Why some wildfires become megafires: compound short-term fire weather and antecedent drought controls in Mediterranean Europe.

Extreme wildfires and megafires in Mediterranean Europe generate disproportionate ecological, social, and economic impacts, yet the processes that govern transitions from large fires to the most extreme events remain insufficiently constrained. In particular, it is unclear whether the emergence of megafires is primarily controlled by short-term atmospheric fire-weather anomalies, antecedent drought-driven fuel preconditioning, or their compound interaction. Clarifying these mechanisms is critical for improving impact-oriented wildfire risk assessment and early warning.

Here, we analyse 11,403 summer wildfires (≥30 ha) that occurred across Mediterranean Europe between 2008 and 2022, including 44 megafires (≥10,000 ha). Fires are classified into four size categories (30–100 ha, 100–1,000 ha, 1,000–10,000 ha, and ≥10,000 ha) to explicitly examine transitions across wildfire size classes. Official fire perimeters from EFFIS are combined with the MESOGEOS environmental–fire datacube (daily, 1 km), integrating meteorological variables, drought indicators, and land-surface conditions.

Fast-reacting atmospheric drivers (air and land-surface temperature, relative humidity, precipitation, and wind speed) are characterized over a ±1-day window around the reported ignition date and aggregated as a 3-day mean to account for start-date uncertainty. Slow-reacting environmental controls are represented using multi-month antecedent drought indicators, including the Standardized Precipitation–Evapotranspiration Index (SPEI), capturing longer-term fuel moisture and stress conditions.

Across increasing fire-size classes, we observe a systematic intensification of hot, dry, and windy conditions near ignition, alongside progressively drier antecedent conditions. Drought indicators show marked stepwise deterioration from medium to very large fires, supporting a strong role of fuel preconditioning driven by prolonged moisture deficits. However, the transition from very large fires to megafires is distinguished less by further increases in drought severity and more by exceptional short-term fire-weather anomalies, particularly strong winds and anomalously high night-time land-surface temperatures.

Using Random Forest classification models with permutation-based feature importance and repeated cross-validation to address class imbalance, we identify a compact and interpretable set of predictors that consistently discriminate transitions toward extreme fire sizes. Night-time land-surface temperature and wind speed emerge as dominant drivers of megafire occurrence, while multi-month drought indicators play a secondary role at the uppermost tail. Complementary logistic regression analyses confirm coherent directions of effect and demonstrate meaningful predictive skill for rare extreme events.

Overall, our results support a compound but non-uniform mechanism: antecedent drought and fuel stress set the stage for very large fires, whereas megafires arise when this preconditioning coincides with extreme short-term fire-weather conditions, particularly persistent nocturnal heat and strong winds. These findings provide actionable insights for extreme-event-focused wildfire early warning and highlight the need to jointly address fuel management and short-term atmospheric extremes under a warming Mediterranean climate.

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