What causes the change in snowpack levels in the Sierra Nevada from year to year?

Sierra Nevada is a Spanish name that means “snowy mountain range.” While the term “snowy” has generally been true for most of American history, the mountain range has seen far less snow accumulation in recent years.
The depth and breadth of the seasonal snowpack in any given year depends on whether a winter is wet or dry. Wet winters (such as 2017 and 2019) tend to stack up a deep snowpack, while dry ones keep it shallow. In 2015, long-term hot and dry conditions in California and Nevada brought snowpack to historically low levels

insertion is the type of mutation