Challenges in assessing Fire Weather changes in a warming climate

The Canadian Fire Weather Index (FWI) is widely used to assess wildfire danger and relies on meteorological data at local noon. However, climate models often provide only daily aggregated data, which poses a challenge for accurate FWI calculations in climate change studies. Here, we examine how using daily approximations for FWI95d -- the annual count of extreme fire weather days exceeding the 95th percentile of local daily FWI values -- compares to the standard noon-based approach for the period 1980--2023. Our findings reveal that FWI95d calculated with noon-specific data increased globally by approximately 65\%, corresponding to 11.66 additional extreme fire weather days over 44 years. In contrast, daily approximations tend to overestimate these trends by 5--10\%, with combinations involving minimum relative humidity showing the largest divergences. Globally, up to 15 million km$^2$, particularly in the western United States, southern Africa, and parts of Asia, exhibit significant overestimations. Among our daily approximation methods, the least biased proxy is the one that uses daily mean data for all variables. We recommend (i) prioritizing the inclusion of sub-daily meteorological data in future climate model intercomparison projects to enhance FWI accuracy, and (ii) adopting daily mean approximations as the least-biased alternative if noon-specific data are unavailable.