What causes droughts in the Sierra Nevada?

Droughts in the Sierra Nevada are primarily caused by the behavior of the Pacific High anticyclone, a high-pressure zone that dominates the region's wind patterns. During the summer, this system naturally draws dry air across the mountains.1

Multi-year droughts occur when this high-pressure zone fails to weaken or shift during successive winters. When the Pacific High maintains its strength, it disrupts the normal winter storm tracks by weakening, splitting, or pushing the mid-latitude jet stream to the north, thereby preventing the arrival of the moisture-laden storms necessary for the region's precipitation.1