Climate Change in Portland, Gresham, and Clackamas County


Morgan, H., G.S. Mauger, and J.S. Won. 2021. Climate Change in Portland, Gresham, and Clackamas County. Report prepared for the City of Portland, City of Gresham, and Clackamas County. Climate Impacts Group, University of Washington.


Recent research suggests that future heavy rain events will be more intense in the Pacific Northwest (e.g., Warner et al. 2015). Past studies have not accounted for this change, either because the methods cannot reliably capture changes in rainfall intensity (e.g., Abatzoglou and Brown 2012), or because previous studies did not evaluate changes in short-duration precipitation (e.g., hourly) that are of relevance to stormwater planning (e.g., Salathé et al. 2010). The purpose of this project was to develop projections of 21st century changes in precipitation that can be used to inform stormwater and wastewater management in the cities of Portland and Gresham, and Clackamas County.

Our results show the potential for large increases in future rainfall intensity. For example, models project an increase of +32% (range: -1% to +94%) for the 10-year event with a 1-hour duration at Portland Airport. Results differ by location, metric, and duration; generally showing a larger change for the more extreme events (e.g., the projected change for the 25-year is generally larger than that for the 2-year event) and for shorter durations (e.g., the projected increase for 1-hour events is generally greater than for 24-hour events). We developed an interactive visualization for browsing the results, based on feedback and testing with prospective users. The tool can be used to browse results for any location within the full model domain, allowing users to quickly view results and download images and data for offline use.