Trends in spring snow cover retreat over the U.S. and the effect of observation time bias

Our analysis of in-situ snow depth data from several thousand cooperative stations that have long-term snow on the ground observations during the post- World War II period shows a significant and countrywide retreat of spring snow cover extent during the past 50 years. This retreat is not spatially homogeneous and has been most pronounced along the Pacific Coast of the United States from Alaska to California. In these regions the earlier spring snow cover retreat by up to 20 days / 50 years has been documented.

At the U.S. cooperative network the observations are made once per day. Our analysis of the dynamic of the time of these observations reveals a systematic shift from the afternoon (PM) to the predominantly early morning time (AM) of observation during the 20th century. In the regions and seasons with ephemeric snow cover (e.g., spring season over most of the U.S.) this switch artificially increases the amount and even the occurrence of snow on the ground. We found that this effect masks/reduces the observed changes in snowfall and snow on the ground over the contiguous United States.