Transcript Seattle Public Library Talk

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)
Senior Environmental Policy Specialist
National Wildlife Federation, Pacific Region
YYYY (you know her!)
Climate Scientist
National 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 >): 1931-96
3 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 (1948-2006)
• 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 19502008.
60year
Trends
for
Top-60
Events
Individual
Stations
Two-degree
bands
60year
Trends
for
Top-20
Events
Individual
Stations
Two-degree
bands
Trend in Maximum Annual Two-Day
Precipitation
Trends on
Unregulated
Rivers
1950-2009:
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