Source: Geophysical Research Letters
“Dry” lightning is an important wildfire ignition source, accounting for about 70% of wildfire burned area in the western United States. Dry lightning is commonly defined as occurring with less than 2.5 millimeters of daily precipitation, but a rigorous quantification of the precipitation threshold associated with lightning-ignited wildfires (LIWs) is lacking.
Combining wildfire, lightning, and precipitation datasets, Kalashnikov et al.  analyzed the environmental conditions associated with 3,726 LIW events during 2015-2020, and found that ignition precipitation amounts range from 1.7 to 7.7 millimeters in the western U.S. LIWs with low amount of precipitation are typically immediately detectable, while wetter conditions tend to cause “holdover” fires that take days to detect. Most of the holdover events occur in forested areas where the dense organic layer on the forest floor and canopy interception of water can sustain ignition and smoldering of wildfires despite a considerable amount of precipitation.
The commonly used 2.5 millimeter as ignition precipitation threshold underestimates wildfire risks. Accounting for the differing ignition precipitation especially for holdover fires can advance the prediction, suppression, and management of wildfires across the western U.S. and possibly in other regions as well.
Citation: Kalashnikov, D. A., Abatzoglou, J. T., Loikith, P. C., Nauslar, N. J., Bekris, Y., & Singh, D. (2023). Lightning-ignited wildfires in the western United States: Ignition precipitation and associated environmental conditions. Geophysical Research Letters, 50, e2023GL103785. https://doi.org/10.1029/2023GL103785
— Guiling Wang, Editor, Geophysical Research Letters