Extreme Precipitation in the Pacific Northwest:

Is there a trend?

Cliff MassUniversity 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

Dark Green: about 20 inches

November 6-9, 2006

Mount Rainier National Park18 inches in 36 hr (Nov 8, 2006)

Mt. Rainierdamage

December 2-3, 2007

15-20 inchesof rain

Dec. 3, 200720 inches in two days over coastal

terrain of SW Washington

The results: massive landslides and river flooding

Pictures courtesy of WSDOT

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

Nearly 1 inch in an hour

Convective precipitation on a slow-moving occluded front

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

Precipitable WaterFrom Mike Warner

Associated with extraordinarily narrow filaments of moisture

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 >): 1931-963 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 (31-96)

Decreasing trends in western Washington

Trends in 7-day precip events for 1-yr recurrence interval (significant

with z-scores 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 (1970-2007):

• 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 (1950-2008) two-day precipitation events at stations up and down the coast.

NOTE! The 2000s are missing 2009!

Top 50 Two-Day Storms!

South North

Eureka Brookings North Bend Newport Astoria Forks (Twilight)

Extreme Precipitation (2-day amounts) 60-yr Trends along the

West Coast (Adam Skalenakis)

CA OR WA BC

Three-degree latitude avg

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 StreamflowDownward Trend in the NW(1934-93 ,1924-93,1914-93)

Precipitation Extremes and the Impacts of Climate Change on Stormwater Infrastructure in Washington

StateEric A. Rosenberg, Patrick W. Keys, Derek B. Booth, David

Hartley, Jeff Burkey, Anne C. Steinemann, and DennisP. 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!

• 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.

Annual DJF Summer

Simulated seasonal precipitation changes (mm/day) from 1989-1999 to 2045-2055 from the ECHAM5-MM5 regional model for a) DJF b) MAM c) JJA and d) SON

There are several other GCM-RCM-downscaling 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!