New Projections of Changing Heavy Precipitation for the City of Everett


Mauger, G.S., J.S. Won, K. Hegewisch, C. Lynch, 2017. New Projections of Changing Heavy Precipitation for the City of Everett. Report prepared for the City of Everett. Climate Impacts Group, University of Washington, Seattle.


Changes in the intensity, duration, and frequency of precipitation may negatively affect stormwater facilities, exacerbate landslide and urban flood risk, and lead to other public safety and water quality concerns. Recent research has shown that heavy rain events are projected to become more intense with climate change (e.g., Warner et al. 2015, Trenberth 2011). This has altered the calculus regarding climate change impacts in the Pacific Northwest, since previous research suggested very little change in precipitation for the region. This is in part due to new methods of “downscaling” the large-scale changes projected by global climate models (GCMs) to smaller-scale changes of relevance to impacts assessment. Studies have shown that a physics based approach (“dynamical downscaling”), is needed to capture changes in precipitation extremes and the associated impacts (Salathé et al. 2014). Previous approaches relied primarily on an empirical approach (“statistical downscaling”), which does not provide reliable estimates of changes in extremes. In dynamical downscaling, a regional climate model is used to simulate local-scale changes in climate, leading to a better representation of changes in the physical processes at these scales. This distinction is particularly important for precipitation, since dynamical downscaling can explicitly represent the interactions of weather systems with the complex terrain of the Pacific Northwest.

In 2015, King County awarded funding to the UW Climate Impacts Group (CIG) to develop new regional climate model simulations of changing precipitation. The City of Everett currently uses a set of climate projections showing a 9% increase in winter precipitation extremes and an 18% increase in summer extremes. These numbers are unlikely to be applicable to all precipitation intensities and durations. For example, the 100-year event may not change by the same amount as the 10-year event. Similarly, the 1-hour extreme may change more rapidly than the 24-hour maximum in precipitation. In addition, a median climate change estimate may not be suitable for mitigating the risks to stormwater facilities. As described below, the new projections funded by King County include a high-end and a low-end projection. For the current project, these new projections are used to evaluate changes in heavy rain events for a variety of intensities and durations.