Available research has extensively examined the spatiotemporal patterns of fire‐weather regime in Portugal, but a comprehensive climatology of extreme wildfires is still under development. This study calls for different strategies and scales of analysis aiming to describe the relationships between medium and low troposphere weather conditions and severe fire behaviour in mainland Portugal, between 1980 and 2018. In particular, critical fire‐weather patterns and thresholds that can contribute to operational and forecasting know‐how in short and medium time ranges are presented. We updated the general trends in the fire regime with a new, longer daily burned area series and developed a method that identifies Extreme Wildfire Periods (EWP) that form the basis for climate analysis. Synoptic analysis using Circulation Weather Types (CWT) showed that the northeasterly and easterly directional flows are significantly associated with EWP and produce the most severe fire‐weather conditions. The four main CWT related to extreme fire are driven from anticyclones over the eastern Atlantic between the Azores and the British Isles. However, severe situations can also be regulated by CWT with marginal presence in both summer and EWP: low systems located to the west and northwest of Iberia carrying air masses from the south quadrant are related to catastrophic events. Regarding the antecedent climate, the results indicate that the coincident meteorological drought, whether weak or intense, is a necessary but not sufficient condition for the development of an EWP. An increasing relevance of water stress for shorter intervals preceding EWP, in the order of days and weeks, is apparent. Following these results, fine dead fuel moisture thresholds related to transitions in fire behaviour in Portuguese landscapes are computed using a promising predictive moisture content model. Finally, the different methods used are summoned for the detailed analysis of an EWP starting under unusual synoptic circulation.

A conceptual model based on the dynamic interaction between fire, the fuel bed and the surrounding flow to explain the non-monotonic or intermittent behaviour of fires is proposed. According to the model, even in nominally permanent and uniform boundary conditions, the fire-induced flow modifies the geometry of the flame and its rate of spread. After an initial acceleration, there is a reduction in the rate of spread followed by one or more cycles of growth. Carefully controlled experiments of fires in slopes and canyons show that the evolution of fire properties, namely flame angle and rate of spread, have high-frequency oscillations superimposed on the low-frequency fire growth cycle described above.

Based on analysis of the interaction between a spreading fire and its surrounding environment, in nominally constant and uniform boundary conditions, it is observed that the evolution of the fire front is characterised by fluctuations of its properties, including (in particular) its rate of spread (ROS). Using a database with a wide range of fires with different time–space scales, it is shown that the amplitude of the fluctuation in ROS is proportional to the average value of the ROS and that the frequency of oscillation varies with the type of fire, and for a given fuel, increases with the average ROS. In fast-spreading fires, the large amplitude of ROS increase and sudden decrease promote the intermittent behaviour of the fire. In general, the amplitude and period required for the ROS increase are larger than for its decrease. However, the acceleration and deceleration phases in junction fires do not follow this rule, suggesting the existence of different convective processes of interaction between the flow and fire. This oscillation explains the variability in many fires at all scales and challenges the current interpretation based on the three factors affecting fire spread and the classification of wind or topography-driven fires.