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Precipitation declines in Pacific Northwest mountains

Posted date: January 23, 2015
Publication Year: 
Authors: Luce, Charles H.;
Document type: Briefing Papers


A rapidly warming climate across the Pacific Northwest is altering the volume, timing, and quality of water received from winter snowpack. Historic observations show increased dryness accompanying more widespread wildfire and forest die-off. These trends have generally been attributed to warming temperatures because measurement gauges at lower elevations throughout the region showed no significant decrease in precipitation.

Key Findings:

  • Prevailing winter winds blowing from the west to the east push water vapor off the North Pacific into the Cascade and Northern Rocky Mountain Ranges. These westerly winds play a key role in carrying precipitation into the mountains.
  • Climate change is likely responsible for the steady slowdown of the westerlies observed from 1950-2012. Their average speed during the 1950s was about 9.5 m/s, but has declined to 8.5 m/s today.
  • Decreased westerlies are hypothesized to have reduced orographic precipitation, yielding differential trends in precipitation across elevations and contributing to the decline in annual streamflow.
  • Additional research is needed to confirm the overall impact of weaker westerlies on the Pacific Northwest's water supply.

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