Antecedent moisture enhances early warning of atmospheric river flood hazards

March 2, 2026

Figure 1. A modified atmospheric river (AR) scale for flood hazards accounting for antecedent moisture. A landfalling AR is ranked according to the maximum integrated vapor transport (IVT) magnitude (y-axis), AR duration in hours (x-axis), and the Seasonal Standardized Precipitation Index (SSPI). Note that the locations of the three x-axes correspond to rank adjustments for neutral, dry, and wet antecedent moisture conditions as defined by SSPI thresholds. From Figure 4 of Webb et al. (2026)

A new study titled “Antecedent moisture enhances early warning of atmospheric river flood hazards” by Mariana Webb, Christine Albano, Deniz Bozkurt, Rene Garreaud, Anna Wilson, Guo Yu, Michael Anderson, and F. Martin Ralph has been published in Nature Communications. The study shows that incorporating information about antecedent moisture conditions into the atmospheric river (AR) scale substantially improves early warning of flood hazards.

Using more than 71,000 AR landfalls across California and central Chile, the analysis demonstrates that a large fraction of the variability in flood response not captured by the existing AR scale is explained by runoff efficiency processes driven by antecedent moisture conditions. Building on this physical understanding, the authors introduce a simple, operationally scalable enhancement to the AR scale that incorporates antecedent moisture, represented by precipitation over the previous 90 days via the Seasonal Standardized Precipitation Index (SSPI). For the modified AR scale, AR ranks are increased by one when antecedent moisture values indicate especially wet conditions and decreased by one for especially dry conditions.

Across both regions, the modified AR scale nearly doubles correlations with peak streamflow and increases the early-warning identification of flood-generating ARs by more than 30%. Together, the paper shows that incorporating antecedent moisture improves the ability of the AR scale to reflect flood hazard potential while preserving its simplicity and early-warning value of the original scale. More broadly, this work demonstrates that hazard classification tools can be enhanced by explicitly accounting for the physical processes that link atmospheric forcing to on-the-ground impacts.

This work, led by researchers at the Desert Research Institute, strengthens collaborations with strategic partners across the western US and internationally, while advancing research on atmospheric river impacts and early-warning tools. The Desert Research Institute has also highlighted the findings and implications of this work with a press release. It supports the Atmospheric Rivers and Extreme Precipitation Research, Prediction, and Applications priority area of the CW3E Five-Year Strategic Plan.

Citation:

Webb, M. J., Albano, C. M., Bozkurt, D., Garreaud, R. D., Wilson, A. M., Yu, G., Anderson, M. L., & Ralph, F. M. (2026). Antecedent moisture enhances early warning of atmospheric river flood hazards. Nature Communications (published online ahead of print 2026). https://doi.org/10.1038/s41467-026-69286-3