Identifying a Thunderstorm's First Lightning Flash From the GOES‐R GLM

The first lightning flash within a developing thunderstorm (“first flash”) is separated spatially and temporally from nearby thunderstorms with previously recorded lightning flashes, and can indicate that deep, moist convection with a robust updraft has initiated. An identification method for initiating convection over large spatial and temporal domains was developed using a thunderstorm's first lightning flash detected by the Geostationary Lightning Mapper (GLM). Over three‐thousand lightning flashes considered as candidate first flashes were manually categorized and used to validate 50 combinations of spatial and temporal identification criteria. The selected criteria were the first lightning flash with a separation of at least 20 km—plus the radius of its flash footprint—from all flashes that occurred over the previous 20 min. This method had a probability of detection of 30%–45% and a false alarm ratio of 5%–15%. First flashes from the GLM were identified over the conterminous United States for all of 2022 to demonstrate the method's ability to characterize the initiation of deep convection. The ability to identify first flashes on increased scales provides a more complete picture of initiating convection, with future research and operational benefits.