UC Berkeley

Abstract Atmospheric Rivers (ARs) often co‐occur with other weather systems, including fronts, low‐pressure systems (LPSs), and mesoscale convective systems (MCSs), which can amplify precipitation and flood risk. While extreme rainfall is frequently linked to multiple interacting features, how these systems co‐occur with AR intensity scales and regions remains unclear. Using ERA5‐derived feature masks and satellite data, we quantify spatiotemporal overlaps between ARs and other systems. AR + Front combinations dominate across much of the conterminous U.S. In higher AR scales (4–5), more complex MCS‐related combinations become increasingly common, particularly in DJF and MAM and along the West Coast, while limited LPS‐related co‐occurrences appear mainly in JJA and SON and are confined to regions such as the Gulf of Mexico and offshore Pacific genesis regions. In contrast, AR‐only occurrences dominate in some inland western regions. Extreme precipitation exhibits seasonal variation, with distinct co‐occurring systems contributing across seasons. Plain Language Summary Atmospheric Rivers (ARs) transport large amounts of moisture and are major contributors to heavy rainfall and flooding. ARs often occur alongside other weather systems, such as fronts, low‐pressure systems (LPSs), and mesoscale convective systems (MCSs), which can influence where and when heavy rainfall occurs. Here, we examine how ARs overlap with these systems across different AR intensity scales and regions. Our results show that AR–Front interactions are most common across much of the United States. Higher AR scales are more likely to co‐occur with MCSs and, in some regions, LPSs as well, particularly during summer and autumn and near the Gulf of Mexico and offshore Pacific genesis regions. The patterns of overlapping weather systems change with the seasons, and stronger ARs often occur alongside more overlapping systems. Key Points AR + Front combinations dominate across much of the conterminous U.S., while Gulf and offshore Pacific regions show greater complexity In DJF and MAM, higher AR scales more often co‐occur with fronts and MCSs; in JJA and SON, additional LPS co‐occurrence becomes evident Each season has different mixtures of weather phenomena associated with extreme precipitation