24.5 trillion gallons of rain have pelted California, student scientist estimates

For weeks, California has faced a barrage of fatal storms that have caused roads to partially collapse, neighborhoods to flood and thousands to evacuate. But just how much water has pelted the state?

Colin McCarthy, a freshman atmospheric science student at the University of California, Davis, who has become somewhat of a public figure on the topic, may have the answer. According to his calculations, at least 24.5 trillion gallons of water have been dumped in the 16 days since the storm series began on Dec. 26. 

He used a calculation method that has been touted by the Farmer’s Almanac to determine the number, as well as a determination from the National Weather Prediction Center that estimated the entire state received an average of 8.61 inches of water during that period. 

“One inch of rainfall in an acre equals about 27,154 gallons of water,” he told CBS News Thursday. “And so I did that 27,000 times 8.61, and then [multiplied] it by the amount of acres in California, and that’s how I came to that conclusion.” 

The storm fronts which have hit California over the past several weeks have been driven by multiple atmospheric rivers, long regions in the atmosphere that transport water. According to the National Oceanic and Atmospheric Administration, these weather events hold water vapor that is about the same amount as the average flow of water at the mouth of the Mississippi River. So when they hit land, much of that vapor is released as either snow or rain. 

These events have essentially drenched California’s coastline, with multiple atmospheric rivers since the day after Christmas pounding the state. As of Wednesday, the California Governor’s Office of Emergency Services has reported at least 18 deaths because of the latest storm. 

The NOAA told CBS News it does not measure rainfall in the way that McCarthy calculated. However, Sarah Rogowski of the National Weather Service’s Regional Operations Center was able to provide CBS News with some additional context on the deluge of rain, including maps which might indicate just how much rain was calculated in various areas of the state from Dec. 26 to Jan. 12. 

“Widespread heavy rain was observed beginning in late December and continuing through early to mid-January,” Rogowski said. 

“Areas of the central and northern California Coast as well as the Sierra Foothills measured between 15 and 20 inches of rain over the past few weeks,” she added. 

Several areas experienced record-breaking rainfall. San Francisco, for example, rang in the New Year with 5.46 inches of rain in the 24-hour period of Dec. 31. That marked “the second wettest day in the 170+ years of records at that site,” the weather service said. That amount accounts for nearly 47% of the rainfall for the entire month of December. 

At the same time, Oakland had its wettest day on record since 1970, with 4.75 inches of rain recorded on Dec. 31, the weather service reported. 

The rain, Rogowski said, is expected to stick around for at least another week, until around Jan. 19. After that, she said, “the Climate Prediction Center shows that below normal precipitation is favored across the state” through Jan. 26. 

“It was a historically significant period of rainfall, for sure,” McCarthy told CBS News. 

And though it’s proven deadly, the rainfall has also provided a respite for the severe drought that has been plaguing the state for months. 

McCarthy, who has been following and sharing information on extreme weather for years, and garnered national attention during Hurricane Ian, said that since the storms began, “extreme drought has pretty much been completely removed from California.” 

“We went from a huge amount of the state experiencing extreme drought from about the southern central valley all the way up to the Oregon border, and now the Bay area, Los Angeles, San Diego are only experiencing moderate drought,” McCarthy said. 

“We need wetter than average conditions to continue throughout the rest of winter if we want drought to be removed across a large area of California,” he added.  

According to the latest numbers from the U.S. Drought Monitor Thursday, no areas of California were in the “exceptional drought” category this week. Just a month ago, 7.2% of the state was in that category. 

The “extreme drought” category has also seen a significant drop – from more than 35% a month ago, to just 0.3% today. 

About 95% of California is now considered in the “moderate drought” category, meaning that pasture growth is stunted, landscapes and gardens need earlier irrigation, and ponds and creeks are lower than usual.  

While there has been some improvement, Rogowski said that long-term drought remains an ongoing problem.

“Reservoir storage deficits, deep soil moisture deficits, and significant groundwater depletions that have built up over many months to years, will require additional precipitation to overcome,” she said. “It took years to get into this drought, and will potentially take years to fully recover.”