On the Meteorological Conditions Influencing Prescribed Burn Windows Across the United States

Prescribed (Rx) burns are intentionally ignited, low intensity fires that consume surface fuels on the forest floor while minimizing mortality of overstory trees. Rx burns function to restore critical ecological processes, reduce future extreme wildfires, and provide safety for human communities. They help in forest restoration by cycling nutrients, preparing sites for seedling establishment, and improving habitat for endangered species. Rx burns aid in reducing future extreme wildfires by reducing hazardous fuels and promoting fire-dependent species. They also dispose of debris from mechanical treatments, removing obstacles and improving accessibility for firefighters during fire suppression efforts.

In order to ensure safe burn practices and limit overstory tree mortality, Rx burns are conducted on days when the meteorological thresholds of maximum daily air temperature, relative humidity, and wind speeds are all met (meteorological burn window). Limited meteorological burn windows are identified as one of the most important constraints for conducting Rx burns in California. Therefore, we investigate the spatiotemporal changes in historical (1980­–2019) meteorological burn windows across California and the subsequent changes in expected future (2021–2060) burn windows across North America. We explore whether burn windows across California can be extended from the typical fall season to include other opportune seasons for facilitating specific management objectives. We determine the seasons and regions most conducive for Rx burns, as well as those exhibiting the greatest decrease in opportune burn window days and identify what meteorological conditions are driving these changes.

We establish burn window days by determining whether three meteorological conditions are met: maximum daily air temperatures between 0ºC–32ºC; relative humidity between 30%-50%; and 10m height wind speed between 2m^-s –10m^-s. For the historical analysis, we use an aggregate of datasets from the California Department of Forestry and Fire Protection, Prescribed Fires Incident Reporting System, and the Monitoring Trends in Burn Severity. We also use gridded interpolated data from gridMET at 4km spatial resolution. For future projections we use an ensemble of the Coupled Model Intercomparison Project Phase 6 (CMIP6) under the shared socioeconomic pathway (SSP3-7.0).

Results from our historical analysis indicate that while fall burns have been most frequently executed (40% of the time), the spring (and to a lesser extent winter) seasons yield efficient Rx burns similar to fall because greater acres are being consumed with fewer burns. In addition, winter and spring seasons experience meteorological burn window opportunities (70-90% of the time) over larger areas than the other seasons, and this is predominantly over forested regions in Northern California. Our results also indicate that meteorological burn windows in the winter and spring are decreasing at a rate of one day per year over a larger spatial area than that of summer and fall. This decrease is primarily driven by changes in the number of days the relative humidity thresholds are met. Preliminary results for future projections indicate that the months of December and February will exhibit the greatest number of burn window days across the United States. The Northwest will see the most frequent burn windows occurring in October, while California’s ideal time to execute burns will be in November and December. Results from this work can help guide policymakers in enacting appropriate mitigation strategies for addressing future wildfire risk.