Investigating the Atmospheric Conditions Associated with Impactful Shallow Landslides in California (USA)

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Abstract Shallow landslides are often triggered during rainfall events, which can increase subsurface soil water pressure and destabilize hillslopes. The likelihood of regional shallow landslide initiation is often assessed through a comparison of rainfall intensity and duration to pre-established thresholds. While informative for landslide warning, this exclusive focus on rainfall exceeding thresholds does not consider the meteorological conditions producing the rainfall. Here, we ask the question, are there common meteorological characteristics that lead to landslide-triggering precipitation? We develop a catalog of 18 post-1995 widespread, impactful shallow landslide events occurring within 13 storms across California, USA, where initiation time could be constrained to a ≤6-h window. We examine storm characteristics during the landslide initiation window using atmospheric reanalysis products, radar observations, and quantitative precipitation estimates. We find that, while there are some common atmospheric characteristics across landslide events, they can occur under a range of atmospheric conditions. For example, all Northern California landslide events assessed are associated with moderate to strong atmospheric rivers (ARs), while Southern California landslides feature non-AR to strong AR conditions. The storm events evaluated herein share many characteristics of hydrologically important storms in California that did not necessarily result in landslides; thus, atmospheric characteristics alone may not be sufficient to determine whether landslides will occur. However, documenting the characteristics of landslide-triggering storms defines the conditions under which landslides tend to occur, provides analog events that can be useful in forecast applications, helps define future research directions relating to atmospheric conditions and landslides, and supports interdisciplinary research efforts. Significance Statement Rainfall-triggered landslides pose a threat to communities situated in and around California’s steep terrain. Thresholds related to measured antecedent rainfall accumulation and anticipated rainfall intensity over various durations are typically used for predicting landslide occurrence. Here, we assess various atmospheric characteristics of 18 storms that triggered landslides to determine whether there are common characteristics that could provide insight into landslide occurrence beyond precipitation information. Our results indicate while there are some common characteristics across landslide events, the events can occur under a range of conditions. This finding and the documentation of these events are useful for communicating weather forecasts and potential hazards, help build an interdisciplinary understanding of landslide-triggering precipitation, and highlight future research needs.