Large‐Scale Moisture Sources and Delivery Pathways Contributing to Winter Floods in the US

Abstract Previous seminal studies on US flood hydroclimatology have recognized large‐scale seasonal precipitation water from the lower atmosphere as a significant factor influencing flood patterns and have identified potential moisture delivery pathways associated with seasonal flooding across the conterminous United States (CONUS). Given the changing climate, however, an update of these studies is warranted, leveraging the more extensive and longer series of data sets currently available on floods and exogenous climate drivers. Using a Lagrangian particle tracking model (HYSPLIT), this study explores the large‐scale moisture delivery pathways associated with monthly floods during the winter season across the CONUS. The contributions of various moisture sources (i.e., land, extratropical Pacific, tropical Pacific, extratropical Atlantic, tropical Atlantic, and Arctic) to winter floods are summarized for each hydrologic region (HUC02) based on the moisture delivery pathways identified by the HYSPLIT model. Our results show that major oceanic sources are the primary contributors to winter floods in coastal regions, whereas land remains a significant source for hydrologic regions over the Midwest. Humidity profiles along the trajectories further indicate that air masses from the Pacific lose substantial moisture when crossing western mountain ranges and subsequently gain over east of the Rockies, making land a key contributor to floods over the upper Midwest. We also find substantial variability in moisture source contributions depending on flood severity in specific regions. In addition, the circulation patterns associated with extreme flood trajectories reveal consistent large‐scale features, typically involving a deep low‐pressure system delivering moisture to hydrologic regions.