The Upper Colorado River Basin (UCRB) is a vital source of freshwater for over 10 million people in the southwestern United States. Much of this water comes from snow, which acts like a natural reservoir by slowly melting and feeding rivers. This study looks at how much of the winter precipitation (from November to March) falls as snow versus rain in the UCRB, and how this balance has changed from 1980 to 2022.

We found that northern parts of the basin receive more snow than southern areas. We also discovered that changes in snowfall are closely linked to a climate pattern called the Pacific Meridional Mode (PMM), which affects ocean temperatures and atmospheric circulation. When the PMM is in a positive phase, the region tends to be warmer, leading to more rain and less snow.

These findings suggest that large-scale climate patterns can significantly influence whether precipitation falls as rain or snow—information that is crucial for managing water resources in the face of climate change.