Long-term trends in snowpack Long-term trends in snowpack in the in the

Pacific NorthwestPacific Northwest

Philip Mote, Alan Hamlet, and Dennis Lettenmaier

20th century decline in NH snow cover

R.D. Brown, J. Climate, 2000

Elevation (m)

Areas with December temperaturesnear freezing

Topography of the Pacific Northwest

Trends in snowpack

Use VIC and met data to evaluate trends, roles of temperature and precipitation

Outline

• Snow course results• VIC results• Wild speculation

Trends in April 1 snow water equivalent, 1940-1992

Trends in April 1 snow water equivalent, 1940-1992

Precipitation trend (%)

Tem

pera

ture

tren

d (C

)

Attributing proportion of trend

<S>T = r(S,T) <T>

where<T> = trend in T<S>T = proportion of trend in S

attributable to trend in Tr(S,T) = regression of S on T,

interannual

Outline

• Snow course results• VIC results• Wild speculation

VIC simulation

• 1/8° long x 1/8° lat• 47°N to 49°N, coast to Continental

Divide• Daily weather data, 1/1/1915 -

9/30/98 interpolated to VIC grid points from USHCN stations

+: observed o: nearest VIC grid point

Outline

• Snow course results• VIC results• Wild speculation

Northwest warming

44

45

46

47

48

49

50

51

52

53

54

1900s1920s1940s1960s1980s2000s2020s2040s

Degrees F

warmest scenarioaverage coolest scenarioobservedCGCM1

Conclusions

• Observations show substantial declines in Northwest snowpack; corroboration with VIC suggests they are real

• Trends in temperature are partly responsible

• A foretaste of things to come?