A Synoptic Approach to Studying Snow Drought in Oregon's Clackamas River Watershed in Observations and Climate Models

Anthropogenic climate warming is projected to result in changes to several factors that affect the quality and quantity of fresh water across the Pacific Northwest. These include changes in the timing and intensity of precipitation, a higher proportion of precipitation falling as rain instead of snow, and decreases in the extent and persistence of snowpack. However, the effects of climate change can vary across relatively small spatial scales, particularly in orographic regions such as the Cascade Range. Areas with complex topography are often difficult to resolve in relatively coarse resolution general circulation models, which hinders translating precise results into actionable mitigation and adaptation plans. Therefore, we characterize the synoptic drivers of snowpack and extreme precipitation variability as it pertains to water availability in the Clackamas River Watershed (CRW), which is located in the southern portion of the Portland Metropolitan Area in Oregon. Using a suite of models from the Coupled Model Intercomparison Project Phase 5 (CMIP5), we discuss climatological processes that contribute to temporal and spatial variability in snowpack extent, as well as patterns in the rain-to-snow ratio over the watershed. The ability of CMIP5 to capture key meteorological drivers of snow drought in the CRW is evaluated using observations and reanalysis products. Results from this evaluation will help constrain uncertainty in climate model projections of these drivers under future climate conditions, which enables a better understanding of how climate change may impact the CRW and surrounding communities that rely on this water source.