Extreme
Precipitation in
the Pacific
Northwest:
Is there a trend?
Cliff Mass
University of
Washington
The most costly
and damaging
weather
phenomena in the
Northwest are
associated with
extreme
precipitation
Two Major Classes of Damaging Extreme
Precipitation Events in the Northwest
• The one to three day event associated with
widespread and sustained flooding: these
represent the big, billion dollar events.
• Intense, short-period downbursts, usually
convective, that cause localized flooding,
slope failures, damage, and injury.
• Antecedent wet conditions can set the stage
for these events, particularly the first.
Two Recent Multi-day Events
• November 6-9, 2006
• December 2-4, 2007
November 6-9,
2006
Dark Green: about 20 inches
Mount Rainier National Park
18 inches in 36 hr (Nov 8, 2006)
Mt.
Rainier
damage
December 2-3, 2007
15-20 inches
of rain
Dec. 3, 2007
20 inches in two
days over coastal
terrain of SW
Washington
Pictures courtesy of WSDOT
The results:
massive
landslides
and river
flooding
Short-term Extreme
Precipitation Events
Heppner, Oregon 1903
Convective flash flood in the Blue Mountains
247 people lost their lives at Heppner in the
Willow River Valley.
December 13, 2006: The Madison
Valley Event
Convective precipitation on a slow-moving occluded front
Nearly 1 inch in an hour
From general principles one could argue
BOTH for increasing and decreasing
threats.
There have been a lot of talk in
the media, by
“environmentalists”, and even
among some scientists, that the
frequency of Northwest heavy
precipitation events have already
increased or will soon increase
under anthropogenic global
warming
NY Times
From the NY Times Article
Not so simple…through
A number of studies have indicated
that extreme precipitation scales
more closely with annual
precipitation than water vapor
content
• ..and of course there is large, coherent
spatial variability in the trends of both
annual and extreme precipitation.
What do we really know?
What don’t we know?
In today’s talk we will deal with the
more serious and widespread 1-3
days events.
Nearly all are associated with the
“Pineapple Express”
a.k.a. atmospheric rivers
A relatively narrow current of warm, moist air from the
subtropics…often starting near or just north of Hawaii.
A Recent Devastating Pineapple Express:
November 6-7, 2006
Associated with
extraordinarily
narrow filaments of
moisture
Precipitable Water
From Mike Warner
We know quite a bit about
atmospheric rivers and heavy NW
precipitation events, although
there are still gaps in our
knowledge
Synoptic Set-Up for Top Fifty
Events at Forks
Courtesy of Michael Warner
Precipitable Water
500 mb height
SLP
850 mb Temp
Extreme Precipitation Events
• The current of warm,
moist air associated with
atmospheric rivers are
found in the warm sector,
parallel, near, and in front
of the cold front.
• Thus, atmospheric rivers
are closely associated
with the jet core and the
region of large
baroclinicity.
Orographic Enhancement
• Upslope flow
greatly increases
precipitation rates
on terrain.
• Thus, wind speed
and angle of attach
can greatly modify
the extreme nature
of the
precipitation.
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)
Trends in Annual Precipitation (3196)
Decreasing trends in western Washington
Trends in 7-day precip events for 1yr recurrence interval (significant with zscores greater than 1.98)
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.
State by State Comparison
They ran contemporary dynamical downscaling (19702007):
• Hadley regional model forced by Hadley Center GCM
• WRF forced by ECHAM5 GCM
“The lack of correspondence between observed
and simulated trends for extreme precipitation
likely results from the dominance of natural
variability over anthropogenic trends during the
period 1970-2007.
Thus, the lack of correspondence between
observed and simulated trends for extreme
precipitation likely results from the dominance of
natural variability over anthropogenic trends
during the period 1970-2007.”
Are there trends in major
precipitation events?
• Mike Warner has
determined the top
50 recent (19502008) two-day
precipitation events
at stations up and
down the coast.
Top 50 Two-Day Storms!
NOTE! The 2000s are missing 2009!
South
Eureka
North
Brookings North Bend
Newport
Astoria
Forks (Twilight)
Extreme Precipitation (2-day
amounts) 60-yr Trends along the
West Coast (Adam Skalenakis)
Threedegree
latitude
avg
CA
OR
WA
BC
Trends in Maximum Annual
Streamflow
Lins, H., and J.R. Slack, 1999: Streamflow
trends in the United States. Geophysical
Research Letters, 26, 227-230
Maximum Daily Streamflow
Downward Trend in the NW
(1934-93 ,1924-93,1914-93)
Precipitation Extremes and the Impacts of Climate
Change on Stormwater Infrastructure in Washington
State
Eric A. Rosenberg, Patrick W. Keys, Derek B. Booth, David Hartley,
Jeff Burkey, Anne C. Steinemann, and Dennis
P. Lettenmaier
Precipitation Extremes and the Impacts of Climate Change on
Stormwater Infrastructure in Washington State
• Few statistically significant changes in
extreme precipitation have been observed to
date in the state’s three major metropolitan
areas, with the possible exception of the
Puget Sound.
Some Conclusions
• A variety of studies have come to same
conclusion: there is no coherent, spatially
uniform trend towards more extreme
precipitation over the region during the past
30-60 years.
• No reason to suggest, state, hint, or imply that
anthropogenic global warming is causing more
extreme precipitation in the NW.
Future Trends of Extreme
Precipitation over the Northwest
NEXT TALK!
Annual
•
DJF
Summer
Figure 11.12. Temperature and precipitation changes over North America from the
21 MMD-A1B GCM simulations. Top row: Annual mean, DJF and JJA temperature
change between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Middle
row: same as top, but for fractional change in precipitation. Bottom row: number of
models out of 21 that project increases in precipitation.
Simulated seasonal precipitation changes (mm/day) from 1989-1999 to 2045-2055 from the ECHAM5MM5 regional model for a) DJF b) MAM c) JJA and d) SON
There are several other GCM-RCMdownscaling studies I could quote
• Bottom line: still a great deal of uncertainty
on the future precipitation of our region and,
even more so, the trend in extremes
• If jet stream moves northward, we could less
atmospheric rivers!