TRENDS IN U.S. EXTREME SNOWFALL SEASONS SINCE 1900

Kenneth E. KunkelNOAA Cooperative Institute for Climate and Satellites - NC

David R. EasterlingNational Climatic Data Center

Michael Palecki and Leslie EnsorIllinois State Water Survey

David RobinsonRutgers University

Kenneth HubbardHigh Plains Regional Climate Center

Kelly RedmondWestern Regional Climate Center

Supported by NOAA Climate Program Office

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

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

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

Red Lodge, MT

0

50

100

150

200

250

300

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

Sn

ow

fall

(in

ches

)

Is this real?

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

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

Red Lodge, MT Snowfall Anomaly Minus Station Anomaly

-150

-100

-50

0

50

100

150

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year

Sn

ow

fall

An

om

aly

(in

ches

) Mystic Lake, MT

Rapelje, MT

Flatwillow, MT

Roberts, MT

Clark, WY

Columbus, MT

Fishtail, MT

Bridger, MT

Joliet, MT

Heart Mountain, WY

Buffalo Bill Dam, MT

Crandall Creek, MT

Cody, WY

Edgar, MT

Result Types

• Total annual snowfall

• Years with annual snowfall above 90th percentile threshold

• Years with annual snowfall below 10th percentile threshold

Total Annual Snowfall Trends

Homogeneous station trends for 1930-2006

Years with annual snowfall above 90th percentile and below

10th percentile thresholds

HIGHSNOWFALL

LOWSNOWFALL

HIGH LOW

HIGH

LOW

Relationships with temperature and precipitation conditions

HIGH SNOWFALL

LOW SNOWFALL

HIGH SNOWFALL

LOW SNOWFALL

r2 = 0.38

r2 = 0.38

Relationships with ENSO

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 Region High Low High Low

Northeast 4.1 24.3 7.7 11.5

East North Central 8.3 11.6 7.6 12.3

Central 0.7 27.8 8.4 13.6

Southeast 6.0 25.9 2.1 17.4

West North Central 6.6 14.3 9.1 12.9

South 17.4 12.9 2.3 17.9

Southwest 22.3 3.5 6.5 14.2

Northwest 0.0 33.3 14.5 4.3

West 7.6 20.4 10.0 12.1

 

Conterminous U.S. 8.3 17.8 7.8 12.9

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

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

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