Issues with Identification of Trends in the 20th Century U.S. Snowfall

Daily data from the National Weather Service cooperative observer network taken prior to 1948 was recently put into digital form, opening the door to analyses spanning the entire 20th Century. An examination of the U.S. daily snowfall records for the 20th Century revealed numerous apparent inconsistencies. For example, in many cases long-term snowfall variations and trends among neighboring stations differ greatly. Given the close proximity, such differences most likely are not the result of atmospheric processes, but instead probably reflect inhomogeneities in the snowfall records. Internal inconsistencies in the snow records of some of these stations also point to inhomogeneities. For example, the frequency of daily observations with a 10:1 snowfall to liquid equivalent ratio declined nationwide from 30% in the 1930s to a current value of around 10%, a change that is clearly due to observational practice. Since the change in the ratio of snowfall to liquid equivalent is due to observational practices, there must be biases in cold season liquid equivalent precipitation, or snowfall, or both. An empirical adjustment of snow-event, liquid-equivalent precipitation indicates that the potential biases can be statistically significant.