Climatology, Variability, and Extremes of the Initial Spread Index Across Alaska’s Boreal and Tundra Regions

Abstract The Initial Spread Index (ISI), a component of the Canadian Forest Fire Danger Rating System, quantifies the expected rate of fire spread and is used operationally across Alaska. Despite its importance, ISI's historical behavior, variability, and environmental sensitivity have not been systematically characterized for Alaska. This study presents the first Alaska-wide climatology, trend analysis, and extremes assessment of ISI using gridded reanalysis data spanning 1991–2024, organized by Predictive Service Areas (PSAs) and fire season sub-seasons. ISI climatology peaks during the wind-driven and duff sub-seasons, with the highest values at low elevations in interior Alaska. Decadal trends show increasing early-season ISI statewide, with the strongest, most statistically significant increases in the Upper Yukon Valley (≥0.5 per decade), consistent with an earlier onset of fire activity. ISI variability has increased across all PSA groups, with a statistically significant rise in the East Boreal, and the frequency of days exceeding the 95th percentile of historical ISI has increased in 17 of 18 PSA regions. Sensitivity analysis showed that wind speed increases lead to the greatest ISI variability (+12% to +60%), while the largest impacts on mean ISI occurred when relative humidity was decreased or increased by 30% (+39% to +77% and −27% to −72%, respectively); high ISI values did not occur when precipitation exceeded 2 mm. Case studies from June 2013 and May 2021 show ISI spikes aligned with surges in acres burned, supporting its value as a diagnostic indicator of rapid fire spread. These findings provide a foundation for improved fire season planning and management across Alaska's boreal and tundra regions.