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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!