Extreme Precipitation over the West Coast of North America Is There a Trend? Cliff Mass, Mike Warner, and Adam Skalenakis University of Washington

Extreme Precipitation and Its Effects (flooding and slope failures) are Generally The Most Costly Weather Features for Much of the West Coast.

There has been a lot discussion in the media and the scientific literature suggesting that western U.S. extreme precipitation events have already increased in intensity or will soon increase under anthropogenic global warming

Press Advisory National Wildlife Federation November 17, 2009 Contacts: XXXX (name removed) YYYY (you know her!) Senior Environmental Policy SpecialistClimate Scientist National Wildlife Federation, Pacific RegionNational Wildlife Federation … Global warming is exacerbating extremely heavy rainfall events, and recent climate change modeling suggests that these pineapple express storms are no exception. “Heavier rainfall events combined with significant snowmelt in midwinter is just what the Pacific Northwest should expect from global warming,” said Dr. YYYY, climate scientist, National Wildlife Federation. …. Recent data[1] suggests that as a result of climate change, the largest storms (i.e., those that are most likely to cause flooding) will trend toward producing increasingly larger quantities of precipitation.

What do we really know?

Studies of Trends of Extreme Precipitation “Long-term trends in extreme precipitation events over the conterminous U.S. and Canada” – Kunkel, Andsager, and Easterling, J. of Climate, 1999 Examined 1-7 day extreme precipitation events (greater than 1 yr return interval)

Trends of 7-day extreme (1 yr or >): and 1-day extremes were similar Little trend in the NW. Suggesting of small increases in western WA and decreases in western Oregon (tail indicates significant at 5% level)

When it Rains it Pours (not reviewed) Used the Kunkel Approach for 1-day rainfall ( ) More over Wa, LESS over Oregon, little trend, N. CA.

Are there trends in major precipitation events? Step 1: Determined the top 60 two-day precipitation events at stations along the coast for

60- year Trends for Top-60 Events Individual Stations Two-degree bands

60- year Trends for Top-20 Events Individual Stations Two-degree bands

Trend in Maximum Annual Two-Day Precipitation

Trends on Unregulated Rivers : Max Annual Daily Discharge

Trends in Maximum Daily Discharge of Unregulated Rivers

Some Conclusions During the past 60 years there has been: – a modest increase in heavy precipitation events over southern and central coastal California, – a decline in heavy events from northern California through the central Oregon coast, – a substantial increase in major events over Washington, and – a modest increase over coastal British Columbia.

Conclusions Most of these trends are not significantly different than zero at the 95% level. The trends in maximum daily discharge of unregulated rivers are consistent with the above pattern, with increasing discharges over the past three decades over Washington and northern Oregon and declines over the remainder of Oregon and northern California.

Bottom Line No spatially uniform upward precipitation trend over the U.S. West Coast.

The End