A 20-Year Climatology of Critical Fire Weather in the Northern Rockies

This presentation is based on a study that constructed a 20-year (2005–2025) climatology of critical fire weather across the NWS Missoula Fire Service Area, establishing quantitative baselines to differentiate standard Red Flag Warning (RFW) events from extreme events. We obtained and normalized historical observations into hourly distributions to identify historical RFW events. The intersection of extreme weather (95th percentile of RFW days) and critically dry wildland fuels (ERC-G ≥ 97th percentile) was used to isolate historical extremes, establishing an objective foundation to evaluate candidates for future Particularly Dangerous Situation (PDS) RFWs.

Results demonstrate that standard RFWs and PDS events cannot be statistically differentiated by relative humidity alone. Vapor Pressure Deficit (VPD) exhibits a profound, highly significant divergence. Furthermore, evaluating exclusively within the PDS event dataset revealed two distinct boundary-layer archetypes. VPD-Driven PDS events—associated with ridge breakdowns and immense atmospheric moisture demand—featured significantly higher temperatures. Conversely, PDS events driven by relative humidity were characterized by dry cold frontal passages, plunging dew points, and sudden, wind surges. We developed a framework that provides meteorologists and wildland fire stakeholders with a definitive pattern-recognition toolbox to confidently anticipate and communicate the highest echelons of fire weather threats.