California Faces Drenching Rains from Back-to-Back Atmospheric Rivers

By Steve Gorman

California is bracing for two consecutive Pacific storms forecasted to bring heavy rainfall across much of the state, raising concerns about potential widespread flooding. Meanwhile, snowfall in higher elevations may help replenish critical freshwater resources.

The first storm, driven by significant atmospheric rivers carrying dense moisture, made landfall on the West Coast, delivering showers and gusty winds from Oregon down through Northern California and the San Francisco Bay area.

These storms also fit the description of a "Pineapple Express," which are Pacific storms sourced from the warm, subtropical waters surrounding Hawaii, explained Daniel Swain, a climate scientist and meteorologist associated with the University of California, Los Angeles.

As the first storm is anticipated to move into Southern California on Thursday, residents—still recovering from recent flooding—have been diligently preparing by filling sandbags and clearing storm drains to brace for the impending deluge.

"We’re going to be filling 40 tons of sandbags at this recreation center," stated Scott Webber, a construction contractor aiding in storm preparations in a San Diego County community that experienced severe flooding earlier this month. "We did 25 tons this morning" at another local sandbagging site, he noted, stressing the importance of community support.

These winter storms, marking the season’s first major event, represent a significant shift for California, which had been enjoying unusually warm weather over recent days.

The storm’s intensity was expected to escalate in the Bay area on Wednesday evening, with projections of 2 to 5 inches of rain in San Francisco by Thursday, and snow anticipated in the surrounding mountains.

The National Weather Service issued a flood watch for the Bay area and California’s Central Coast, along with high-wind warnings for much of the region. While some flooding of roads and streams in Southern California was expected, experts considered the likelihood of major inundation to be low. Heavy rains could persist in parts of Northern California for six to twelve hours, according to Swain.

A Second, Stronger Storm on the Horizon

A second, potentially stronger storm is expected to strike California on Sunday, bringing strong, gusty winds to the north and heavier rainfall in the south, along with additional snowfall in mountain areas.

Although many details about this storm’s path remain uncertain, Swain noted that it appears to carry a denser plume of moisture which could result in heavier and more sustained rainfall. "Suffice it to say there will be some flooding in Southern California," he remarked. "The question is whether it will be typical street flooding from any significant rainstorm or something more severe."

San Diego County has already suffered from record-setting rains and severe flash flooding due to a localized storm the previous week, while parts of Ventura County experienced considerable flooding after an hour’s rainfall equaled a month’s worth in December.

Despite the imminent storms, the precipitation expected in California does not compare to the extreme conditions in Anchorage, Alaska, where a recent storm has pushed the seasonal snowfall total past the 100-inch milestone earlier in the winter than previously recorded.

Last winter, California faced a series of about a dozen atmospheric river storms in rapid succession, leading to mass evacuations, power outages, levee breaches, and road closures in a state that has long battled drought and wildfires. Those storms resulted in the deaths of at least 20 individuals but also significantly helped to alleviate a prolonged drought.

The upcoming storms are similarly expected to benefit the state’s water supply by enhancing mountain snowpacks, which are currently below average.

Over the decades, the U.S. West Coast has seen an average of 10 or 11 atmospheric river storms each year since 1980. However, scientists project that these events will become increasingly frequent and intense over the next century if climate change continues at its current pace.

The current storms also align with the El Niño weather pattern, which reflects a natural variation in the Pacific jet stream resulting in warmer-than-normal sea temperatures along the West Coast of North and South America, according to Swain.