WWETAC Projects

Project Title: East Cascades Extreme Precipitation

Principal Investigators: Nicholas Bond and Andy Chiodi, University of Washington, Seattle/JISAO; Sim Larkin, U.S. Forest Service, Pacific Northwest Research Station, Seattle, WA

nab3met[at]u.washington.edu

Key Issues/Problems Addressed:
The progression of the fire season the Eastern Cascades is impacted by moderate/heavy rainfall events that occur on occasion, yet characteristics and patterns of these rainfall events are not well documented with direct observations and in complex terrain.  

Setting and Approach:

Precipitation data were used to evaluate summertime rainfall events along the East Slopes of the Cascades including: daily precipitation on 0.25 x 0.25 degree grid (NOAA Climate Prediction Center), daily USGS streamflow records on selected rivers with moderate-sized watersheds (Stehekin, American, Klickitat, Metolius), and daily 500 hPa Geopotential height from the NCEP Reanalysis. Smoothed daily climatologies of streamflow and precipitation were created at a grid cell level corresponding to each selected river’s watershed. Daily deviations from these climatologies were then computed for June-August of 1949-2009 and the top 40 events were chosen for analysis. Events were sorted (based on streamflow and precipitation separately) with respect to synoptic pattern based on surface air temperature and 500 hPa Z. A composite was computed for each river and results compared.

Key Findings:

  • Events defined as one or more consecutive days of rainfall exceeding 0.25 inch occur in most locations about 2-3 times per summer (July-September).
  • A relatively high proportion of the events in the northern portion of the domain occur during winter-like westerly flow anomalies; a high percentage of the rainfall events in the southern and low elevation portions of the domain occur during anomalous easterly flow.
  • The events associated with anomalous easterly flow are much more likely to be accompanied by lightning.

Impacts/Applications:

The nature of rainfall events in eastern Oregon and Washington is affected by North Pacific atmospheric circulation. Likely shifts in spatial and temporal distribution of atmospheric circulation in association with climate change may affect the distribution, frequency and timing of rainfall events.

Publication

Bond, N.A., A. M. Chiodi, N.K. Larkin and R.J. Barbour, 2012: Summertime Rainfall Events in Eastern Oregon and Washington (In Revision)

WWETAC Project ID:  FY08JB42