Lightning-Ignited Wildfires in the Western United States: Ignition Precipitation and Associated Environmental Conditions

Cloud-to-ground lightning with minimal rainfall (“dry” lightning) is a major wildfire ignition source in the western United States (WUS). Although dry lightning is commonly defined as occurring with <2.5 mm of daily-accumulated precipitation, a rigorous quantification of precipitation amounts concurrent with lightning-ignited wildfires (LIWs) is lacking. We combine wildfire, lightning and precipitation data sets to quantify these ignition precipitation amounts across ecoprovinces of the WUS. The median precipitation for all LIWs is 2.8 mm but varies with vegetation and fire characteristics. “Holdover” fires not detected until 2–5 days following ignition occur with significantly higher precipitation (5.1 mm) compared to fires detected promptly after ignition (2.5 mm), and with cooler and wetter environmental conditions. Further, there is substantial variation in precipitation associated with promptly-detected (1.7–4.6 mm) and holdover (3.0–7.7 mm) fires across ecoprovinces. Consequently, the widely-used 2.5 mm threshold does not fully capture lightning ignition risk and incorporating ecoprovince-specific precipitation amounts would better inform WUS wildfire prediction and management. Key Points The widely-used 2.5 mm daily precipitation threshold to define “dry” lightning does not fully capture fire ignition risk across the WUS Ignition precipitation amounts range from 1.7 to 7.7 mm depending on ecoprovince and whether the fire was promptly-detected or a holdover Holdover fires occur when conditions are cooler and wetter compared to promptly-detected fires Plain Language Summary Cloud-to-ground lightning with minimal rainfall, also known as “dry lightning,” is a major wildfire ignition source in the western United States (WUS). Typically, daily-accumulated precipitation of less than 2.5 mm is used to identify dry lightning occurrence. However, there is limited knowledge of (a) the true precipitation amounts that occur with lightning-ignited wildfires (LIWs), and (b) how these amounts vary across different landscapes and vegetation types. We combine wildfire, lightning and precipitation data sets to quantify these ignition precipitation amounts across different regions of the WUS. Although we find a 2.8 mm median ignition precipitation for all LIWs, we show that “holdover” fires not detected until 2–5 days following ignition occur with significantly higher precipitation (5.1 mm) compared to fires detected promptly after ignition (2.5 mm). Holdover fires also occur with cooler and wetter environmental conditions. Further, ignition precipitation amounts associated with promptly-detected and holdover fires vary substantially across ecoprovinces. Consequently, the widely-used 2.5 mm threshold does not fully capture lightning ignition risk. WUS wildfire prediction and management could be improved through incorporating ecoprovince-specific precipitation amounts and accounting for differing characteristics of holdover fires.