Flood events across Europe are shaped by a range of atmospheric, geographical and hydrological factors. Rainfall patterns, which play a significant role, differ greatly across the continent and fluctuate with the seasons. This report explores the main differences in seasonal rainfall patterns among the UK and other parts of Europe.
Autumn and Winter
During autumn and winter, flooding in Europe–particularly northwestern Europe–shares similar characteristics with flooding in the UK. Precipitation across Europe during these seasons is significantly influenced by Atlantic low-pressure systems that track from west to east. Typical autumn and winter flooding in the UK and Europe predominantly results from extended periods of rainfall, leading to saturated soils and high rates of runoff. When low-pressure systems cluster (several cyclones occurring in quick succession) or stall (move slowly), the risk of flooding is exacerbated due to extremes in accumulated precipitation that can occur in such events.
In contrast, in southern Europe–especially around the Mediterranean and southern Iberian Peninsula–organised and isolated convective storms play a larger role in producing rainfall extremes and flooding compared with Atlantic low-pressure systems (Da Silva & Haerter, 2023).
Case Studies: Storm Babet (October 2023) and Storm Henk (January 2024)
Occasionally, autumn and winter storms deviate from the typical west-to-east trajectory. For instance, Storm Babet moved from south to north across the UK, drawing in moisture-laden air as it crossed the Bay of Biscay. The storm eventually stalled over the UK, unable to progress eastward due to a high-pressure block over Scandinavia, leading to significant rainfall (Mishra, 2023). This situation exemplifies how a stalled cyclone can result in substantial flooding.
In contrast, Storm Henk, which occurred in January 2024, followed a more conventional path for an Atlantic storm, moving swiftly eastward across southern England. Henk’s arrival was preceded by a period of wet weather in autumn and December 2023. Just 5 days before Henk, Storm Gerrit delivered 30-50 mm of rain to large parts of UK, leaving soils saturated. This pre-existing soil saturation, combined with Henk's heavy rainfall, led to severe surface water flooding, significant property damage, and widespread disruption, highlighting how the clustering of cyclones can amplify flood risks.
Spring and Summer
During spring and summer, the UK sees a significant proportion of flooding events occur due to intense, short-duration convective rainfall that leads to flash floods. In July 2021, a series of heavy thunderstorms brought prolonged downpours to southeast England, flooding roads and underground stations in London (Holley et al., 2022).
Severe convective storms in the UK are most common from summer through early autumn, with the highest proportion of events occurring in the south and east of England due to the highest levels of convective available potential energy (CAPE) (Darwish et al., 2018). Severe convection in the southeast of the UK is generally associated with slack winds and more localised sources of moisture than in autumn/winter events (Allan et al., 2015). Additionally, the presence of cities can generate or intensify convective storms due to the urban heat island effect and increased concentrations of cloud-forming particles.
Short duration convective rainfall events also cause severe flooding across Europe with large regional variation in their seasonal peak activity (Da Silva & Haerter, 2023). However, many of the most severe spring and summer flooding events across Central Europe (return period > 100 years) tend to have a more complex genesis. These events often involve low-pressure systems that typically emerge in the western Mediterranean and track northeast into central Europe, known as Vb cyclones. The track of Vb cyclones facilitates the advection of moist air from various sources, including the Mediterranean, Eastern Europe and the Baltic Sea, toward Central Europe. Moisture transport for orographic precipitation plays a critical role in driving extreme floods in these regions (Hundecha et al., 2020).
Case Studies: Bernd (July 2021), Emilia-Romagna (May 2023) and Boris (September 2024)
In July 2021, the extreme flood event that occurred across parts of Belgium, Germany and neighbouring countries was one of the most severe natural catastrophes of the last half-century. The event resulted from a low-pressure system that stalled over western Europe, blocked by an area of high pressure across northeastern Europe. A northeasterly airflow on the northern flank of the low brought warm, moisture-laden air from the Baltic Sea, which was experiencing warm sea-surface-temperature (SST) anomalies of up to 8°C (Mohr et al., 2023). The increased evaporation served as a major moisture source, and the alignment of the extreme rainfall area near the low’s movement resulted in extreme rainfall totals. 4 key factors contributed to the extremity of the flooding:
- Above-average SSTs in the Baltic Sea.
