Heavy Precipitation Projections for use in Stormwater Planning

  • On-Going
  • King County Department of Natural Resources and Parks
  • Washington State Department of Ecology
  • City of Everett
  • Thurston County
  • Port Gamble S’Klallam Tribe
  • Portland’s Bureau of Environmental Services
  • Clackamas County
  • City of Gresham

Recent work has shown that dynamically downscaled projections  – in which Regional Climate Models (RCMs) are used to simulate local scale changes in climate – are needed to accurately capture changes in heavy rainfall events in the Pacific Northwest (e.g., Salathé et al. 2014). This page summarized the projected changes in heavy rainfall events, from a new set of RCM projections, for use in stormwater and CSO planning. The results presented here stem from a series of projects, in which tailored projections were produced for each participating jurisdiction.

Results are designed to support both municipal and rural stormwater planning. The online tool allows users to evaluate projections as a function of precipitation intensity, duration, and frequency. The online repository includes summary statistics as well as time-series data for use in continuous simulation modeling.

Extreme Precipitation Tool

The purpose of this tool is to visualize projected changes in heavy rainfall (or extreme precipitation) events across the Pacific Northwest. This tool provides extreme precipitation projections as a function of decade, duration, and frequency (or return interval).

Rain Gauges & Data Access

Use this map to access observations and projections for rain gauges in Washington and Oregon. Station Networks available on this google map include: NOAA HPD, NOAA LCD, HYDRA, King County Network, Seattle, Thurston County, Everett, Snohomish County.

Observed Rainfall Statistics

Use this visualization to explore observed precipitation statistics for all rain gauges. Results are shown for a variety of statistics, including annual totals, monthly totals, and three extreme metrics which represent the top 1% of annual events for durations of 1, 2, 6, 24, and 72 hour durations.

Model Evaluation

Use this visualization to view comparisons between model hindcasts and the observations. This data can be used to assess how well the model is able to reproduce observed precipitation statistics for each rain gauge, across monthly precipitation totals and across six extreme statistics.

Explore the Tool  Explore the Map Access the Visualization Access the Visualization


RCM simulations were performed using the Weather Research and Forecasting (WRFSkamarock et al. 2005) community mesoscale model, with much of the modeling led by Cliff Mass in the UW Department of Atmospheric Sciences. Building on extensive UW experience with this model, WRF was configured using the same approach used in previous work (Salathé et al. 20102014). WRF simulations extended from 1970-2099 at a spatial resolution of 12 km, covering all of the Pacific Northwest. In order to facilitate stormwater analyses, results were saved at an hourly time step.

Global models are needed to drive the RCM simulations. All global climate model scenarios were obtained from the Climate Model Intercomparison Project, phase 5 (CMIP5Taylor et al., 2012). All but one projection are based on a high-end greenhouse gas scenario (Representative Concentration Pathway (RCP) 8.5; van Vuuren et al., 2011). One additional simulation was performed for the low-end RCP 4.5 greenhouse gas scenario, for the ACCESS 1.0 global model.

Results were used to evaluate projected changes in the intensity of precipitation for a variety of durations (from 1 to 360 hours), return intervals (from 2- to 100-year events), and time periods (from the 1990s to the 2080s). The interactive web tool allows users to browse the results, expressed as the percent change relative to the 1990s, for any location within the model domain. For most applications, this will be the best way to make use of the projections. However, in some applications (e.g., for use in a continuous stormwater simulation) it may be worth exploring WRF projections that have been bias-corrected to match the statistics of a particular rain gauge. These rain gauges, their observed statistics, and comparisons between WRF and the observations, can all be accessed via the links highlighted in the sidebar to the right.


Portland, Clackamas, Gresham

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.
Read the Report

City of Everett

Errata: An error was found in the GFDL CM3, RCP 8.5 projection. The error has been corrected in this link version, but the previous results should be discarded and replaced. Table 3 has been updated to reflect the corrected GFDL CM3 projections. Updated 8/20/2019.
Read the Report

King County

Errata: The Phase 1 King County report (Mauger et al., 2018) described results from two WRF projections: (1) ACCESS 1.0, RCP 4.5, and (2) GFDL-CM3, RCP 8.5. In creating the new larger ensemble of 12 RCP 8.5 projections (see Phase 2 King County report), an error was found in the WRF boundary conditions used for the GFDL-CM3 simulation. The error has been corrected in the new ensemble, and all of the reports have been updated to reflect this change. Previous results, based on the older GFDL CM3 RCP 8.5 WRF projection, should be discarded. Updated 8/20/2019)
Read Phase 1 ReportRead Phase 2 Report

Port Gamble S'Klallam Tribe

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Thurston County

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Peer-Reviewed Publications

Lorente-Plazas, R., Mitchell, T.P., Mauger, G., Salathé, E.P., 2018. Local Enhancement of Extreme Precipitation during Atmospheric Rivers as Simulated in a Regional Climate Model. American Meteorological Society, 19:1429-1446. https://doi.org/10.1175/JHM-D-17-0246.1
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The original source of funding for this work was the King County Department of Natural Resources and Parks, with additional provided by the Washington State Department of Ecology. Since then, the work has been refined and expanded with funding from the City of Everett, Thurston County, the Port Gamble S’Klallam Tribe, Portland’s Bureau of Environmental Services, Clackamas County, and the City of Gresham.

The regional climate model simulations were produced by Professor Cliff Mass of UW Atmospheric Sciences, with funding from the Amazon Catalyst program.