Identifying, Observing, and Forecasting Thunderstorms Producing Deep Surface Hail Accumulations

In recent years, deep hail accumulations from thunderstorms have occurred frequently enough in metropolitan areas to raise the attention of the National Weather Service (NWS), the general public, and news agencies to this elusive phenomenon. Despite the extreme nature of these thunderstorms with deep hail accumulations, no comprehensive reports, measurements, or forecasting of accumulated hail depth, hail size distributions, and median hail size are currently in place. Hail accumulations from these storms, which had reports of up to 25 cm in depth, have resulted in have resulted in motor vehicle accidents, road closures, airport delays, urban and river flooding, and water rescues impacting thousands of people and causing millions of dollars in damage. To better identify, observe, and forecast significant hail accumulations on the ground forecasters from the NWS Forecast Office Boulder Colorado in collaboration with researchers from the University of Colorado started the Colorado Hail Accumulation from Thunderstorm (CHAT) project aiming at collecting improved and more frequent hail depth reports on the ground as well as studying thunderstorm characteristics of storm with large accumulations in order to identify good predictors that might be used as operational products in the future. This research highlights the increased importance of observing and reporting hail depth in thunderstorms and the need to bring new ideas and state-of-the-art practices for identifying, tracking, and forecasting the potential of deep hail accumulations in thunderstorms. Hail accumulation at the surface depends on three main components: i) hail mass concentration or hail production in the cloud, ii) hailfall duration, which is related to the storm propagation speed, and iii) rapidity of hailstone reaching the ground without being melted and sublimated. This presentation shows how these three components can be tracked throughout the lifetime of a thunderstorm using dual-polarization radar and lightning data. We show how to identify and observe hail accumulations at the surface and how remote observation-based hail accumulations compare to insitu reports from trained spotters, the general public, and new agencies.