Moisture Source and Isotopic Signatures of Extreme Rainfall in the Washington, D.C. Area

The Washington, D.C. area is located at the intersection of multiple air masses, which fuel the intensity of storms and make this region vulnerable to extreme rainfall events. Previous studies have identified four major storm types in this area: general storms, tropical storms, hybrid storms, and local storms. However, the relationship between moisture sources and these storm types, as well as their isotope signatures in rainfall, remains poorly understood. Here, we use Hybrid Single Particle Lagrangian Integrated Trajectory Model to track the moisture origins of extreme rainfall events from 2023 to 2025 and investigate their relationship to storm types. We identified the top 10% of extreme rainfall events during the summer (June-August) and winter (October-December) months, based on meteorological data. Multiple statistical methods were applied to investigate backward moisture trajectories related to these rainfall events, including backward ensemble trajectories, frequency analysis, and cluster analysis. We found that summer rainfall was primarily sourced from the Gulf of Mexico, the Mid-Atlantic Ocean, and land transport from the Northern Pacific. Main winter moisture sources showed more diverse moisture origins, including the North Atlantic, the Great Lakes region, and the North Pacific. Such seasonal shifts in the dominant sources are generally consistent with the monthly average of sparse existing rainwater ¹⁸O observations, which show more enriched (positive) values during the summer months. Moreover, the rainwater ¹⁸O values from individual events exhibit significant variability, underscoring the need for more rainwater isotopic analysis to better understand the link between storm types, moisture sources, and isotopic composition.