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

The first of two atmospheric rivers is set to hit California on Wednesday and Thursday, bringing heavy rain and snow that may lead to flash flooding, landslides, and hazardous whiteout conditions.

What is an Atmospheric River?

An atmospheric river is a large airborne current that carries moisture over long distances, often from the Pacific Ocean, leading to significant rainfall and snowfall when it reaches land. These phenomena can manifest as long, wispy trails of clouds extending for hundreds of miles. While some atmospheric rivers are relatively weak and provide beneficial precipitation that supports water supplies, others can be far more intense and disruptive.

Current Water and Snowpack Levels in California

The incoming rains and snow are much needed in California, which has been facing below-average precipitation since early October. The state has recorded about 9.81 inches of rain in this timeframe, which is approximately 82% of its historical average. While major reservoirs are at 116% above their average capacity, snowpack across California is at just 32% of normal levels, with the Sierra Nevada experiencing particularly low amounts compared to historical averages.

How Common Are Atmospheric Rivers?

Atmospheric rivers are fairly common, with around 11 occurring on Earth at any given time, as reported by NASA. Though most are weak and do not cause damage, they play a crucial role in providing essential precipitation.

Size and Potential Dangers of Atmospheric Rivers

Atmospheric rivers can carry moisture volumes up to 15 times greater than that of the Mississippi River. The storm expected to impact California could produce heavy rains of up to 6 inches (15 cm), increasing the risk of flash flooding and road washouts. Wind gusts could also result in downed trees and power lines. Additionally, some high-elevation areas may receive up to 3 feet (91 cm) of snow, leading to dangerous whiteout conditions. Last winter, a series of atmospheric rivers caused significant urban flooding, landslides, and road closures, resulting in over $3 billion in damages and 21 fatalities.

The ample rainfall from previous atmospheric rivers promoted vegetation growth, contributing to a less severe wildfire season in 2023. However, heavy rains can also pose risks around areas affected by past wildfires, where burned land is susceptible to mudslides.

Historical Context and Future Outlook

In 2019, an atmospheric river known as the "Pineapple Express" impacted California, bringing significant rainfall that led to mudslides and dangerous conditions for residents.

Experts warn that climate change may lead to larger and potentially more destructive atmospheric rivers in the future. Studies indicate that these events will become more frequent and possibly occur in succession, resulting in economic damage that could be four times greater than individual storms would inflict.

Current Drought Conditions in California

Despite recent rainfall, small areas in northern and southeastern California remain classified as "abnormally" dry, according to the U.S. Drought Monitor. From 2020 to 2023, the state has experienced periods of extreme drought. Research shows that California has not escaped the long-term drying trends that led to the driest 22-year span in over a millennium from 2000 to 2021.