1. Declines in mountain snowpack Philip Mote, Alan Hamlet, Dennis Lettenmaier University of Washington With thanks to NRCS and Iris Stewart ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/

2. Trends 1950-1997, relative to 1950 value Data from NRCS, CA DWR, BC SRM 824 snow courses/SNOTEL Relative to 1950 value VIC simulation 1/8° long x 1/8° lat, west of Continental Divide Daily weather data, 1/1/1915 - 9/30/97 interpolated to VIC grid points from Coop stations Long-term trends interpolated from USHCN stations

3. Trends 1950-1997, relative to 1950 value

4. 1950-1997 relative trends vs DJF temperature Obs VIC

5. Correlations between Nov-Mar climate and Apr 1 SWE X-direction: precip Y-direction: temp Coldest locations insensitive to temperature Cascades very sensitive

6. At almost every USHCN station, winters warmed + signs: warming but not statistically significant

7. Winters wetter in much of the West Drier in some of Northwest (PDO)

8. April 1 SWE trends, 1950-2002 a P a T

9. Regional average April 1 SWE Obs o VIC x

10. Changes in SWE vs changes in precip 1930s to 1990s1945-55 to 1990s Obs SWEVIC SWEPrecipObs SWEVIC SWEPrecip Cascades-14%+1%+4%-29%-16%-5% Rockies+11%+2%+9%-16%-9%+1% California+3%-14%+10%-2%-25%-1% Interior+9%-6%+10%-22%-18%+2%

11. Trends in Simulated Average APR 1 SWE for the Cascades in WA and OR (1950-1995) Effects of TMP and PCP -54% Effects of TMP -26%Effects of PCP -28% SWE (mm)

12. Stewart et al., 2004; Stewart et al., in press, J. Clim. Spring-pulse dates Centers of Mass By several measures, Western snowfed streamflow has been arriving earlier in the year in recent decades Spring pulse Center time

13. Trends in timing of peak snowpack Change in Date

14. As the West warms, winter flows rise and summer flows drop Figure by Iris Stewart, Scripps Inst. of Oceanog. (UC San Diego)

15. March June Relative Trend (% per year) Trends in fraction of annual runoff for cells with more than 50 mm of SWE on April 1

16. Trends/15yr in snow days in Switzerland From Scherrer et al., GRL 2004

17. Northern Hemisphere spring snow extent Courtesy Ross Brown

18. Conclusions Observations show substantial declines in western snowpack Changes in timing of melt, peak snowpack, and seasonality of flow Dependence on elevation shows temperature playing a dominant role ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/