Atmospheric rivers (ARs) drive most riverine floods on the United States (U.S.) West Coast. However, it is challenging to estimate flood risk based solely on AR intensity and duration because precipitation phase, antecedent conditions, and physical watershed characteristics (e.g. slope and soil depth) can influence the magnitude of flood peaks. In this project, I analyzed how antecedent soil moisture (ASM) conditions contribute to variability in flood peaks during AR events and how that changes across climatic regimes and physiography in 122 U.S. West Coast watersheds. I identified a robust non-linear relationship between event maximum streamflow and ASM during ARs in 87% of watersheds. The inflection point in this relationship represents a watershed-specific critical ASM threshold, above which event maximum streamflow is, on average, three times larger. Wet ASM conditions amplify the impacts of more frequent but weaker, lower moisture transport AR events, while dry ASM conditions attenuate the flooding that stronger, higher moisture transport AR events could otherwise cause. This research shows that watersheds prone to ASM-influenced flooding have higher evaporative indices, lower cold season precipitation, lower snow-to-rain ratios, and shallower, clay-rich soils. Higher evaporation and lower precipitation lead to greater ASM variability during the flood season, increasing flooding during wet periods and buffering flooding during dry periods. Lower snow fraction and shallower soils limit the antecedent water storage capacity of a watershed, contributing to greater sensitivity of flood peaks to ASM variability. Incorporating ASM thresholds into flood models in these regions prone to AR-influenced flooding could improve forecasts and decrease uncertainty.

The frequency of multiple atmospheric rivers (ARs) occurring in sequence is projected to increase due to climate change. Sequential AR scenarios are needed to understand regional flooding impacts and inform effective mitigation and response. This work investigates the flooding impacts of ARkFuture–a modeled month-long sequence of ARs making landfall on the West Coast of the U.S.–specifically focusing on the Truckee Watershed. ARkFuture is the climate change scenario in the ARkStorm 2.0 model, forced by 2072 climate conditions and with an estimated recurrence interval exceeding 400 years. The Truckee Watershed, located on the eastern slope of the northern Sierra Nevada and encompassing Lake Tahoe, has experienced multiple large and damaging historical floods, most recently in 1997 and 2017. With a large area of the watershed at elevations close to the rain-snow line, warming may have major impacts on how the basin responds to future storm events. Moreover, the region has experienced significant population growth in recent decades, increasing impervious surfaces and development in and near the floodplain. Using a series of hydrologic and operational models, we estimated the local flows and inundation resulting from the ARkStorm 2.0 ARkFuture scenario. We then compared these hydrologic impacts with those from estimates of shorter-duration higher-frequency events such as those used to generate FEMA 100-year flood maps and the historical 1997 flood extent. This comparative analysis sheds light on the unique challenges posed by long-duration events and provides valuable insights for emergency planning and response efforts. Ultimately, this research serves as a critical case study and demonstration of how the ARkStorm 2.0 scenario can be used in consultation with a group of local stakeholders to facilitate conversations around, and promote community resilience to, climate change impacts on flooding events.

Atmospheric rivers (ARs) are a dominant driver of water-related benefits and hazards in Central Chile, a region that hosts the majority of the country’s population and economic activities. However, existing literature on floods caused by ARs has primarily focused on the West Coast of the U.S. and has not addressed how flood magnitudes vary relative to the Ralph et al. (2019) AR scale in other geographies. Much like the Western U.S., Central Chile has Mediterranean climate characteristics, prominent topography, and has experienced recent megadroughts and floods. However, Central Chile has distinct AR storm types influenced by tropical-subtropical origins, orographic enhancement, and coastal-valley transitions within the Andes. The unique interplay between large-scale climate patterns and regional geographical features is further compounded by local hydrological conditions and water management practices. Here, we quantify the relative contribution of ARs to flooding in Central Chile. We characterize the storm and hydrologic conditions that result in higher, often damaging, streamflows. Using the AR scale, we assess how flooding increases with AR strength and explore the influence of other AR intensity features such as storm orientation and temperature. Furthermore, we examine how differences in local climate, land surface, and water management practices may mediate or enhance flood risk. This research aims to enhance knowledge of AR hydrologic impacts along this Chilean transect, contributing to improved forecasting, risk communication, and mitigation of flooding during ARs globally.