Along with the strange weather phenomena occurring in the southeast region of the U.S. the Bay Area is also being hit by large storms. Under normal circumstances, California’s wettest parts of the year are between October and April as storms cross the Pacific and make their way through the state of California including the Bay Area.

Some of the largest downpours that we experience are fueled by what are called atmospheric rivers which are bands of moisture that are able to flow for thousands of miles from Hawaii, all the way to the West Coast. Alexander Gershunov is a research meteorologist at UC San Diego’s Scripps Institution of Oceanography and he has said that, “An average atmospheric river instantaneously carries two to three times what the Amazon River typically carries,” he tells the San Francisco Chronicle.  Even if you can’t visibly see them, these atmospheric rivers in the sky transport massive amounts of water around the Earth. When this moisture from the rivers falls as rain, they can have destructive consequences.  

Many scientists believe that a series of atmospheric rivers were the cause of the Great Flood of 1862, which was a megaflood that left around 6,000 square miles of California’s Central Valley underwater.  These rivers often cause about $1.1 billion in flood damage yearly only across the Western United States.  A series of atmospheric rivers that lasted 3 weeks that began in December of 2022 fueled storms that doused California in 32 trillion gallons of water.  The storms caused catastrophic floods across the Bay Area and the Central Coast.  

Scientists tell the difference between atmospheric rivers from other storms based on the large amounts of water they transport through the atmosphere, which is also referred to as vertically integrated water vapor transport. Daniel Swain is a climate scientist at UCLA and he has said, “It’s water moving fast above your head”, and the more moisture that is being carried within these rivers, the more precipitation could potentially follow. Atmospheric rivers don’t look like what you imagine when you think of a typical storm. Rivers stretch hundreds or even thousands of miles across, and on average span around 500 miles in width.  The rivers transport their moisture relatively close to the surface, within the lowest mile or two of Earth’s atmosphere.

Recently in California, a high-powered atmospheric river has made its way through and dropped several inches of rain across the state in the process. Rachel Kennedy, a meteorologist for the National Weather Service, has reported 2 to 3 inches of rain in San Mateo County, 2.14 inches of rainfall in downtown San Francisco and about 2.5 inches throughout the rest of the city, the East Bay Hills have received around 3 to 4 inches, and the inland areas had 1.5 to 2.5 inches. 

The South Bay received the least amount of rainfall with only about 1.5 to 2 inches in most of the areas, and the most intense rainfall has hit the Santa Cruz Mountains with about 5.5 to 6 inches with some areas seeing up to 8 inches of rain, and in the higher elevations of Santa Cruz County and the Santa Clara Valley 4.5 to 5.5 inches of rain fell.  Both Sonoma and Alameda County received flood warnings as there were some reports of streams in creeks flooding within those areas.  

Although the atmospheric rivers can be disastrous, atmospheric rivers have some benefits as well.  Swain has said that, “They’re responsible for the majority of California’s water supply.” During the seasons that the Bay Area experiences storms, agriculture and urban water supplies are replenished, and they also help to build up the snowpack that gives California water after the end of the wet season.  Atmospheric rivers can also end droughts, and according to a 2013 study, between 33% and 40% of droughts in California from 1950 to 2010 ended due to landfalling atmospheric rivers.  However, with these drastic changes to our world, climate change is causing the nature of these atmospheric rivers to transform – which is only making California’s relationship with them more dangerous than ever. 

Additional Sources: 

www.sfchronicle.com/projects/2022/atmospheric-river-california/ 

www.sfgate.com/weather/article/rainfall-totals-california-atmospheric-river-20166489.php