Throughout the Holocene, bison have always been more abundant east of the Rocky Mountains with considerably fewer bison found west of the Rocky Mountains. It is likely that drought frequency and snowfall characteristics have influenced the pattern of historical bison occurrence across the northwest United States. Using monthly average snow and precipitation data from the past several decades, average April snow water equivalent (SWE) and summertime drought frequency were analyzed at sites across the northwest United States. A climatic stress index (CSI) was developed by combining average SWE and drought frequency for sites, as these are the climate factors that will most likely affect bison success. The results of the CSI revealed that locations west of the Rockies experience heavier snowfall and a greater frequency of droughts, thus presenting a “double whammy” of climate conditions that bison would have to endure. The locations of highest combined snow and drought frequencies coincide with locations of low bison occurrence.