University of WA Climate Impact Group 4/15/2016

Exhibit One Scope of Work New projections of changing heavy precipitation in the City of Everett Investigator Guillaume Mauger, Climate Impacts Group, UW Seattle Proposal Use new regional climate model simulations of 21st century changes in precipitation to develop projections that are specific to the City of Everett. Results will be tailored to address the decision needs of city planners. Background 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. King County has recently awarded funding to the UW Climate Impacts Group (CIG) to develop new regional climate model simulations of changing precipitation.These simulations are an improvement over statistically-based methods for "downscaling" global climate model output because they explicitly simulate the physics of changing local processes—for precipitation, the key distinction is that they simulate the interactions of weather systems with the complex terrain of the Pacific Northwest.' The City of Everett currently uses a previous set of climate projections, with a middle estimate of a 9%increase in winter precipitation extremes and an 18% increase in summer extremes. These numbers are unlikely to hold for all precipitation intensities—for example, the 100-year event may not change by the same amount as the 10-year event. In addition, a median climate change estimate may not be suitable for mitigating the risks to stormwater facilities. Finally, new projections have since been developed, such that even the middle estimates, averaged over all precipitation quantiles, may no longer match the 9%and 18%estimates in use today. The proposed work will develop updated heavy precipitation projections that are specific to the city of Everett. These will be evaluated for changes in user-specified metrics, selected for direct application to risk assessment and decision-making. Methods Project researchers will leverage existing regional climate model simulations to produce hourly precipitation time series and exceedance probabilities for specific locations of interest. The analysis would be based on two climate simulations that bracket the low and high end of precipitation projections for Puget Sound: one based on a cooler model and a low greenhouse 1 Eric P.Salathe Jr.,Alan F.Hamlet,Clifford F.Mass,Se-Yeun Lee,Matt Stumbaugh,and Richard Steed,2014:Estimates of Twenty-First-Century Flood Risk in the Pacific Northwest Based on Regional Climate Model Simulations.J.Hydrometeor, 15, 1881-1899.doi:http://dx.doi.org/10.1175/JHM-D-13-0137.1 Impacts from Climate Change On Stormwater in Everett KC—UW Agreement Page 6