An Hourly Wildfire Potential Index for Predicting Short-Range Fire Activity Based on Rapidly-Updating Convection-Allowing Model Forecasts

Many fire weather index products have been developed to assist land managers, weather forecasters, and firefighters with anticipating weather conditions that may impact existing or potential new wildland fires in coming days. Most of these indices are designed to provide a single value for an entire 24h period. Extreme fire activity in the western US in recent years, including the impact of mesoscale and microscale phenomena such as thunderstorm gust frontal passages, radiative shading by dense smoke plumes, and pyrocumulonimbus development and collapse, as well as the advent of operational convection-allowing model forecasts, has highlighted the need for a more frequently updated index. In this study, we present an hourly wildfire potential (HWP) index developed specifically for application within a rapidly-updating convection-allowing model (CAM). The HWP is derived based on comparison of forecasts from the High-Resolution Rapid Refresh (HRRR) and fire activity as observed by polar-orbiting satellites during a number of major western US wildfires from 2018-2021. The HWP includes variables related to hot / dry / windy conditions (a 10-m wind gust potential diagnostic, and surface dewpoint depression), as well as a soil moisture availability term from the HRRR’s land surface model. The HWP is evaluated against observations of fire radiative power from geostationary satellites, and is compared with existing indices and other baseline forecasts. The HWP generally outperforms existing frequently used indices from the perspective of its ability to highlight fire weather conditions and rapidly-changing fire weather, particularly for overnight fire activity. The HWP can help forecasters anticipate dangerous fire weather conditions, and also paves the way for improved prediction of wildfire smoke emissions for use in the coming hours and days.