Matthieu Lacroix

Tornadoes and severe convective storms in Europe pose a significant yet historically under-recognised risk. Synoptic forecasting successfully captures severe convective environments at short lead times, but sub-seasonal to seasonal (S2S) predictability still remains limited. This study investigates the S2S modulation of European tornadic environments by stratospheric polar vortex disruptions, specifically major Sudden Stratospheric Warming (SSW) events. Departing from tornado environment indices (TEIs) elaborated in North America where convective and kinematic terms are multiplied, we calibrate an additive TEI suited to low-CAPE (convective available potential energy) European conditions using a Poisson Generalised Linear Model trained on European Severe Weather Database (ESWD) reports and ERA5 reanalysis. Applying this novel index to a 75-year climatology of cold-season tornadoes (1950–2025), we evaluate the lagged surface response to 50 major SSW events. We demonstrate that the downward-propagating Northern Annular Mode (NAM) signal does not yield a uniform response across the continent. Instead, the tropospheric impact is conditioned by the low-frequency East Atlantic (EA) pattern. During an EA-positive regime, SSW-induced equatorward jet displacement, coupled with warm, moist advection, doubles the baseline probability of extreme tornadic environments (up to + 15 percentage points) across southern Europe and around the Mediterranean Basin. In contrast, EA-negative regimes suppress instability across the entire continent as a result of atmospheric blocking. A combination of stratospheric monitoring with EA phase tracking thus provides a robust framework for anticipating high-impact European severe weather windows three to four weeks in advance.

High-intensity tornadoes in western Europe occurring under weakly unstable atmospheric conditions represent a disproportionate share of recorded events, but their dynamics remain poorly understood and their numerical simulation has rarely been attempted at convection-permitting resolution. This study presents the first high-resolution numerical reproduction of a violent tornado using the Meso-NH atmospheric model at 500 m resolution. It simulates the EF3 tornado of 23 October 2022 at Bihucourt, France – a long-track event (206 km) produced by a discrete supercell occurring in a high-shear, low-CAPE (HSLC) environment. Forced by the ECMWF Integrated Forecasting System (IFS, ~9 km), the simulation successfully reproduced the tornadic vortex signature with peak 10-m wind speeds of 77.4 m·s⁻¹, maximum low-level vorticity of 0.1325 s⁻¹, and a simulated vortex centroid offset by only 9.6 km (median) from the observed damage track. This output constitutes the first documented numerical reproduction of a tornadic signature for an HSLC-type tornado in Europe. Companion WRF-ARW simulations at 333 m resolution were conducted in an independent replication attempt but consistently failed to reproduce a tornadic vortex signature across all tested reanalysis forcing environments (ERA5, GDAS/FNL, CERRA) and microphysics schemes, despite the similar or finer resolution of CERRA compared to the IFS forcing used in Meso-NH. This systematic failure, discussed alongside the possible role of boundary-layer turbulence treatment, underscores the high performance of the Meso-NH result and highlights open questions regarding the relative roles of forcing resolution, model structure, and turbulence closure in constraining the predictability of this class of European tornadic event.

Whereas convective available potential energy (CAPE) is a well-established factor in supercell development in the United States, its role in European tornadogenesis remains less clear. This study examines significant tornadoes (≥EF2) in continental western Europe (1940–2022), revealing that the majority of autumn and winter events occur under high-shear, low-CAPE (HSLC) conditions, with most unstable CAPE ≤ 500 J/kg. Using ERA5 reanalysis data, principal component analysis, and k-means clustering, the highly variable atmospheric states conducive to tornadogenesis over the study area were successfully reduced to three statistically robust synoptic regimes, respectively labelled as the Deep Atlantic Low, Iberian Cut-off Low, and Northeast Atlantic Trough scenarios. Across all patterns, thermodynamic instability is profoundly deficient (medians are frequently less than 100–250 J/kg), thus with high chances of destructive entrainment impeding updraft survival. We demonstrate that tornadogenesis in these adverse environments relies on critical compensatory mechanisms: extreme near-ground streamwise vorticity (0–1 km-level storm-relative helicity often > 170 m2/s2), deep synoptic lift generated by upper-level jet streaks, and exceptionally low cloud bases (lifted condensation level heights < 400 m) that prevent updraft undercutting by mitigating cold pool development. A high-resolution sensitivity analysis using the 5.5 km CERRA dataset confirms that this marginal instability is a physical reality of the European cool season, not an ERA5-related grid-smoothing artifact. These findings suggest that operational forecasting would gain from pivoting away from buoyancy-centric paradigms towards kinematic diagnostics to effectively anticipate dynamically driven tornado events in Europe.

