for a range of possible future climates.
This report is accompanied by a flood footprint covering affected areas in seven countries - detailing extents and depths of the flooding. Download it via our Client Portal or request a copy by emailing eventresponse@jbarisk.com.
