Expanding the ensemble of precipitation projections for King County


Mauger, G.S., J.S. Won, 2019. Expanding the ensemble of precipitation projections for King County. Report prepared for the King County Department of Natural Resources. Climate Impacts Group, University of Washington, Seattle.


Recent research suggests that future heavy rain events will be more intense in the Pacific Northwest (e.g., Warner et al. 2015). The UW Climate Impacts Group (CIG) recently developed hourly climate projections for use in stormwater planning and design at King County (Mauger et al. 2018). These were limited to just two new regional climate model projections, but recent work by Cliff Mass (UW Atmospheric Sciences) has resulted in 12 additional regional climate model projections. This is a major improvement, since previous work has shown that two model projections are insufficient to accurately characterize the mean and range of projected changes in climate. This technical memo is an addendum to Mauger et al. (2018), in which the same methodology is applied to the new WRF ensemble.

Since all of the projections are based on the high-end RCP 8.5 scenario, an additional analysis is included to identify time periods in the RCP 8.5 projections that can be used as an analog for a different time period in the RCP 4.5 projections. These comparisons suggest that the changes projected for the 2080s in the RCP 4.5 scenario roughly to those projected for the 2050s in the RCP 8.5 scenario.

Our results show the potential for large increases in future rainfall intensity. For example, models project an increase of +35% (+8 to +81%) in the 10-year hourly precipitation extreme at Sea-Tac, when considering the full water year (as opposed to a specific season). Results differ by location, metric, and season; generally showing a larger change for the more extreme events (e.g., the projected change for the 5-year is generally larger than that for the 2-year event). All data, documentation, and findings are made available online, including an interactive tool that allows users to easily navigate to the station and results of interest.