1. Alan F. Hamlet Nate Mantua Todd Mitchell JISAO/CSES Climate Impacts Group Dept. of Civil and Environmental Engineering University of Washington ENSO Transition and the Potential for Extremely Wet Winters in the PNW

2. Natural Flows at Milner 1928-1989

3. Nino 3.4 Forecast (2010)

4. Nino 3.4 Forecast (2011)

6. Historic Dalles Apr-Sept streamflow anomalies for water years with previous winter warm ENSO, current winter cool ENSO Stats: 12 of 14 above 0.39 11 of 14 above 0.49 9 of 14 above 0.87 Water Year Average April- Sept Flow (cfs) Anomaly (cfs) Std. Anomaly (number of standard deviations from the mean) 1904 381769 825951.465941 1907 355726 565520.998325 1921 355607 564330.996192 1925 327301 281270.487946 1932 321917 227430.391269 1943 348757 495830.873195 1965 350140 509660.898033 1971 386065 868911.543085 1974 423785 1246112.220358 1984 332255 330810.576899 1989 277693 -21481-0.40279 1996 353565 543910.959533 1999 350530 513560.905032 2006 310757 106310.190885

7. Natural Streamflow (cfs) Based on Dalles Climatology 1901-2003 Warm season flow in 2008 was slightly above the long term mean.

8. 1932-0.235 19431.137 19651.238 19712.170 19741.686 19841.883 1989-0.080 Historic Milner Apr-Sept streamflow anomalies for water years with previous winter warm ENSO, current winter cool ENSO WY Std Deviations from the Mean Stats: 5 of 7 above 1.1 3 of 7 above 1.7

9. enso_2008enso_trans_2008 Based on Milner ESP 1961-2000 Natural Flow (KAF)

10. Emerging Prospects for Long-Term Predictability

11. Source: Okumura, Y.M. and C. Deser, 2010: Asymmetry in the Duration of El Niño and La Niña, http://journals.ametsoc.org/doi/pdf/10.1175/2010JCLI3592.1 12 3 4567 89101112 7/12 warm ENSO events above 1.0 threshold are followed by cool ENSO events 5/12 warm ENSO events above 1.0 are followed by cool ENSO events below 1.0

12. Conclusions Water year pairs with ENSO transitions from warm to cool phase in a single year are strongly associated historically with above average flow in Columbia and Snake Rivers in the second (cool ENSO) year. Historically such paired (warm/cool) events are more likely to occur when a strong (above 1.0 std deviation) warm event occurs. Thus increased odds of above average flow are predictable with very long lead times of up to 24 months under certain conditions. In June 1997, with a strong warm ENSO event clearly underway, elevated odds of high flows in warm season 1999 would have been predicted, for example. These kinds of long range forecasts have potentially useful application to water resources management problems. In particular, the amount of storage that can be safely used during low flow conditions in warm ENSO years is arguably a function of the strength of the warm ENSO event.