What California’s Atmospheric Rivers Mean for Water and Snowpack Levels

A second atmospheric river storm, referred to as a Pineapple Express weather system, has made its way to California and is predicted to persist through Tuesday. This storm is bringing heavy rain that could lead to flash floods and mudslides.

What Is an Atmospheric River?

An atmospheric river is a substantial current of moisture suspended in the atmosphere, traveling across hundreds of miles from the Pacific Ocean and depositing as intense rain and snow upon land. These weather systems can often be identified as long, thin swaths of clouds stretching for great distances.

While some atmospheric rivers are weak and provide beneficial precipitation to replenish water supplies, they play a significant role in the global water cycle.

Current Water and Snowpack Levels in California

The heavy precipitation is a welcome development for California, which has seen below-average rainfall since the start of October. During this period, the state has recorded 9.81 inches of precipitation, approximately 82% of its historical average. Major reservoirs in California are currently at 116% above their typical levels, though the snowpack across the state stands at only 32% of average levels. Specifically, snowpack in the Sierra Nevada mountains is notably below average, while areas around Mount Shasta are closer to average.

Both reservoirs and melting snowpack are crucial for countering dry conditions during the warmer months.

How Common Are Atmospheric Rivers?

Atmospheric rivers, sometimes referred to as "rivers in the sky," are relatively common phenomena, with around eleven existing on Earth at any given moment, according to NASA. Most of these atmospheric rivers are weak and do not cause significant damage; instead, they often provide essential rainfall or snowfall.

Size and Potential Dangers

These atmospheric rivers can carry up to fifteen times the volume of the Mississippi River, as noted by the National Oceanic and Atmospheric Administration. The current storm is expected to bring heavy rainfall to California with projected totals of up to 6 inches, leading to potential flash flooding and road washouts. Wind gusts may topple trees and power lines, resulting in power outages. In higher elevations, snowfall could reach up to 3 feet, potentially creating hazardous whiteout conditions on roadways.

Last winter, California experienced a series of twelve atmospheric rivers that resulted in urban flooding, landslides, and road closures, leading to the deaths of 21 individuals and over $3 billion in damages, according to researchers from Stanford University. This significant rainfall promoted substantial growth in grasses and scrub, contributing to a less severe wildfire season in 2023. Heavy rains can, however, pose threats around areas affected by previous wildfires, as the altered landscape may become vulnerable to mudslides.

In 2019, a notable atmospheric river dubbed the "Pineapple Express" brought moisture from near Hawaii to California, resulting in torrential rain and mudslides that forced some motorists to escape into floodwaters.

Impact of Climate Change

Research indicates that atmospheric rivers affecting California and areas like British Columbia are likely to become larger and potentially more destructive due to climate change. They are expected to occur more frequently and in greater clusters, potentially causing up to four times the economic damage compared to earlier individual occurrences.

Is California Free of Drought?

According to the U.S. Drought Monitor, minor regions in northern and southeastern California are currently classified as "abnormally dry." From early 2020 through 2023, the state has endured periods of both "extreme" and "exceptional" drought conditions. Experts warn that California and the broader Western region have not escaped the long-term dry patterns that, as a study published in Nature Climate Change suggests, created the driest span in 12 centuries from 2000 to 2021.