Snow is one of the most important wintertime land surface characteristics and is crucial for water resources over western United States. However, regional snow mass variability and its drivers remain uncertain. Based on our recently developed high‐quality gridded snow mass product, here, for the first time, we quantify and understand the snowpack change from 1982 to 2016 over conterminous United States (ConUS) at 4‐km pixels. Annual maximum snow mass decreases significantly by 41% on average for 13% of snowy pixels over the western United States (or the size of South Carolina). Over ConUS, snow season was shortened significantly by 34 days on average for 9% of the snowy pixels (or the size of Virginia). October–March mean temperature and accumulated precipitation largely explain the temporal variability of 1 April snow mass over western United States, and considering temperature alone would exaggerate the warming effect on the snow decrease. For the snowpack projection in the next few decades over United States to be reliable, Earth system models need to demonstrate their capability in reproducing historical snowpack variabilities and trends and their different relations with temperature and precipitation over western versus eastern ConUS, as reported here.