Citation: Silva, P.; Carmo, M.; Rio, J.; Novo, I. Changes in the Seasonality of Fire Activity and Fire Weather in Portugal: Is the Wildfire Season Really Longer?. Meteorology 2023, 2, 74–86. https://doi.org/10.3390/ meteorology2010006 Academic Editors: Andreas Matzarakis and Paul D. Williams Received: 18 November 2022 Revised: 30 January 2023 Accepted: 31 January 2023 Published: 2 February 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Changes in the Seasonality of Fire Activity and Fire Weather in Portugal: Is the Wildfire Season Really Longer? Pedro Silva 1 , Miguel Carmo 2, * , João Rio 1 and Ilda Novo 1 1 Departamento de Meteorologia e Geofísica, Instituto Português do Mar e da Atmosfera (IPMA), Rua C do Aeroporto, 1749-077 Lisbon, Portugal 2 Institute of Contemporary History, School of Social Sciences and Humanities, NOVA University Lisbon (IHC-NOVA FCSH), Campus de Campolide, 1749-077 Lisbon, Portugal * Correspondence: miguelcarmo@fcsh.unl.pt Abstract: The length of the fire season has not garnered much attention within the broad field of meteorological research on fire regime change. Fire weather research on the Iberian Peninsula is no exception in this case; there is no solid understanding on fire season lengthening in Portugal, although recent decades do suggest ongoing transitions. Based on a complete record of fire occurrence and burned area between 1980 and 2018, we first searched for consistent trends in the monthly distribution of fire activity. To determine day-scale changes, an exceedance date method based on annual cumulative burned area was developed. Results show an early onset of fire activity in a range of 23–50 days and no significant extension into autumn, suggesting that existing projections of the lengthening of the fire season in Portugal over the present century have been already achieved. Fire weather results show a trend in the cumulative Daily Severity Rating (DSR), with the last two decades (2000–2018) displaying an early build-up of meteorological fire danger in late spring and early summer. The detailed spatio-temporal analysis based on the daily Fire Weather Index (FWI) shows that June stands out with the largest increase (year-round) in days per month with an FWI above 38.3, the threshold above which fire conditions make suppression uncertain. This aggravated fire weather is likely sustaining early fire activity, thus contributing to a longer critical fire season. Keywords: wildfire seasonality; extended burning season; pre-season fire weather; daily severity rating; climate change; western Iberia 1. Introduction The inter-annual variability in the incidence of wildfires can be observed in the uneven distribution of fire occurrence and burned area, which are largely derived from the annual cycles of temperature and precipitation and the corresponding seasonal dynamics of the ecosystems (e.g., [1–3]). This close relationship between the seasonal patterns of fire and weather has long been studied and parameterized, being reflected in meteorological indices of fire danger, as is well exemplified by the Canadian Forest Fire Weather Index (FWI) system and its long- to short-memory moisture sub-indices [4,5]. Regarding the Iberian Peninsula, where Portugal is located, different burned area models largely reproduce (in the order of 2/3 or more) annual and monthly variations in burned area based solely on meteorological variables [2,6–8]. This research field has received increasing attention over the last decade or two in the context of noticeable changes in both climate and fire regimes (e.g., [9–14]). Recent work has discussed emerging interactions between specific weather conditions and aggravated fire behavior that had not been identified before in fire weather studies [15–18]. In this context, the increasing length of the fire season can be seen as a seasonal-scale outcome of a changing climate, as discussed early by Wotton and Flannigan [19]. Indeed, the lengthening of the fire season observed in different regions of the world can be studied Meteorology 2023, 2, 74–86. https://doi.org/10.3390/meteorology2010006 https://www.mdpi.com/journal/meteorology