Lightning-Ignited Fires in the Northwest United States and SPC Dry Thunderstorm Precipitation Thresholds

Lightning-ignited fires pose a serious threat to life and property in the northwest United States. From 2002-2015, lightning-ignited fires represented over half of the total fires and three-fourths of the area burned. This is due to both the frequency of dry thunderstorms (currently defined by the Storm Prediction Center as at least one cloud-to-ground (CG) lightning flash with less than 0.10” of precipitation) and the abundance of fuels in the region. In this study, each lightning-caused fire listed in the Forest Research Data Archive was paired with the Vaisala National Lightning Detection Network CG lightning flashes that were observed within 5.68 km of the fire’s discovery location. The closest CG lightning flash on the day nearest to the fire discovery date (up to 14 days prior) and location was used as the ignition source. Of the fires that had corresponding lightning flashes identified, 23% occurred between 0.10” and 0.50” of rain and less than 3% had greater than 0.5” of rain based on NWS NCEP Stage IV QPE data.The Energy Release Component Percentiles and 100-hr fuel moistures followed similar distributions for all sizes of fires, suggesting that the fuel receptiveness at ignition matters less for the eventual spread of the fire, than the fuel/meteorological conditions that follow afterwards. While any lightning over dry fuels in the Northwest United States carries a threat for wildfire ignitions, the vast majority (97%) occur with less than 0.50”. To increase the probability of detection of lightning-ignited fires, the authors recommend the precipitation criteria for issuing SPC Dry Thunderstorm Outlooks be increased to 0.50”, with greater emphasis on the availability of dry fuels and consideration of subsequent meteorological conditions.