Analyzing Geostationary Lightning Mapper (GLM) Gridded Product Values throughout the Tornado and Extremely Large Hailstone Lifecycles

Forecasters across the Western Hemisphere routinely use lightning observations to augment operational warning and forecast decisions in operations. Gridded Geostationary Lightning Mappers (GLMs) products display the spatial extent of lightning flashes, as well as aggregated flash characteristics such as the count, size, and brightness within a given time window. Several years of GLM research and operational applications have shown a strong correlation between increasing flash counts within a thunderstorm core and the propensity for that storm to produce severe weather, such as damaging winds, heavy rain, hail, and tornadoes. This study investigates the three main GLM gridded products of Flash Extent Density (FED - flash counts), Minimum Flash Area (MFA - area of the smallest flash), and Total Optical Energy (TOE - accumulated flash brightness) from observed tornadoes (n=21) in a late-winter 2024 tornado outbreak in the Midwest United States and from most of the extremely large hailstones (≥4 inches, n=44) observed in the United States from April to June, 2024. The maximum/minimum values of the GLM gridded products within a 5-minute window were tracked from the parent storms from ten minutes prior to tornadogenesis to ten minutes after tornado cessation and from thirty minutes prior through to thirty minutes after each large hailstone. In the ten minutes following tornado cessation, maximum FED and maximum TOE markedly decreased while FED, MFA, and TOE all had little change in the ten minutes prior to tornadogenesis. In contrast, the maximum TOE increased substantially in the minutes leading up to the observance of large hailstones, while the maximum FED typically increased in the thirty minutes after these hailstones. An inverse relation was found between the change in average TOE values after each hailstone was observed and the hailstone diameter, possibly indicating that on average larger hailstones were more closely associated with a drop in the accumulated lightning flash brightness as detected by the GLM instruments. These trends in the lightning activity reinforce the utility of incorporating GLM observations into the warning decision-making process, especially for determining if an ongoing tornado is likely to continue or dissipate and if a severe-warned hail storm might produce extremely large hailstones.