The climatology of severe convective storms is well documented in the United States, but the large-scale atmospheric drivers of European tornadoes remain less well understood. This study investigates the interannual variability and environmental forcing of tornadoes across the Euro-Atlantic domain using observational reports and ERA5 reanalysis over the 1950–2022 period. Statistical analysis reveals a moderate multidecadal increase in tornado frequency. Sensitivity tests restricting data to significant events (F2 and above on the Fujita scale) confirm this trend is not merely an observational artifact: it displays robust links with the positive phase of the Atlantic Multidecadal Oscillation (AMO), which provides the essential thermodynamic backdrop for deep convection. A sharp seasonal dichotomy is additionally identified in synoptic forcing controlled by the North Atlantic Oscillation (NAO). Winter events prevail during positive NAO phases, with strong zonal flow driving a high-shear, low-CAPE (convective available potential energy) kinematic regime. Summer events, conversely, are tied to negative NAO phases, where atmospheric blocking fosters a low-shear, high-CAPE thermodynamic regime. Spring exhibits a distinct continental susceptibility to NAO-related blocking. Previously assumed Mediterranean multidecadal teleconnection (WeMO) were also examined, but are shown to be spurious artifacts linked to tornado reporting trends. These findings demonstrate that European tornadogenesis hinges on seasonally dependent atmospheric environments modulated by low-frequency, AMO-related climate variability with NAO-driven overprints. Recognising these distinct teleconnection signatures provides a potentially fruitful framework for anticipating interannual to decadal fluctuations in European severe weather risk.

The study discusses in an exploratory way the atmospheric conditions leading to several fires in the Pantanal on November 12, 2023. These episodes were marked by two periods of maximum fire expansion, first in the early afternoon and another in the evening. The study is based on a set of observations from satellites and weather stations, which helped to identify these fires and some meteorological conditions at the surface. In addition, the Fire Weather Index (FWI) in the Pantanal was analyzed for a 44-year period. However, this dataset was not enough to completely explain the fire behavior on that day. In this context, atmospheric modeling was applied to find the possible causes of fire behavior in these two periods. The Meso-NH model was run with two nested domains of horizontal resolutions of 2500 m and 500 m. The results showed a positive trend of FWI in the last decades and a clear seasonality for the maximum values in 2023. Also, the simulation indicated favorable weather conditions for fire ignition, given the high temperatures and low relative humidity. However, the wind gust field showed moderate gusty winds in both periods but caused by different forces. In the early afternoon, the larger scale circulation favored fire propagation, whereas in the evening fire spread by a gust front. Furthermore, the findings highlight the role of weather conditions on a sub-daily scale, with sudden changes in surface winds throughout the day, which is strongly recommended to be considered when examining fire danger and firefighting action plans in the region.

