Precipitation associated with lightning ignited wildfires in Arizona and New Mexico

From 1990 through 1998, over 15,000 naturally ignited wildfires were observed on the United States Department of Agriculture Forest Service and Department of Interior land in Arizona and New Mexico. In spite of this high number, these wildfires only accounted for slightly more than 0.3% of all detected lightning strikes in this region. In this arid region, these ignitions are often attributed to “dry lightning”, or lightning in the absence of abundant precipitation. This implies a significant role of rainfall events in fire suppression either by preventing ignition or extinguishing the fire prior to detection. Can a threshold value of precipitation be identified that is associated with the likely ignition of wildfire? Analysis was performed using daily gridded precipitation data at a 0.25 degree spatial resolution, cloud-to-ground lightning data, and federal, natural wildfire data to determine the amount of precipitation occurring on days with lightning that either (a) ignited an observed fire (fire lightning, FL), or (b) did not coincide with any reported fires (no fire lightning, NFL). Daily precipitation amounts associated with FL cases ranged from 0 to 35 mm (1.4 in) and from 0 to 70 mm (2.8 in) for NFL cases. Precipitation amounts on the day of FL events were predominantly lower than for NFL events. When comparing the daily precipitation amounts for FL and NFL events, a threshold of less than or equal to 1.3 mm (0.05 in) provided the most representative separation between FL and NFL populations. This threshold was relatively consistent when the datasets were grouped by elevation and vegetation type.