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Has spring snowpack decline in the Washington Cascades?

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Our best estimates of 1 April snow water equivalent (SWE) in the Cascade Mountains of Washington State indicate a substantial (roughly 15?35%) decline from mid-century to 2006, with larger declines at low elevations and smaller declines or increases at high elevations. This range of values includes estimates from observations and hydrologic modeling, reflects a range of starting points between about 1930 and 1970 and also reflects uncertainties about sampling. The most important sampling issue springs from the fact that half the 1 April SWE in the Cascades is found below about 1240 m, altitudes at which sampling was poor before 1945. Separating the influences of temperature and precipitation on 1 April SWE in several ways, it is clear that long-term trends are dominated by trends in temperature, whereas variability in precipitation adds "noise" to the time series. Consideration of spatial and temporal patterns of change rules out natural variations like the Pacific Decadal Oscillation as the sole cause of the decline. Regional warming has clearly played a role, but it is not yet possible to quantify how much of that regional warming is related to greenhouse gas emissions.
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... Approximately 70% of the region's precipitation occurred during the wet season (October-April) when the North Cascades were on the receiving end of the Pacific storm track [11,12]. From late spring to early fall, high pressure to the west kept the Pacific Northwest comparatively dry. ...
... Rain on snow events had increased in frequency, this led to an increase in the ratio of the winter precipitation falling as rain versus it falling as snow. Mote [11] noted a decline in the snowpack storage efficiency in the Pacific Northwest, with the ratio between the total accumulation season precipitation and April 1 having retained snowpack SWE (snow water equivalent). Pelto [8] used the Diablo Dam and concrete weather stations for the total accumulation season precipitation for November-March-the period of accumulating the snowpack at the six SNOTEL sites, with long term records in the North Cascades-in order to determine the snowpack storage efficiency (Fish Lake, Lyman Lake, Park Creek, Rainy Pass, Stampede Pass, and Stevens Pass). ...
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