O estudo discute de forma exploratória as condições atmosféricas favoráveis à evolução dos incêndios florestais no Pantanal em 12 de novembro de 2023. Esses episódios foram marcados por dois períodos de rápida expansão do fogo, primeiro no início da tarde e outro à noite. O estudo usa um conjunto de observações de satélite e estações meteorológicas, as quais ajudaram a identificar o fogo e algumas condições meteorológicas na superfície. Além disso, o Fire Weather Index (FWI) no Pantanal foi analisado para um período de 44 anos. No entanto, esse conjunto de dados não foi suficiente para explicar completamente o comportamento do fogo naquele dia. Nesse contexto, a modelagem atmosférica foi aplicada para encontrar as possíveis causas do comportamento do fogo em dois períodos. O modelo Meso-NH foi configurado com dois domínios aninhados e resoluções horizontais de 2500 m e 500 m. Os resultados mostraram uma tendência positiva do FWI nas últimas décadas, bem como uma clara sazonalidade para os valores máximos no ano de 2023. A simulação indicou condições favoráveis à ignição do fogo, e o campo de rajadas de vento mostrou ventos moderados em ambos os períodos, mas causados por diferentes forçantes. No início da tarde, a circulação em grande escala favoreceu a propagação do fogo, enquanto à noite uma frente de rajada foi observada. O estudo destaca o papel das condições meteorológicas na escala sub-diária, em particular para mudanças repentinas do vento à superfície ao longo do dia. Esse resultado deve ser considerado ao examinar o perigo de fogo e o planejamento das ações de combate aos incêndios na região.

Although Convective Available Potential Energy (CAPE) is a well-established factor in supercell development in the United States, its role in European tornadogenesis—particularly under low CAPE conditions—remains poorly documented. This study examines tornado occurrences in western Europe between 1940 and 2022, using a dataset of 1,675 tornadoes recorded in Portugal, Spain, France, Belgium, and the Netherlands, of which 255 were classified as intense (≥EF2). Using ERA5 reanalysis data, the thundeR package, and convective diagnostics, we find that 58% (n = 148) of these intense tornadoes occurred with Most Unstable CAPE (MUCAPE) values ≤ 500 J/kg. The spatial and seasonal distribution of these low-CAPE, high-intensity events highlights a marked autumn and winter signal. Autumn tornadoes were consistently associated with the left-exit region of 300 hPa jet streaks, strong vertical wind shear, and enhanced low-level storm-relative helicity (SRH). While also frequently occurring near jet streak exit regions, winter events showed a less systematic relationship suggesting the contribution of additional factors such as weaker upper-level forcing and colder low-level environments. Five distinct synoptic patterns were identified, modulated in part by the North Atlantic Oscillation (NAO). Positive NAO phases favoured tornado activity in northern France and western Spain during autumn, whereas negative phases were linked to winter tornadoes across the Iberian Peninsula. The overall findings underscore the importance of dynamic forcing—particularly low-level shear and helicity—in driving intense tornadogenesis in low-CAPE environments across western Europe, with clear seasonal and synoptic-scale variability partly driven by jet stream behaviour.

High-intensity tornadoes in western Europe occurring under weakly unstable atmospheric conditions (Most Unstable CAPE ≤ 500 J/kg) are more frequent than commonly assumed, representing 58% of all recorded EF2+ tornadoes between 1940 and 2022. These events challenge conventional paradigms of tornadogenesis, which typically emphasize high buoyancy environments. This study explores the atmospheric mechanisms that enable such intense tornadoes in low-CAPE settings by focusing on a well documented case: the EF4-rated Bihucourt tornado that struck northern France on 03 August October 2008. Using a combination of ERA5 reanalysis data and numerical modeling, we identified the synoptic and mesoscale features conducive to tornadogenesis under limited atmospheric instability. Particular attention was paid to the influence of upper-level jet stream dynamics, which enhance vertical wind shear and helicity, especially within the left-exit region of a jet streak. However, synoptic-scale conditions alone fail to fully account for the tornadic potential observed. A key hypothesis explored in this study was the role of external or sustained forcing mechanisms such as local convergence zones, gravity waves, or terrain-induced circulations, which contribute to maintain or enhance mesocyclonic structure within a convective storm. To test these mechanisms, we employed a dual modelling approach: Cloud Model 1 (CM1), a 3D model for simulating mesoscale convective processes, was used to resolve Hautmont storm-scale and boundary-layer processes at high spatial and temporal resolution; meanwhile, the Weather Research and Forecasting (WRF) model provided insight into the broader mesoscale environment surrounding the storm event. Together, these simulations aim to shed light on how strong dynamic forcing can compensate for weak thermodynamic instability, and under what conditions tornadogenesis is favoured. Ultimately, the findings of this study aim to improve the understanding and forecasting of tornadoes in marginal environments, which remain underrecognized in current European operational frameworks.