A Trajectory-Based Method for Estimating the Contribution of Landfalling Atmospheric Rivers to Top-Decile Precipitation Across Colorado

September 2, 2025

A new paper, “A Trajectory-Based Method for Estimating the Contribution of Landfalling Atmospheric Rivers to Top-Decile Precipitation Across Colorado” by Deanna Nash (CW3E), Jon Rutz (CW3E), Jason Cordeira (CW3E), Zhenhai Zhang (CW3E), F. Martin Ralph (CW3E), Kris Sanders (NWS Grand Junction), and Erin Walter (NWS Grand Junction), was recently published in Journal of Geophysical Research: Atmospheres. This research, supported by the Miramar Charitable Foundation on behalf of Eaton and Margaret Scripps, uses backward trajectories to quantify how much of Colorado’s most extreme precipitation (top 10% of daily totals within sub-basins) is linked to landfalling atmospheric rivers (ARs).

This work addresses one of CW3E’s Strategic Plan priorities: Atmospheric Rivers and Extreme Precipitation Research Prediction and Applications. It advances understanding of inland-penetrating ARs and their role in Colorado’s heaviest precipitation events, while strengthening Research and Operations Partnerships through collaboration with the National Weather Service (NWS) in Grand Junction, CO.

Results show that ARs contribute 21–78% of western Colorado’s cool-season top-decile precipitation, far exceeding earlier estimates of up to 30% (Fig. 1). This wide range reflects Colorado’s complex topography and the influence of storm track variations among inland-penetrating ARs. In western Colorado, most AR-related precipitation is tied to landfalling ARs near Southern California and the Baja Peninsula (Fig. 2). While fewer trajectories originated from the Pacific Northwest and Gulf of Mexico, these events also produced some of the state’s wettest days. Additionally, local factors such as slope orientation and moisture loss likely affect the magnitude of contributions. Because western Colorado snowpack is a critical source of water for the broader Southwestern United States, understanding the AR contribution to these extreme precipitation events is essential for regional water resource planning.

By demonstrating the significant role of landfalling ARs in Colorado’s cool-season extremes, this study underscores the importance of accurately representing ARs in forecast models to improve precipitation prediction. Building on these results, future work will focus on developing a historical outlook tool to provide forecasters with probabilistic guidance on sub-basin precipitation based on the location and strength of landfalling ARs.

Figure 1. The fraction of top-decile precipitation associated with landfalling atmospheric rivers (ARs) [shaded; % of total top-decile precipitation for the number of trajectories ran for the cool season (NDJFMA) between 2000 and 2023] for each sub-basin (contour, gray) in CO based on the Guan & Waliser (2024) AR detection tool.

Figure 2. A schematic showing the three most frequent pathways of cool-season landfalling AR trajectories. The bubbles on the coast show the relative number of landfalling ARs associated with top-decile precipitation days in Colorado sub-basins. Credit: Deanna Nash.

Nash, D., Rutz, J. J., Cordiera, J., Zhang, Z., Ralph, F. M., Sanders, K., & Walter, E. (2025). A Trajectory-Based Method for Estimating the Contribution of Landfalling Atmospheric Rivers to Top-Decile Precipitation Across Colorado. Journal of Geophysical Research: Atmospheres, 130(17), e2025JD043580. https://doi.org/10.1029/2025JD043580