A year ago, California was experiencing one of its driest wet seasons on record. The dryness turned into tragedy, with devastating wildfires in and around Los Angeles. This year, the climate has whiplashed back to wet. In southern California, it’s the wettest start to the water year in more than 20 years, according to the National Weather Service, and the 4th-wettest on record dating back to 1877.

GOES satellite images from dawn to dusk on December 31, 2025 (7 am to 6 pm PST). Sunrise reveals a bank of clouds flowing north over California. An atmospheric river (right of bottom center of the frame) feeds the storm from the south over the course of the day. The moisture is being drawn north and west (counterclockwise) by an area of low pressure that occupies roughly the center of the frame. Credit: Climate.us/Robert Simmon.

This animation captures the natural phenomenon at the heart of the region’s big swings in precipitation from year to year: transient atmospheric rivers that carry massive amounts of water vapor into the mid-latitudes from as far away as the tropics. When the warm, wet air climbs West Coast mountains, it cools and unleashes huge amounts of snow and rain. These rivers of water vapor can flow for hours or even days at a time, providing a steady supply of moisture to mid-latitude weather systems such as cold fronts, cut-off lows, and extratropical cyclones. 

Not only can atmospheric rivers deliver huge amounts of precipitation in a single event, they also tend to cluster, occurring virtually back to back within a season. It’s this behavior—and whether a season has a lot of atmospheric rivers or few—that is largely responsible for the huge spread between California’s wettest and driest years.

GOES satellite images from dawn to dusk on January 5, 2026 (7 am to 6 pm PST). A swath of clouds stretches from the bottom center of the frame toward California at image right. Beneath those clouds, an atmospheric river is petering out. The comma-shaped feature near the center of the image explains the motion of clouds across the whole frame; air is being drawn toward an area of low pressure in a counterclockwise direction. Credit: Climate.us/Robert Simmon.

The event captured in the January 5 animation was the second of two that struck the U.S. West Coast in the first week of January. According to an event summary from the Center for Western Weather and Water Extremes:

The remnants of the first AR [atmospheric river] and the second AR impacting northern California and southern Oregon in quick succession led to ~70–85 hours of AR conditions… . 10+ inches of precipitation was received in portions of the California Coast Ranges, the highest elevations of the Transverse Ranges and in the northern Sierra Nevada… . An estimated 24–48 in. of snow fell across much of the Sierra Nevada, with snowfall totals exceeding 72 in. over the highest elevations.

By bringing repeated rounds of heavy precipitation, atmospheric rivers can cause flooding, avalanches, and landslides. They can increase fire activity in the following dry season. But they also recharge the region’s surface water supplies and can bring an end to deep drought. Given their importance to extreme events and water supplies in the West and other parts of the country, climate and weather scientists are working to understand every aspect of atmospheric rivers, including how to detect and describe them in a globally consistent way, if they can be predicted farther in advance, and how climate change might affect their strength or frequency.

Pair of Sentinel-2 satellite images capturing the snowy transformation of Lassen Volcanic National Park between December 14 (top) and 29 (bottom).
Three back-to-back atmospheric river events produced heavy rain over the Pacific Northwest and central California in late December 2025. A fourth storm carried the precipitation into the mountains, where it fell as snow. This pair of Sentinel-2 satellite images captures the snowy transformation of Lassen Volcanic National Park between December 14 and 29. Ten-day precipitation totals exceeded 15 inches in the southern Cascade Mountains, where the park is located. In the nearby Sierra Nevadas, snowpack increased from ~7–30% of normal to 57–100% of normal between December 22 and 27, according to the Center for Western Weather and Water Extremes.

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