Our best estimates of 1 April snow water equivalent (SWE) in theCascade Mountains of Washington State indicate a substantial (roughly15–35%) decline from mid-century to 2006, with larger declines atlow elevations and smaller declines or increases at highelevations. This range of values includes estimates from observationsand hydrologic modeling, reflects a range of starting points betweenabout 1930 and 1970 and also reflects uncertainties aboutsampling. The most important sampling issue springs from the fact thathalf the 1 April SWE in the Cascades is found below about 1240 m,altitudes at which sampling was poor before 1945. Separating theinfluences of temperature and precipitation on 1 April SWE in severalways, it is clear that long-term trends are dominated by trends intemperature, whereas variability in precipitation adds "noise" tothe time series. Consideration of spatial and temporal patterns ofchange rules out natural variations like the Pacific DecadalOscillation as the sole cause of the decline. Regional warming hasclearly played a role, but it is not yet possible to quantify how muchof that regional warming is related to greenhouse gas emissions.
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