FWI 2025
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In 2025, the Fire Weather Index (FWI) received its first major update in over 30 years as part of the modernised Canadian Forest Fire Danger Rating System (NG-CFFDRS). The update maintains the overall structure of the original FWI system while introducing significant improvements to address long-standing limitations.
Why it matters
Section titled “Why it matters”The original FWI (Van Wagner, 1987) was designed around once-daily noon observations in Canadian boreal forests. Fire weather conditions can change dramatically within a single day, and the system’s generic fuel model was never calibrated for the range of vegetation types where FWI is now applied worldwide. The 2025 update addresses both issues.
What changed
Section titled “What changed”Hourly weather data
Section titled “Hourly weather data”The original FWI used daily noon observations — a single snapshot per day. FWI 2025 uses hourly weather data, providing much finer temporal resolution. This matters because:
- Fire behaviour changes dramatically within a day — calm mornings can become dangerous afternoons
- Wind events can develop and pass within hours
- The daily minimum relative humidity (usually mid-afternoon) may differ significantly from the noon value
Updated reference fuel model
Section titled “Updated reference fuel model”The classic FWI used a generic fuel model. FWI 2025 introduces a mature jack pine forest as the explicit reference fuel type, better aligning the index with specific fire behaviour characteristics and making the physical basis more transparent (Wotton et al., 2025).
Grassland extension
Section titled “Grassland extension”FWI 2025 adds optional indices specifically calibrated for grassland areas — a fuel type poorly represented in the original system. Work is underway to extend coverage to other fuel types including peatlands.
Simplified fuel moisture calculations
Section titled “Simplified fuel moisture calculations”The 2025 update includes improved methods for calculating live and dead fuel moisture, drawing on decades of research since the original 1987 formulation (Jolly et al., 2024).
What stayed the same
Section titled “What stayed the same”The six-component structure remains unchanged:
The interpretation logic and danger class thresholds are designed to be compatible with the original system, so existing assessment procedures can continue with minimal adjustment during the transition.
How to read it in Wildflyer
Section titled “How to read it in Wildflyer”Wildflyer currently computes FWI using the classic (1987) formulation. As the FWI 2025 specification is finalised and validated by European agencies, we plan to offer it as an alternative computation mode alongside the original.
Sources
Section titled “Sources”- Natural Resources Canada. (2025). Canadian Forest Fire Weather Index (FWI) System — Next Generation. Canadian Forest Service.
- Van Wagner, C.E. (1987). Development and structure of the Canadian Forest Fire Weather Index System. Forestry Technical Report 35, Canadian Forest Service.
- Jolly, W.M. et al. (2024). Modernizing the US National Fire Danger Rating System (version 4): Simplified fuel models and improved live and dead fuel moisture calculations. Environmental Modelling & Software.