Potential for Changing Extreme Snowmelt and Rainfall Events in the Mountains of the Western United States

The intensity of mountain precipitation is often modified by snow accumulation and melt, yet rainfall-based observations are widely used in planning and design. Comparisons of extreme rainfall versus snowmelt intensities are needed because they have different predictability and hazard implications. Regional warming is expected to intensify not only rainfall and snowfall but also slow snowmelt, which could further challenge intensity duration frequency (IDF) techniques. We use observations from 379 mountain sites across the western U.S. to estimate the 10 and 100year intensity at 1, 2, and 30day durations for historical snowmelt (SM), precipitation occurring during snow cover (SCP), and precipitation during the snow-free period (SFP). At 1day durations, 100year SCP was greater than SM and SFP at 40% of sites, while SM was larger than SCP and SFP at 39% of sites. At 30day durations, SM was greater than SCP and SFP at 95% of sites. The continental sites are generally insensitive to increased water input intensity from SCP occurring as rainfall. In contrast, the maritime mountains are relatively insensitive to changes in SM but have the potential for increased water input intensity from greater SCP occurring as rainfall. Standard precipitation intensity data sets accurately estimated the 100year, 1day SCP and SM but underestimated SM at 78 continental sites where SM was greater than SCP and SFP. These results confirm that snow processes modify IDF estimates and highlight regional sensitivity to increased winter rainfall and slower snowmelt that may necessitate local adaptation strategies. Plain Language Summary Extreme precipitation can cause floods, landslides, and other natural hazards in the mountains of the western United States. Predicting extreme precipitation intensity is therefore a critical tool for protecting life and property. Most of our standard prediction tools do not differentiate snow and rain or track the effects of snowmelt. Deficiencies in standard estimates could be amplified by increased winter rainfall and slowing snowmelt rates that are expected from regional warming. We investigated 379 mountain sites over 30+years to estimate the 100year intensity (i.e., statistically a 1/100 likelihood of occurring in any given year) at 1, 2, and 30day durations. We found that snowmelt and precipitation during the snow cover season were the main drivers of extreme water input intensity. Changes to slower snowmelt rates were more likely to affect extreme water input intensity at continental sites like the southern Rocky Mountains. Conversely, changes to rainfall during the snow cover season were more likely to affect water input intensity at maritime sites like the Cascades. These regional differences give a framework to understand vulnerability to changing extreme water input intensity that local resource managers and planners could use to adapt standard estimates to their areas.