- Orientation of the extreme rainfall.
- Anomalously wet conditions through June and July in the region.
- Orographic enhancement from the local hills.
In May 2023, extreme rainfall occurred in northern Italy, leading to severe flooding that caused significant disruption and damage in the region. A vast area of low pressure across the central Mediterranean basin channeled moisture-laden air from North Africa and the Mediterranean towards the Italian peninsula. The moisture encountered cooler air from the Apennines mountains, resulting in heavy rainfall across the Emilia-Romagna and Marche regions (Arrighi & Domeneghetti, 2024). Although atmospheric disturbances often cross the Italian peninsula during springtime, this event was unique due to its unusually slow northward track, North African origin and precipitation intensity.
More recently, a severe flooding event impacted central Europe in mid-September 2024. This event was characterised by a slow-moving circulation pattern, which led to a prolonged period of heavy rainfall. Large waves in the jet stream resulted in cold air plunging southward across Europe, where it clashed with warm, moist air from the Mediterranean and Black Sea. High-pressure systems over western and eastern Europe blocked a surface low-pressure system from moving away from central Europe. Heavy precipitation developed along the warm/cold boundary, enhanced by mountainous terrain and fueled by warm sea-surface temperatures.
Climate Change
Climate change is anticipated to increase the frequency and intensity of extreme rainfall events, leading to a greater risk of flash flooding. The occurrence and impact of these events are influenced by factors such as atmospheric circulation patterns, antecedent conditions and local terrain. Other elements, including urbanization, exposure growth, changes in land use, flood defense improvements and evolving building practices play significant roles in shaping present and future flood losses.
As the atmosphere warms by 1°C, its capacity to hold moisture increases by about 6-7%, which directly affects rainfall patterns across Europe. Climate model and observational analyses indicate that regions north of the Mediterranean basin are becoming wetter, while the southernmost areas of Europe are becoming increasingly drier.
Regarding extremes, there is a growing risk of exceptionally wet seasons across Europe, particularly in regions away from the Mediterranean. Additionally, slow-moving extreme precipitation events are projected to occur 31-65% more frequently between August and November by 2100 (Kahraman et al., 2021). This trend suggests that storms like Babet (2023) and Boris (2024) could become a more prevalent pattern of European flooding in the future, leading to more frequent and severe flood events.
Conclusion
Flooding mechanisms in the UK and Europe exhibit similarities and differences shaped by distinct seasonal and geographical factors. In the UK, extreme flood events are more commonly triggered by prolonged rainfall from clustered or stalling low-pressure systems during autumn and winter. These systems lead to saturated soils and high runoff rates, increasing the likelihood of flooding, whereas Central Europe frequently experiences extreme flooding in the summer, driven largely by slow-moving cyclones that tap into widespread moisture sources across the continent, which are often influenced by the Mediterranean and other regional bodies of water.
While both regions face significant flood risks, the underlying drivers differ due to local atmospheric conditions and terrain. As climate change continues to intensify rainfall events, understanding these mechanisms is essential to developing effective flood prevention and management strategies across the continent.
Questions? Please contact the authors Samuel Bray and Edward Wilson.
References
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Kahraman, A. et al. (2021) “Quasi‐stationary intense rainstorms spread across Europe under climate change,” Geophysical Research Letters, 48(13). doi:10.1029/2020gl092361.
Mishra, S. (2023) “What is fueling the “extraordinary” Storm Babet?” The Independent, 20 October.
Mohr, S. et al. (2023) “A multi-disciplinary analysis of the exceptional flood event of July 2021 in Central Europe – part 1: Event description and analysis,” Natural Hazards and Earth System Sciences, 23(2), pp. 525–551. doi:10.5194/nhess-23-525-2023.