1. TRENDS IN U.S. EXTREME SNOWFALL SEASONS SINCE 1900 Kenneth E. Kunkel NOAA Cooperative Institute for Climate and Satellites - NC David R. Easterling National Climatic Data Center Michael Palecki and Leslie Ensor Illinois State Water Survey David Robinson Rutgers University Kenneth Hubbard High Plains Regional Climate Center Kelly Redmond Western Regional Climate Center Supported by NOAA Climate Program Office

2. U.S. Snowfall Data National Weather Service Cooperative Observer Program (1888-present) Climate Database Modernization Program has put all of the COOP data into digital form, allowing century-long analyses. We have performed an assessment of variability and change in snowfall extending back to 1900

3. Stations with less than 10% missing snowfall data for 1930-present

4. Quality Issues Inhomogeneities due to changes in station location, observer, site characteristics, observer practices, etc. Discontinuities in time series are not always detectable or distinguishable from real changes

6. Is this real?

7. Data Quality Assessment Identify a station set suitable for trends analyses - No systematic biases - Absence of station change inhomogeneities Expert assessment of quality by authors using a number of statistical and graphical tools Assessment of the quality of >1100 long-term stations with annual snowfall > 12.5 cm

8. Spatial Coherence Analysis Use neighboring stations with long records For each time series, calculate snowfall anomalies (annual snowfall minus long-term mean snowfall) Create time series of annual values (reference station anomaly minus neighboring station anomaly): “Anomalies of anomalies” If reference station’s behavior is similar to neighboring station’s, then values will be small and fluctuate around zero

11. Result Types Total annual snowfall Years with annual snowfall above 90 th percentile threshold Years with annual snowfall below 10 th percentile threshold

12. Total Annual Snowfall Trends

13. Homogeneous station trends for 1930-2006

14. Years with annual snowfall above 90 th percentile and below 10 th percentile thresholds

15. HIGH SNOW FALL LOW SNOW FALL

16. HIGHLOW

17. HIGH

18. LOW

19. Relationships with temperature and precipitation conditions

20. HIGH SNOWFALL LOW SNOWFALL

21. HIGH SNOWFALL LOW SNOWFALL

22. r 2 = 0.38

26. Relationships with ENSO

27. U.S. and regional high and low extreme snowfall percentages for moderate to strong winter El Niño (ONI > +1.5) and La Niña (ONI <- 1.5) events from 1900-1901 to 2006-2007 El Niño La Niña NCDC RegionHigh LowHighLow Northeast4.124.37.711.5 East North Central8.311.67.612.3 Central0.727.88.413.6 Southeast6.025.92.117.4 West North Central6.614.39.112.9 South17.412.92.317.9 Southwest22.33.56.514.2 Northwest0.033.314.54.3 West7.620.410.012.1 Conterminous U.S.8.317.87.812.9

28. Conclusions The quality of snowfall data is a major challenge to assessment of extremes. We rejected about half of the available stations because of quality uncertainties and concerns We hope that major homogeneity issues are random and that spatial averaging/aggregation minimizes the effect on identification of real climatic effects

29. Conclusions Since the late 1980s, extreme high snowfall years have been rather infrequent, while extreme low snowfall years have occurred at a somewhat above-average frequency Statistically significant relationships between temperature and the frequency of extreme snowfall years –Negative correlations with high snowfall –Positive correlations with low snowfall

30. Conclusions Precipitation also correlated, but less so than for temperature ENSO signal in some regions Given the sensitivity of extreme snowfall seasons to temperature, and the signs of recent trends observed since 1950, it is likely that the increasing frequency of low- extreme snowfall years and decreasing frequency of high- extreme snowfall years are at least partially a consequence of the general warming that has occurred over that time period